<Page>

                                                               EXHIBIT 10(a)(28)

              SEPARATION AGREEMENT AND WAIVER AND RELEASE OF CLAIMS
                               REVISED 10/11/2002

     This SEPARATION AGREEMENT AND WAIVER AND RELEASE OF CLAIMS (the
"AGREEMENT") is entered into this 11th day of October, 2002 by and between
Aquila, Inc. and Aquila Merchant Services, Inc. (collectively, "AQUILA") and
Edward K. Mills ("YOU") a Texas resident.

                                    RECITALS

     WHEREAS, Your employment with Aquila will be terminated effective November
15, 2002 as part of a strategic repositioning of the business; and

     WHEREAS, You desire to cooperate with Aquila in investigations and lawsuits
against Aquila under the terms and conditions of this Agreement; and

     WHEREAS, in consideration for this Agreement, You desire to release Aquila
from any and all current or potential liability stemming from Your employment
with Aquila;

     NOW THEREFORE, in consideration of the mutual undertakings hereinafter set
forth, and for other good and valuable consideration, which each party hereby
acknowledges, it is agreed as follows:

                                    AGREEMENT

     1.   TERMINATION OF EMPLOYMENT. As part of a strategic repositioning of the
business, Your employment with Aquila will be terminated effective November 15,
2002. From today until Your final date of employment, you will be considered an
employee and afforded all the privileges of full employment, but will not be
required to be present at any office or to have any further commercial
responsibilities except as is mutually agreed between Aquila and You.

     2.   CONSIDERATION. As consideration for Your execution of this Agreement,
Aquila agrees to make payments and provide the benefits set forth in EXHIBIT A.
You are receiving payments and benefits in addition to any payments or benefits
for which You would be eligible if You did not sign this Agreement; these are
the entire payments and benefits that You will receive pursuant to this
Agreement.

     3.   EMPLOYEE BENEFITS. The benefits provided under any Aquila-sponsored
employee benefit plan will be subject to all terms and conditions set forth
therein. Aquila recognizes that termination of employment does not affect any of
the benefits described in Exhibit A.

     4.   CONTINUED COOPERATION. You agree reasonably to: (i) cooperate and
assist Aquila and its affiliates in the orderly transition of management, and
(ii) cooperate and assist Aquila and

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its affiliates in the investigation and handling of any pending or future
litigation, regulatory proceeding, investigations, or administrative or other
hearing, whether formal or informal, initiated by Aquila or by any person,
entity or governmental body against Aquila or any subsidiary or affiliate of
Aquila (a "HEARING"). Your obligations to cooperate and assist Aquila include
any Hearing that relates to (in whole or in part) the period of time during
which You were an employee of Aquila. Aquila agrees to pay You $300 per hour for
time working with Aquila and to reimburse You for all reasonable out-of-pocket
expenses incurred by You in connection with such cooperation and assistance,
including reasonable attorneys' fees approved in advance by Aquila. This
Continued Cooperation will terminate at the end of the severance period
described in Exhibit A, unless otherwise required by law.

     5.   RELEASE AND WAIVER OF CLAIMS. In exchange for this Agreement, You (on
behalf of You and anyone claiming through or on behalf of You), release Aquila
and Aquila's affiliates, their successors and assigns, and all of their past and
present employees, officers, directors, attorneys, stockholders, insurers,
fiduciaries of benefit plans and agents from any and all claims and potential
claims, known or unknown, demands and causes of action You have or may have had
against any of them arising out of Your employment or termination from Aquila,
as well as all future employment-related claims, including claims not currently
known to or contemplated by the parties, to the maximum extent permitted by law.
This release includes, but is not limited to, any and all claims, demands and
causes of action which are related to or concern: any employment agreement; Your
employment and Your termination; attorneys' fees or costs; the Aquila Workforce
Transition Program; discrimination under local, state or federal law; the
Missouri Service Letter Statute; the Age Discrimination in Employment Act; Title
VII of the Civil Rights Act of 1964; the Civil Rights Act of 1991; the Americans
With Disabilities Act; the Employee Retirement Income Security Act; the Family
and Medical Leave Act; severance pay; tort claims including invasion or privacy,
defamation, fraud and infliction of emotional distress; disputed wage claims;
and all other claims, demands, and causes of action, whether they arise in the
United States of America or elsewhere, to the maximum extent permitted by law.
This Release does not include (a) any rights or benefits as set forth in EXHIBIT
A to this Agreement, or (b) any vested benefits, as of Your termination date,
under any Aquila-sponsored employee benefit plans, in accordance with the terms
of such plans. In exchange for this Agreement, Aquila and Aquila's affiliates,
their successors and assigns, release You, Your heirs, successors and assigns,
from any and all claims, demands and causes of action which they have or may
have against You or them prior to the date this Agreement is signed to the
maximum extent permitted by law. This Release does not include any obligations
You or they have under this Agreement.

     6.   NO ADMISSION OF WRONGDOING. This Agreement is not admission of
wrongdoing or liability by You, Aquila, or any of the individuals or entities
set forth in Paragraphs 5, above. Any and all such wrongdoing or liability is
expressly denied.

     7.   RETURN OF COMPANY PROPERTY. Except as otherwise agreed in writing by
the parties, You represent that You will have returned all Aquila's and Aquila's
affiliates' files, records, documents, plans, drawings, specifications,
equipment, software, pictures, videotapes, or any property or other items of
Aquila or Aquila's affiliates in Your possession or concerning the business of
Aquila or Aquila's affiliates, whether prepared by You or otherwise coming into
Your possession or control as of the date you sign this Agreement. This
paragraph does not apply to

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data or information that is in the public domain, or to property, data or
information that You and Aquila agree is immaterial.

     8.   PROPRIETARY INFORMATION. You agree that You shall not at any time,
except as authorized by the Chairman of Aquila or his authorized designee,
communicate, divulge or use, for Your own benefit or for the benefit of any
other person, firm, or corporation, any confidential or proprietary information
concerning Aquila's and Aquila's affiliates' business, including but not limited
to Aquila's and Aquila's affiliates' operations, services, materials, policies,
and the manner in which they are developed, marketed, and provided, and such
other information regarded as trade secrets or confidential or proprietary
information under any applicable law, including without limitation information
that is attorney work product or attorney-client privileged. These provisions do
not apply to data or information that are compelled to be released by law or
judicial process, or to data or information that are in the public domain, or
are subsequently released by Aquila to the public domain.

     9.   NON-DISPARAGEMENT. The parties agree that they will not in any way
disparage each other. Further, the parties agree that they will not make nor
solicit any comments, statements, or the like to the media or to third parties
that may be considered derogatory or detrimental to the good name or business
reputation of the other.

     10.  INDEMNIFICATION. Aquila agrees to the maximum extent permitted by
applicable law, to defend and indemnify You and pay all costs and expenses as
they become due (including any reasonable attorneys' fees approved in advance by
Aquila) in any lawsuit and/or administrative proceeding pending against You as
of Your termination date, and to further defend and indemnify You in any lawsuit
and/or administrative proceeding filed in the future, so long as Your actions
which are or may be the subject of litigation and/or administrative proceedings
were taken: (i) within the course and scope of Your employment with Aquila or as
a director or officer with any Aquila affiliate; (ii) in good faith and in the
manner You reasonably believe to be in or not opposed to the best interests of
Aquila; and (iii) with respect to any criminal action or proceeding, without any
reasonable cause by You to believe Your actions were unlawful. You must notify
Aquila of any written claims made against You within seven (7) calendar days
after You are notified of such claims and to otherwise cooperate and assist
Aquila and its agents in any defense. In accordance with Section 145(e) of the
Delaware Corporation Law, You agree to repay to Aquila any expenses advanced by
Aquila in connection with all indemnified matters, if it shall ultimately be
determined that You are not entitled to be indemnified against such expenses.
Aquila will maintain Director & Officer insurance for a period of three (3)
years provided insurance of this type is available to the company and will
provide You with a letter setting forth the policy number, the policy period,
any limits, and confirming that You are covered by the policy upon written
request by You. Such request should be directed to the Office of the General
Counsel, Aquila, Inc., 20 W. 9th Street, Kansas City, Missouri 64105.

     11.  CONFIDENTIALITY. The content of this Agreement, and Your discussions
with Aquila pertaining to it, are confidential. You will not communicate or
allow the communication in any manner with respect to the content of this
Agreement, and the discussions pertaining to it, except that the Agreement may
be disclosed by You to Your immediate family members, to Your

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attorneys and accountant, tax consultant, financial planner and to governmental
taxing authorities or regulatory agencies, to enforce the provisions hereof, or
as may otherwise be required by law. Aquila agrees that the contents of the
Agreement will not be disclosed to anyone other than those persons and entities
with a need to know for legitimate business purposes, unless otherwise required
by a regulatory agency or by law; disclosure by Aquila to the extent necessary
to implement future obligations of Aquila will not constitute a breach of this
Paragraph. Any publication or disclosure by You of this Agreement, other than as
allowed by this Paragraph, shall be considered a material breach of this
Agreement.

     12.  REMEDIES FOR BREACH OF THIS AGREEMENT. If either You or Aquila
believes that the other party to this Agreement has breached its obligations
under this Agreement, then the party claiming a breach will provide notice to
the other party, in writing, including a statement of the specific manner in
which the party believes that this Agreement has been breached. If the breach is
not cured, or cannot reasonably be cured, within thirty (30) days following
notice, then the parties, subject to Paragraph 16, and at their respective
options, will be entitled to proceed as follows:

     (a)  If Aquila substantially and materially breaches any provision of this
          Agreement, payment of any remaining benefits on EXHIBIT A shall be
          accelerated and not be recoverable by Aquila as long as You have not
          also substantially breached this Agreement. You may also pursue any
          other available remedies for such breach, including but not limited to
          recovery of Your costs and attorneys' fees.

     (b)  If You substantially and materially breach any provision of this
          Agreement, and Aquila has not also substantially and materially
          breached this Agreement, then Aquila, at its option, will be entitled
          to immediately cease all remaining payments and benefits under this
          Agreement. Aquila may also pursue any other available remedies for
          such breach, including but not limited to recovery of its costs and
          attorneys' fees.

     13.  CONSIDERATION PERIOD. You received this Agreement on October 11, 2002.
You have been advised in writing to consult with an attorney before signing this
Agreement. You have been provided twenty-one (21) calendar days after initially
receiving this Agreement within which to sign it and return the signed original
to Aquila, and You agree that the last day You have to sign and return this
Agreement is November 1, 2002. You may revoke this Agreement within seven (7)
calendar days after You sign it, by providing written notice of such revocation
to Mr. Leo Morton, Chief Administrative Officer, Aquila, Inc., 20 W. 9th Street,
Kansas City, Missouri 64105 before the expiration of such seven (7) day period.
This Agreement is effective and enforceable on the eighth (8th) calendar day
after You and Aquila have both signed it, if not revoked prior to that time (the
"EFFECTIVE DATE").

     14.  TAX CONSIDERATIONS. You acknowledge that no representations have been
made to You by Aquila, Aquila's affiliates, or other agents or legal counsel
regarding the tax implications of any payments made pursuant to this Agreement.
All liability for the employee's share of federal, state, and local taxes
(including FICA) remains with You, unless otherwise agreed to in writing by
Aquila, and Aquila shall deduct all required withholdings from the consideration
payable under this Agreement. You will receive all consideration contemplated in
this agreement

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as a Texas resident and Aquila will deduct all required withholdings from the
consideration payable under this Agreement accordingly unless required to do
otherwise by law.

     15.  CHOICE OF LAW. This Agreement shall be construed in accordance with
the laws of the State of Missouri.

     16.  KNOWING EXECUTION/BINDING ARBITRATION. You knowingly and voluntarily
executed this Agreement. You have had the opportunity to review the Agreement
and consult with an attorney. Aquila has made no other promise, inducement or
agreement not expressed in this Agreement. You and Aquila agree that if a
dispute arises out of or is related to this Agreement or Your employment by
Aquila, other than a dispute regarding the obligations under Paragraphs 8, 9 or
11, such dispute shall, if not earlier resolved by negotiations of the parties
or in accordance with Paragraph 12, be submitted to binding arbitration under
the Employment Rules of the American Arbitration Association, or the mutually
agreed equivalent. Following the 30-day period described in Paragraph 12, above,
either party may provide written notice to the other party that the dispute is
not able to be resolved by negotiation and such notifying party shall then
contact the American Arbitration Association for appointment of an arbitrator to
resolve such dispute. Any arbitration hearing shall take place in Kansas City,
Missouri. In addition to all other remedies otherwise available to Aquila,
Aquila shall have the right to injunctive relief to restrain and enjoin any
actual or threatened breach of the provisions of Paragraphs 8, 9, or 11.

     17.  ENTIRE AGREEMENT. This Agreement, including EXHIBIT A, contains the
entire agreement of the parties with respect to the matters contemplated by this
Agreement; provided, however, the terms of any employee benefit plan or
insurance contract under which You are receiving benefits or insurance pursuant
to this Agreement will control if there is any inconsistency between this
Agreement and any employee benefit plan or insurance contract.

     18.  AUTHORSHIP. This Agreement will not be construed against either party
due to authorship.

     19.  SEVERABILITY. If any provision of this Agreement or the application
thereof to any party or circumstance shall be invalid or unenforceable to any
extent, the remainder of this Agreement and the application of such provisions
to other parties or circumstances shall not be affected thereby and shall be
enforced to the greatest extent permitted by law.

     20.  SUCCESSORS AND ASSIGNS. This Agreement is binding on and inures to
the benefit of Aquila's successors and assigns and Your heirs and assigns,
except that any services to be provided by You under this Agreement shall only
be performed by You.

     21.  NOTICE. Unless either party notifies the other contrary, any notice
required by this Agreement shall be duly given if delivered in person or by
certified, first-class mail (a) if to Aquila, Inc., to the Chief Administrative
Officer at 20 West 9th Street, Kansas City, Missouri 64105, and (b) if to You,
at your current address.

     22.  AMENDMENT AND WAIVERS. Except as otherwise expressly set forth in this
Agreement, (a) any term of this Agreement may be amended only with the written
consent of

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each Party, and (b) a Party's observance of any term of this Agreement may be
waived (either generally or in a particular instance and wither retroactively or
prospectively) only by written consent of all the other parties. No waivers of
or exceptions to any term, condition, or provision of this Agreement, in any one
or more instances, shall be deemed to be, or construed as, a further or
continuing waiver of any such term, condition, or provision.

THIS AGREEMENT CONTAINS A BINDING ARBITRATION PROVISION THAT MAY BE ENFORCED BY
THE PARTIES.

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
signed as of the day and year first above written.


EDWARD K. MILLS                             AQUILA, INC.


Signature: /s/ Edward K. Mills              By: /s/ Leo Morton
           ----------------------------         ------------------------------

                                            Name: Leo Morton

                                            Title: Chief Administrative Officer

Date: October 11, 2003                      Date: October 11, 2003
      --------------------------                  ----------------------------

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                                    EXHIBIT A

<Table>
<Caption>
         COMP. / BENEFIT                             ACTION                             DESCRIPTION
-----------------------------------   -------------------------------------   ------------------------------
<S>                                   <C>                                     <C>
Severance Payment                     You will receive 56 weeks of            $484,615.38
                                      severance at Your current base
                                      salary.  Severance payments will be
                                      paid every two weeks according to
                                      the Aquila payroll schedule,
                                      starting the pay period after Your
                                      employment is terminated
                                      (11/15/2002) upon receipt of your
                                      signed severance agreement.

Stock Options                         Vest all unvested options upon          130,471 options
                                      receipt of your signed severance
                                      agreement. Options must be
                                      exercised within three (3) years of
                                      termination, or termination date of
                                      original grant whichever comes
                                      first.

Restricted Stock                      Restrictions lifted as of November      88,465 restricted shares
                                      15, 2002 upon receipt of your
                                      signed severance agreement.

Perquisites                           Pay the cost of financial planning      Financial Planning reimbursed
                                      and tax preparation for the 2002        up to $5,000
                                      calendar year.                          Tax Preparation reimbursed up
                                                                              to $5,000

Vacation                              48 hours paid in a lump sum             $10,384.62

Capital Accumulation Plan             Plan provisions apply

Pension                               Plan provisions apply

SERP                                  Plan provisions apply

401(k) / ESCP                         Plan provisions apply
                                      May maintain account until age 70 1/2
                                      or rollover to IRA.

ESPP                                  Plan provisions apply

BeneFlex                              BeneFlex benefits while on payroll      Plan Provisions apply
                                      or COBRA

Outplacement                          Aquila will provide executive           $30,000.00 paid in cash, less
                                      outplacement support of $30,000.00      taxes
</Table>

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