<PAGE>

                                                                     Exhibit 2.4

                         AGREEMENT AND PLAN OF MERGER

     THIS AGREEMENT AND PLAN OF MERGER is dated as of January 7, 1999, by and
among BSH Holding LLC, a Delaware limited liability company ("BSH Holding"), SIG
Holding LLC, a Delaware limited liability company ("SIG Holding"), and Bronner
Slosberg Humphrey Co., a business trust formed under the laws of The
Commonwealth of Massachusetts ("Parent").


                             W I T N E S S E T H:

     WHEREAS, each of BSH Holding and SIG Holding is a limited liability company
duly formed and validly existing under the laws of the State of Delaware;

     WHEREAS, Parent is a business trust duly organized and validly existing
under the laws of The Commonwealth of Massachusetts and is (i) the sole member
and 100% beneficial owner of BSH Holding, and (ii) the sole member and 100%
beneficial owner of SIG Holding;

     WHEREAS, upon the terms and subject to the conditions of this Agreement and
in accordance with the Delaware Limited Liability Company Act (the "Act"), the
Limited Liability Company Agreement of BSH Holding dated December 30, 1998 (the
"BSH Agreement"), and the Limited Liability Company Agreement of SIG Holding
dated December 30, 1998 (the "SIG Agreement"), BSH Holding, SIG Holding, and
Parent desire to enter into a business combination transaction pursuant to which
SIG Holding will merge with and into BSH Holding, with BSH Holding surviving,
which merger is intended to constitute a merger of one disregarded entity with
another for tax purposes under the Internal Revenue Code of 1986, as amended;
and

     WHEREAS, Parent, as the sole member of BSH Holding and as sole member of
SIG Holding, has adopted and approved this Agreement and the Merger (as
hereinafter defined) in accordance with Section 209 of the Act and, as the case
may be, the BSH Agreement and the SIG Agreement.

     NOW, THEREFORE, in consideration of the foregoing and the mutual covenants
and agreements contained herein, and intending to be legally bound hereby, each
of BSH Holding and SIG Holding agree as follows:

     1.   The Merger.  In accordance with the relevant provisions of the Act, at
          ----------
the Effective Time (as hereinafter defined), SIG Holding shall be merged with
and into the BSH Holding (the "Merger").  As a result of and following the
Merger, the separate existence of SIG Holding shall cease to exist and BSH
Holding shall continue as the surviving entity of the Merger (the "Surviving
Company"), and all identity, rights, powers, privileges, assets and all
obligations, liabilities and debts of SIG Holding shall be vested in the
Surviving Company in accordance with Section 209(g) of the Act.
<PAGE>

     2.   The Effective Time.  The Merger shall become effective (the "Effective
          ------------------
Time") immediately upon the later of (i) the date on which a Certificate of
Merger, together with any other documents required to be filed to consummate the
Merger, is filed with and accepted by the Secretary of State of the State of
Delaware, or (ii) any future effective date or time of the Merger stated in the
Certificate of Merger filed with and accepted by the Secretary of State of the
State of Delaware.

     3.   Certificate of Formation.  The Certificate of Formation of BSH
          ------------------------
Holding, as in effect on the Effective Date, shall be the Certificate of
Formation of the Surviving Company until thereafter changed or amended as
provided therein or by applicable law.

     4.   Limited Liability Company Operating Agreement.  The Limited Liability
          ---------------------------------------------
Company Operating Agreement of BSH Holding as in effect on the Effective Date
and attached hereto as Exhibit A (the "Surviving Company Operating Agreement")
                       ---------
shall be the Limited Liability Company Operating Agreement of the Surviving
Company until thereafter changed or amended as provided therein or by applicable
law.

     5.   Purposes.  The purposes of the Surviving Company shall be as set forth
          --------
in the Surviving Company Operating Agreement as in effect on the date hereof
until such time as such purposes may be changed or amended as provided in the
Surviving Company Operating Agreement and by applicable law.

     6.   Officers of Surviving Company.  The officers of BSH Holding on the
          -----------------------------
Effective Date shall be the officers of the Surviving Company in accordance with
the Surviving Company Operating Agreement and the delegation of authority
thereunder by the sole member of the Surviving Company, until successors
therefor are duly appointed and qualified, or such delegation is revoked, as the
case may be.

     7.   LLC Membership Interests. As of the Effective Date, all of the
          ------------------------
membership interests in SIG Holding and any certificates evidencing the same,
shall be canceled, and the membership interests in the Surviving Company
outstanding immediately prior to the Merger shall remain issued and outstanding
from and after the Effective Date.

     8.   Additional Actions.  If, at any time on and after the Effective Time,
          ------------------
the Surviving Company or its successors and assigns shall consider or be advised
that any further assignments or assurances in law or any organizational or other
acts are necessary or desirable (a) to vest, perfect or confirm, of record or
otherwise, in the Surviving Company title to and possession of any property or
right of SIG Holding acquired or to be acquired by reason of, or as a result of,
the Merger, or (b) otherwise to carry out the purposes of this Agreement, SIG
Holding, its sole member and its officers shall be deemed to have granted to the
Surviving Company an irrevocable power of attorney to execute and deliver all
such proper deeds,

                                       2
<PAGE>

assignments and assurances in law and to do all acts necessary or proper to
vest, perfect or confirm title to and possession of such property or rights in
the Surviving Company and otherwise to carry out the purposes of this Agreement.

     9.   Termination.  Notwithstanding the prior approval of this Agreement by
          -----------
Parent, this Agreement may be terminated by Parent for any reason and at any
time prior to the Effective Time.

     10.  Amendment.  Notwithstanding the prior approval of this Agreement by
          ---------
Parent, any term or provision of this Agreement may be amended by Parent for any
reason and at any time prior to the Effective Time; provided, however, that any
                                                    --------  -------
amendment which directly, materially and adversely affects any right
specifically granted hereunder to a particular party in a manner different than
other parties shall not be effective unless such party has consented to such
amendment.



              [Remainder of this page intentionally left blank.]

                                       3
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement and
Plan of Merger as of the date first set forth above.

                              BSH HOLDING LLC, a Delaware limited liability
                              company


                              By:   /s/ David W. Kenny
                                  -----------------------------------------
                                  Name: David W. Kenny
                                  Title: Chief Executive Officer


                              SIG HOLDING LLC, a Delaware limited liability
                              company


                              By:   /s/ David W. Kenny
                                  -----------------------------------------
                                  Name: David W. Kenny
                                  Title: Chief Financial Officer


                              BRONNER SLOSBERG HUMPHREY CO., a
                              Massachusetts business trust

                              By:   /s/ David W. Kenny
                                  -----------------------------------------
                                  Name: David W. Kenny
                                  Title: Chief Executive Officer


                              Bronner Slosberg Humphrey Co. (the "Trust") is a
                              Massachusetts business trust. A copy of the
                              Trust's Declaration of Trust, as the same may be
                              amended and/or restated from time to time, is on
                              file with the Secretary of State of The
                              Commonwealth of Massachusetts. Each of the parties
                              hereto acknowledges and agrees that this Agreement
                              is not executed on behalf of the trustees or
                              officers of the Trust as individuals or, in the
                              event the trustee is a corporation or other
                              entity, on behalf of the individual owners of such
                              corporation or entity, and the obligations of this
                              Agreement are not binding upon any of the
                              trustees, officers or shareholders of the Trust,
                              or any of their respective trustees, officers,
                              directors, partners, members or shareholders,
                              individually, but are binding only upon the assets
                              and property of the Trust. Each of the parties
                              hereto agrees that no shareholder, trustee or
                              officer of the Trust, or any of their respective
                              trustees, officers, directors, partners, members
                              and shareholders, may be held personally liable or
                              responsible for any obligations of the Trust
                              arising out of this Agreement. With respect to
                              obligations of the Trust arising out of this
                              Agreement, each of the parties hereto shall look
                              for payment or satisfaction of any claim solely to
                              the assets and property of the Trust.
<PAGE>
                                                                       Exhibit A

                                BSH HOLDING LLC

                      Limited Liability Company Agreement
                      -----------------------------------

     This Agreement is made as of December 30, 1998 by and between BSH Holding
LLC, a Delaware limited liability company (the "LLC"), and Bronner Slosberg
Humphrey Co., a Massachusetts business trust with a business office at The
Prudential Tower, 800 Boylston Street, Boston, MA  02199, as the sole member
(the "Member") of the LLC.

     WHEREAS, the LLC was formed as a limited liability company under the
Delaware Limited Liability Company Act (as amended from time to time, the "Act")
on December 28, 1998; and

     WHEREAS, the Member wishes to set out fully its rights, obligations and
duties regarding the LLC and its assets and liabilities.

     NOW, THEREFORE, in consideration of the mutual covenants expressed herein,
the parties hereby agree as follows:

     1.   Purpose; Powers.  The principal business activity and purposes of the
          ---------------
LLC shall initially be to receive, hold and manage the 100% membership interest
of the Member in Bronner Slosberg Humphrey, LLC, a Delaware limited liability
company (which membership interest shall be assigned and transferred by the
Member to the LLC on or immediately after the date hereof), and to engage in any
business related thereto or useful in connection therewith, as well as any and
all other activities permitted under the Act.  The LLC shall possess and may
exercise all the powers and privileges granted by the Act, any other law or this
Agreement, together with any powers incidental thereto, and may take any other
action not prohibited under the Act or other applicable law, so far as such
powers and actions are necessary or convenient to the conduct, promotion or
attainment of the business, purposes or activities of the LLC.

     2.   Principal Place of Business.  The principal office and place of
          ---------------------------
business of the LLC shall be located at The Prudential Tower, 800 Boylston
Street, Boston, Massachusetts  02199. The Member may change the principal office
or place of business of the LLC at any time and may cause the LLC to establish
other offices or places of business in various jurisdictions and appoint agents
for service of process in such jurisdictions.  The initial registered agent of
the LLC shall be The Corporation Trust Company, 1209 Orange Street, Wilmington,
Delaware 19801.  The registered office of the LLC in the State of Delaware, as
required by Section 18-104 of the Act, shall be The Corporation Trust Company at
such address.

     3.   Fiscal Year.  The fiscal year end of the LLC shall be December 31.
          -----------

     4.   Capital Contributions.  The Member is contributing the amount
          ---------------------
specified on Schedule A attached hereto in cash to the capital of the LLC at the
             ----------
date hereof.  Future contributions to capital, if any, shall be made in such
amounts and at such times as the
<PAGE>

members of the LCC may mutually agree. The percentage interest of each member of
the LLC in the profits or losses of the LLC shall be denominated by membership
units (the "Membership Units"), with the Member's initial number of Membership
Units being as listed on Schedule A, which Membership Units shall be in
                         ----------
proportion to each member's capital contributions. The aggregate number of
Membership Units which the LLC shall have authority to issue is 1,000. Each such
Membership Unit shall be entitled to one vote on all matters for which a vote or
decision of the membership of the LLC is required or taken.

     5.   Capital Account; Distributions.  A capital account shall be
          ------------------------------
established for the Member and shall be maintained in accordance with applicable
regulations under Section 704(b) of the Internal Revenue Code of 1986, as
amended (the "Code").  A capital account shall be maintained for the sole
purpose of allocating income, gain, loss and deduction to the Member and shall
have no effect on the amount of any distributions to the Member in liquidation
or otherwise.

     6.   Certificates.
          ------------

          (a) General.  At the option of the Member, interests in the LLC may be
              -------
represented by one or more certificates, in such form as may from time to time
be prescribed by the Member.  Such certificate shall be signed by the Chief
Executive Officer, President, or Vice President of the LLC and by the Chief
Financial Officer, Treasurer, Assistant Treasurer, Secretary or Clerk of the
LLC, which signatures may be facsimiles thereof.  In case an officer of the LLC
who has signed or whose facsimile signature has been placed on such certificate
shall have ceased to serve as such officer of the LLC before such certificate is
issued, it may be issued by the LLC with the same effect as if such person were
an officer of the LLC at the time of its issue.  The certificate shall contain a
legend with respect to any restrictions on transfer.

          (b) Application of Article 8 of the Uniform Commercial Code.  The LLC
              -------------------------------------------------------
hereby irrevocably elects that all interests in the LLC shall be securities
governed by Article 8 of the Uniform Commercial Code in effect in the State of
Delaware.  Each certificate, if any, evidencing an interest in the LLC shall
bear the following legend:

     "This Certificate evidences a membership interest in BSH Holding LLC and
     shall be a security for purposes of Article 8 of the Uniform Commercial
     Code in effect in the State of Delaware."

No change to this provision shall be effective until all outstanding
certificates have been surrendered for cancellation and any new certificates
thereafter issued shall not bear the foregoing legend.

                                       2
<PAGE>

     7.   Management.
          ----------

          (a) General.  The management of the LLC's business and affairs shall
              -------
be vested in the Member.  The Member shall have full power and authority (a) to
take any action and execute any documents on behalf of the LLC, and (b) to
appoint such officers, representatives, or agents for the LLC (each of whom
shall serve in such capacity until the Member removes such person or appoints a
successor for such person), and to grant to any such officer, representative or
agent the power and authority to take such action and execute such documents on
behalf of the LLC as may be determined by the Member. In addition, the Member
may enter into one or more management agreements with one or more third party
managers, and assign such managers the rights, duties and obligations as
provided in such management agreement.  Any delegation of powers and duties
pursuant to this Section 7 may be revoked at any time by the Member.  Any
officer of the LLC may be removed with or without cause at any time by the
Member.

          (b) Officers of the LLC. Unless the Member decides otherwise, the
              -------------------
officers of the Member from time to time shall have the authority to act on
behalf of the Member in managing and conducting the business of the LLC as
provided for in this Section 7. Unless the Member decides otherwise, if an
official title is one commonly used for officers of a business corporation
formed under the Delaware General Corporation Law, the assignment or incumbency
of such title to a person who serves as such officer of the Member shall
constitute assignment of the same title, and delegation of the authorities and
duties normally associated with that office, to such person as such officer of
the LLC.

          (c) Assignment of Membership Interest; Withdrawal of Member.  The
              -------------------------------------------------------
membership interest of the Member in the LLC shall be assignable in whole (but
not in part) by the Member.  At the option of the Member, the person to whom
such membership interest is assigned may be admitted a new member of the LLC (a
"Successor Member"), with all the rights, privileges, duties and obligations
thereof, including without limitation the right to manage the business of the
LLC, provided that (i) the Member executes a signed writing evidencing its
resignation and withdrawal from membership in the LCC in favor of such Successor
Member, and (ii) such Successor Member consents in writing to be admitted as a
member of the LLC and to assume all the rights, privileges, duties and
obligations of the Member thereof.

     8.   Limitation of Liability.  Except as otherwise provided in the Act, the
          -----------------------
Member shall not be obligated personally for any debt, obligation or liability
of the LLC, whether arising in contract, tort or otherwise, solely by reason of
being a member of the LLC. The Member shall have no responsibility to restore
any negative balance in its capital account. The Member shall not be personally
liable to the LLC for acting in good faith reliance upon the provisions of this
Agreement, or for breach of any fiduciary or other duty that does not involve
(i) a breach of the duty of loyalty to the LLC, (ii) acts or omissions not in
good faith or which involve intentional misconduct or a knowing violation of
law, or (iii) a transaction from which the Member derived an improper personal
benefit.

                                       3
<PAGE>

     9.   Other Business.  The Member may engage or have an interest in other
          --------------
business ventures which are similar to or competitive with the business of the
LLC, and the pursuit of such ventures, even if competitive, shall not be deemed
wrongful or improper or give the LLC any rights with respect thereto.  The
Member shall not be obligated to present an investment opportunity to the LLC
even if it is similar or consistent with the business of the LLC, and it shall
have a right to take for its own account or recommend to others any such
investment opportunity.

     10.  Indemnification.  To the fullest extent permitted by law, the LLC
          ---------------
shall indemnify the Member and any and all officers and agents of the LLC and
the officers, directors, trustees, partners, members and shareholders of any
such person which is a corporation, trust, partnership, limited liability
company or other entity, against any and all liability incurred and/or for any
act performed by them in good faith within the scope of the authority conferred
on them by this Agreement, and/or for any act in good faith omitted to be
performed by them (including, without limitation, reasonable legal and other
professional fees and expenses), except for their gross negligence or willful
misconduct.

     11.  Term.  The LLC shall have a perpetual existence and shall not
          ----
terminate upon the resignation, bankruptcy or dissolution of the Member or the
occurrence of any other event which terminates the continued membership of the
Member in the LLC; provided, however, the LLC shall terminate upon the first to
                   --------  -------
occur of the following:

          (a) the written consent of the Member; or

          (b) the entry of a decree of judicial dissolution under Section 18-802
of the Act.

Upon any such dissolution, the Member shall proceed to liquidate the LLC and
wind up its affairs and make final distributions as provided in the Act. The
cost of dissolution and liquidation shall be borne as an expense of the LLC. On
completion of the liquidation, the LLC shall be terminated and the Member (or
such other person or persons as the Act may require or permit) shall file a
Certificate of Cancellation with the Secretary of State of the State of Delaware
under the Act and take such other actions as may be necessary to terminate the
existence of the LLC and cancel any other filings as appropriate.

     12.  Amendment.  This Agreement may only be amended pursuant to an
          ---------
instrument in writing signed by the Member.

     13.  Governing Law.  This Agreement is governed by and shall be construed
          -------------
in accordance with the laws of the State of Delaware, exclusive of its conflict-
of-laws principles.

     14.  Entire Agreement.  This Agreement embodies the entire agreement and
          ----------------
understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior agreements and understandings relating to such
matter.

                                       4
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have executed this Agreement as of
the date set forth above.

MEMBER:                            BRONNER SLOSBERG HUMPHREY CO.,
------

                                   By:
                                       -----------------------------------------
                                       Michael E. Bronner, as sole trustee

                                   Bronner Slosberg Humphrey Co. (the "Trust")
                                   is a Massachusetts business trust. A copy of
                                   the Trust's Declaration of Trust, as the same
                                   may be amended and/or restated from time to
                                   time, is on file with the Secretary of State
                                   of The Commonwealth of Massachusetts. Each of
                                   the parties hereto acknowledges and agrees
                                   that this Agreement is not executed on behalf
                                   of the trustees or officers of the Trust as
                                   individuals or, in the event the trustee is a
                                   corporation or other entity, on behalf of the
                                   individual owners of such corporation or
                                   entity, and the obligations of this Agreement
                                   are not binding upon any of the trustees,
                                   officers or shareholders of the Trust, or any
                                   of their respective trustees, officers,
                                   directors, partners, members or shareholders,
                                   individually, but are binding only upon the
                                   assets and property of the Trust. Each of the
                                   parties hereto agrees that no shareholder,
                                   trustee or officer of the Trust, or any of
                                   their respective trustees, officers,
                                   directors, partners, members and
                                   shareholders, may be held personally liable
                                   or responsible for any obligations of the
                                   Trust arising out of this Agreement. With
                                   respect to obligations of the Trust arising
                                   out of this Agreement, each of the parties
                                   hereto shall look for payment or satisfaction
                                   of any claim solely to the assets and
                                   property of the Trust.

LLC:                               BSH HOLDING LLC, a Delaware
---                                limited liability company

                                   By: BRONNER SLOSBERG HUMPHREY CO.,
                                       as sole member

                                   By:
                                      ------------------------------------------
                                      Michael E. Bronner, as sole trustee

                                   Bronner Slosberg Humphrey Co. (the "Trust")
                                   is a Massachusetts business trust. A copy of
                                   the Trust's Declaration of Trust, as the same
                                   may be amended and/or restated from time to
                                   time, is on file with the Secretary of State
                                   of The Commonwealth of Massachusetts. Each of
                                   the parties hereto acknowledges and agrees
                                   that this Agreement is not executed on behalf
                                   of the trustees or officers of the Trust as
                                   individuals or, in the event the trustee is a
                                   corporation or other entity, on behalf of the
                                   individual owners of such corporation or
                                   entity, and the obligations of this Agreement
                                   are not binding upon any of the trustees,
                                   officers or shareholders of the Trust, or any
                                   of their respective trustees, officers,
                                   directors, partners, members or shareholders,
                                   individually, but are binding only upon the
                                   assets and property of the Trust. Each of the
                                   parties hereto agrees that no shareholder,
                                   trustee or officer of the Trust, or any of
                                   their respective trustees, officers,
                                   directors, partners, members and
                                   shareholders, may be held personally liable
                                   or responsible for any obligations of the
                                   Trust arising out of this Agreement. With
                                   respect to obligations of the Trust arising
                                   out of this Agreement, each of the parties
                                   hereto shall look for payment or satisfaction
                                   of any claim solely to the assets and
                                   property of the Trust.
<PAGE>

                                  Schedule A
                                  ----------


                                  Initial Capital    Membership   Percentage
                                  ---------------    ----------   ----------
Member                             Contribution        Units       Interest
------                             ------------        -----       --------

Bronner Slosberg Humphrey Co.           $10              1            100%