EX-10.3





 Exhibit 10.3 

CHECKMATE PHARMACEUTICALS, INC. 

2020 EMPLOYEE STOCK PURCHASE PLAN 

The purpose of the Checkmate Pharmaceuticals, Inc. 2020 Employee Stock Purchase Plan (the “Plan”) is to provide eligible
employees of Checkmate Pharmaceuticals, Inc. (the “Company”) and each Designated Company (as defined in Section 11) with opportunities to purchase shares of the Company’s common stock, par value $0.0001 per share (the
“Common Stock”). 284,696 shares of Common Stock in the aggregate have been approved and reserved for this purpose, plus on January 1, 2022 and each January 1 thereafter until the Plan terminates pursuant to
Section 20, the number of shares of Common Stock reserved and available for issuance under the Plan shall be cumulatively increased by the least of (i) 284,696 shares of Common Stock, (ii) 1% of the number of shares of Common Stock issued and
outstanding on the immediately preceding December 31, and (iii) such lesser number of shares of Common Stock as determined by the Administrator (as defined in Section 1). 

The Plan includes two components: a Code Section 423 Component (the “423 Component”) and a non-Code Section 423 Component (the “Non-423 Component”). It is intended for the 423 Component to constitute an “employee stock purchase plan”
within the meaning of Section 423(b) of the U.S. Internal Revenue Code of 1986, as amended (the “Code”), and the 423 Component shall be interpreted in accordance with that intent. Under the
Non-423 Component, which does not qualify as an “employee stock purchase plan” within the meaning of Section 423(b) of the Code, options will be granted pursuant to rules, procedures or sub-plans adopted by the Administrator designed to achieve tax, securities laws or other objectives for eligible employees. Except as otherwise provided herein, the Non-423
Component will operate and be administered in the same manner as the 423 Component. 
 Unless otherwise defined herein, capitalized terms in
this Plan shall have the meaningascribed to them in Section 11. 
 1. Administration. The Plan will be administered by the person
or persons (the “Administrator”) appointed by the Company’s Board of Directors (the “Board”) for such purpose. The Administrator has authority at any time to: (i) determine the Offering Periods,
(ii) adopt, alter and repeal such rules, guidelines and practices for the administration of the Plan and for its own acts and proceedings as it shall deem advisable; (iii) interpret the terms and provisions of the Plan; (iv) make all
determinations it deems advisable for the administration of the Plan, including to accommodate the specific requirements of local laws, regulations and procedures for jurisdictions outside the United States; (v) decide all disputes arising in
connection with the Plan; and (vi) otherwise supervise the administration of the Plan. All interpretations and decisions of the Administrator shall be binding on all persons, including the Company and the Participants. No member of the Board or
individual exercising administrative authority with respect to the Plan shall be liable for any action or determination made in good faith with respect to the Plan or any option granted hereunder. 












 2. Offerings. The Company will make one or more offerings to eligible employees to
purchase Common Stock under the Plan (“Offerings”) consisting of one or more Purchase Periods. Unless otherwise determined by the Administrator, the initial Offering will begin on a date determined by the Board and will end six
(6) months from such date (the “Initial Offering”). Thereafter, an Offering will begin on begin on a date determined by the Board and will end six (6) months from such date. The Administrator may, in its discretion,
designate a different period for any Offering, provided that no Offering shall exceed 27 months in duration or overlap with any other Offering. 

3. Eligibility. All individuals classified as employees on the payroll records of the Company and each Designated Company are eligible
to participate in any one or more of the Offerings under the Plan, provided that, unless otherwise determined by the Administrator, as of the first day of the applicable Offering (the “Offering Date”) they are customarily employed
by the Company or a Designated Company for more than 20 hours a week and have completed at least 30 days of employment, provided, however, that employees who are employed for 20 hours or less a week may be eligible to participate in the Plan if
required by applicable law or regulations. Notwithstanding any other provision herein, individuals who are not contemporaneously classified as employees of the Company or a Designated Company for purposes of the Company’s or applicable
Designated Company’s payroll system are not considered to be eligible employees of the Company or any Designated Company and shall not be eligible to participate in the Plan. In the event any such individuals are reclassified as employees of
the Company or a Designated Company for any purpose, including, without limitation, common law or statutory employees, by any action of any third party, including, without limitation, any government agency, or as a result of any private lawsuit,
action or administrative proceeding, such individuals shall, notwithstanding such reclassification, remain ineligible for participation. Notwithstanding the foregoing, the exclusive means for individuals who are not contemporaneously classified as
employees of the Company or a Designated Company on the Company’s or Designated Company’s payroll system to become eligible to participate in this Plan is through an amendment to this Plan, duly executed by the Company, which specifically
renders such individuals eligible to participate herein. 
 4. Participation. 

(a) Participants in the Initial Offering. Each eligible employee as of the date of the Initial Offering shall be deemed to be a
Participant at such time. If an eligible employee is deemed to be a Participant pursuant to this Section 4(a), such individual shall be deemed not to have authorized payroll deductions and shall not purchase any Common Stock hereunder unless he
or she thereafter authorizes payroll deductions by submitting an enrollment form (in the manner described in Section 4(c)) within 60 days of the commencement of the Initial Offering (or by such other deadline as shall be established by the
Administrator for the Initial Offering). If such a Participant does not authorize payroll deductions by submitting an enrollment form within 60 days of the commencement of the Initial Offering (or by such other deadline as shall be established by
the Administrator for the Initial Offering), that Participant will be deemed to have withdrawn from the Plan. 
 (b) Participants in
Subsequent Offerings. An eligible employee who is not a Participant in any prior Offering may participate in a subsequent Offering by submitting an enrollment form (in the manner described in Section 4(c)) at least 15 business days before
the Offering Date (or by such other deadline as shall be established by the Administrator for the Offering). 

  
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 (c) Enrollment. The enrollment form (which may be in an electronic format or such
other method as determined by the Company in accordance with the Company’s practices) will (a) state a whole percentage to be deducted from an eligible employee’s Compensation (as defined in Section 11) per pay period,
(b) authorize the purchase of Common Stock in each Offering in accordance with the terms of the Plan and (c) specify the exact name or names in which shares of Common Stock purchased for such individual are to be issued pursuant to
Section 10. An employee who does not enroll in accordance with these procedures will be deemed to have waived the right to participate. Unless a Participant files a new enrollment form or withdraws from the Plan, such Participant’s
deductions and purchases will continue at the same percentage of Compensation for future Offerings, provided he or she remains eligible. 

(d) Notwithstanding the foregoing, participation in the Plan will neither be permitted nor be denied contrary to the requirements of the Code.

 5. Employee Contributions. Each eligible employee may authorize payroll deductions at a minimum of 1 percent up to a maximum
of 15 percent of such employee’s Compensation for each pay period. The Company will maintain book accounts showing the amount of payroll deductions made by each Participant for each Purchase Period. No interest will accrue or be paid on
payroll deductions, except as may be required by applicable law. If payroll deductions for purposes of the Plan are prohibited or otherwise problematic under applicable law (as determined by the Administrator in its discretion), the Administrator
may require Participants to contribute to the Plan by such other means as determined by the Administrator. Any reference to “payroll deductions” in this Section 5 (or in any other section of the Plan ) will similarly cover
contributions by other means made pursuant to this Section 5. 
 6. Deduction Changes. Except as may be determined by the
Administrator in advance of an Offering, a Participant may not increase or decrease his or her payroll deduction during any Offering, but may increase or decrease his or her payroll deduction with respect to the next Offering (subject to the
limitations of Section 5) by filing a new enrollment form at least 15 business days before the next Offering Date (or by such other deadline as shall be established by the Administrator for the Offering). The Administrator may, in advance of
any Offering, establish rules permitting a Participant to increase, decrease or terminate his or her payroll deduction during an Offering. 

7. Withdrawal. A Participant may withdraw from participation in the Plan at any time prior to the end of an Offering by delivering a
written notice of withdrawal to the Company (in accordance with such procedures as may be established by the Administrator). The Participant’s withdrawal will be effective as of the next business day. Following a Participant’s withdrawal,
the Company will promptly refund such individual’s entire account balance under the Plan to him or her (after payment for any Common Stock purchased before the effective date of withdrawal). Partial withdrawals are not permitted. Such an
employee may not begin participation again during the remainder of the Offering, but may enroll in a subsequent Offering in accordance with Section 4. 

  
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 8. Grant of Options. On each Offering Date, the Company will grant to each
Participant in the Plan an option (“Option”) to purchase, on the last day of a Purchase Period (the “Exercise Date”) and at the Option Price hereinafter provided for, the lowest of (a) a number of shares of
Common Stock determined by dividing such Participant’s accumulated payroll deductions on such Exercise Date by the Option Price (as defined herein), (b) such maximum number of shares as shall have been established by the Administrator in
advance of the Offering; provided, however, that such Option shall be subject to the limitations set forth below. Each Participant’s Option shall be exercisable only to the extent of such Participant’s accumulated payroll deductions on the
Exercise Date. The purchase price for each share purchased under each Option (the “Option Price”) will be 85 percent of the Fair Market Value of the Common Stock on the Offering Date or the Exercise Date, whichever is less.

 Notwithstanding the foregoing, no Participant may be granted an Option hereunder if such Participant, immediately after the Option was
granted, would be treated as owning stock possessing 5 percent or more of the total combined voting power or value of all classes of stock of the Company or any Parent or Subsidiary. For purposes of the preceding sentence, the attribution rules
of Section 424(d) of the Code shall apply in determining the stock ownership of a Participant, and all stock which the Participant has a contractual right to purchase shall be treated as stock owned by the Participant. In addition, no
Participant may be granted an Option which permits his or her rights to purchase stock under the Plan, and any other employee stock purchase plan of the Company and its Parents and Subsidiaries, to accrue at a rate which exceeds $25,000 of the fair
market value of such stock (determined on the option grant date or dates) for each calendar year in which the Option is outstanding at any time. The purpose of the limitation in the preceding sentence is to comply with Section 423(b)(8) of the
Code and shall be applied taking Options into account in the order in which they were granted. 
 9. Exercise of Option and Purchase of
Shares. Each employee who continues to be a Participant in the Plan on the Exercise Date shall be deemed to have exercised his or her Option on such date and shall acquire from the Company such number of whole shares of Common Stock reserved for
the purpose of the Plan as his or her accumulated payroll deductions on such date will purchase at the Option Price, subject to any other limitations contained in the Plan. Unless otherwise determined by the Administrator in advance of an Offering,
any amount remaining in a Participant’s account after the purchase of shares on an Exercise Date of an Offering solely by reason of the inability to purchase a fractional share will be carried forward to the next Purchase Period and, if such
Exercise Date is the final Exercise Date of an Offering, will be carried forward to the next Offering; any other balance remaining in a Participant’s account at the end of an Offering will be refunded to the Participant promptly. 

10. Issuance of Certificates. Certificates representing shares of Common Stock purchased under the Plan may be issued only in the name
of the employee, in the name of the employee and another person of legal age as joint tenants with rights of survivorship, or in the name of a broker authorized by the employee to be his, her or their, nominee for such purpose. 

11. Definitions. For the purposes of this Plan, the following terms shall be defined as set forth below: 

“Affiliate” means any entity that, directly or indirectly through one or more intermediaries, controls, is controlled by or
is under the common control with, the Company. 

  
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 “Compensation” means the amount of base pay, prior to salary reduction
(such as pursuant to Sections 125, 132(f) or 401(k) of the Code), but excluding overtime, commissions, incentive or bonus awards allowances and reimbursements for expenses such as relocation allowances or travel expenses, income or gains
related to Company stock options or other share-based awards, and similar items. The Administrator shall have the discretion to determine the application of this definition to Participants outside the United States. 

“Designated Company” means any present or future Subsidiary or Affiliate that has been designated by the Administrator to
participate in the Plan. The Administrator may so designate any Subsidiary or Affiliate, or revoke any such designation, at any time and from time to time, either before or after the Plan is approved by the stockholders, and may further designate
such companies or Participants as participating in the 423 Component or the Non-423 Component. The Administrator may also determine which Affiliates or eligible employees may be excluded from participation in
the Plan, to the extent consistent with Section 423 of the Code or as implemented under the Non-423 Component, and determine which Designated Company or Companies will participate in separate Offerings
(to the extent that the Company makes separate Offerings). For purposes of the 423 Component, only the Company and its Subsidiaries may be Designated Companies; provided, however, that at any given time, a Subsidiary that is a Designated Company
under the 423 Component will not be a Designated Company under the Non-423 Component. 

“Fair Market Value of the Common Stock” on any given date means the fair market value of the Common Stock determined in good
faith by the Administrator; provided, however, that if the Common Stock is admitted to quotation on the National Association of Securities Dealers Automated Quotation System (“NASDAQ”), NASDAQ Global Market or another national
securities exchange, the determination shall be made by reference to the closing price on such date. If there is no closing price for such date, the determination shall be made by reference to the last date preceding such date for which there is a
closing price. 
 “Parent” means a “parent corporation” with respect to the Company, as defined in
Section 424(e) of the Code. 
 “Participant” means an individual who is eligible as determined in Section 3 and
who has complied with the provisions of Section 4. 
 “Purchase Period” means a period of time specified within an
Offering beginning on the Offering Date or on the next day following an Exercise Date within an Offering and ending on an Exercise Date. An Offering may consist of one or more Purchase Periods. 

“Registration Date” means the date on which the registration statement on Form S-1
that is filed by the Company with respect to its Initial Public Offering is declared effective by the U.S. Securities and Exchange Commission (the “SEC”). 

“Subsidiary” means a “subsidiary corporation” with respect to the Company, as defined in Section 424(f) of the
Code. 

  
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 12. Rights on Termination or Transfer of Employment. If a Participant’s
employment terminates for any reason before the Exercise Date for any Offering, no payroll deduction will be taken from any pay due and owing to the Participant and the balance in the Participant’s account will be paid to such Participant or,
in the case of such Participant’s death, to the legal representative of his or her estate as if such Participant had withdrawn from the Plan under Section 7. An employee will be deemed to have terminated employment, for this purpose, if
the corporation that employs him or her, having been a Designated Company, ceases to be a Subsidiary or Affiliate, or if the employee is transferred to any corporation other than the Company or a Designated Company. Unless otherwise determined by
the Administrator, a Participant whose employment transfers between, or whose employment terminates with an immediate rehire (with no break in service) by, Designated Companies or a Designated Company and the Company will not be treated as having
terminated employment for purposes of participating in the Plan or an Offering; provided, however, that if a Participant transfers from an Offering under the 423 Component to an Offering under the Non-423
Component, the exercise of the Participant’s Option will be qualified under the 423 Component only to the extent that such exercise complies with Section 423 of the Code. If a Participant transfers from an Offering under the Non-423 Component to an Offering under the 423 Component, the exercise of the Participant’s Option will remain non-qualified under the
Non-423 Component. Further, an employee will not be deemed to have terminated employment for purposes of this Section 12, if the employee is on an approved leave of absence where the employee’s right
to reemployment is guaranteed either by a statute or by contract or under the policy pursuant to which the leave of absence was granted or if the Administrator otherwise provides in writing. 

13. Special Rules and Sub-Plans. Notwithstanding anything herein to the contrary, the
Administrator may adopt special rules or sub-plans applicable to the employees of a particular Designated Company, whenever the Administrator determines that such rules are necessary or appropriate for the
implementation of the Plan in a jurisdiction where such Designated Company has employees, regarding, without limitation, eligibility to participate in the Plan, handling and making of payroll deductions or contributions by other means, establishment
of bank or trust accounts to hold payroll deductions, payment of interest, conversion of local currency, obligations to pay payroll tax, withholding procedures and handling of share issuances, any of which may vary according to applicable
requirements; provided that if such special rules or sub-plans are inconsistent with the requirements of Section 423(b) of the Code, the employees subject to such special rules or sub-plans will participate in the Non-423 Component. 
 14.
Optionees Not Stockholders. Neither the granting of an Option to a Participant nor the deductions from his or her pay shall result in such Participant becoming a holder of the shares of Common Stock covered by an Option under the Plan until
such shares have been purchased by and issued to him or her. 
 15. Rights Not Transferable. Rights under the Plan are not
transferable by a Participant other than by will or the laws of descent and distribution, and are exercisable during the Participant’s lifetime only by the Participant. 

16. Application of Funds. All funds received or held by the Company under the Plan may be combined with other corporate funds and may be
used for any corporate purpose, unless otherwise required under applicable law. 

  
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 17. Adjustment in Case of Changes Affecting Common Stock and Transactions. 

(a) If any change is made in the shares of the Company’s Common Stock subject to the Plan, or subject to any Option under the Plan,
without the receipt of consideration by the Company (through merger, consolidation, reorganization, recapitalization, reincorporation, stock dividend, dividend in property other than cash, stock split, liquidating dividend, combination of shares,
exchange of shares, change in corporate structure or other transaction not involving the receipt of consideration by the Company), the Plan will be appropriately adjusted in the class(es) and maximum number of shares of the Company’s Common
Stock subject to the Plan and the share limitation subject to Section 8, if any, and the outstanding Options will be appropriately adjusted in the class(es), number of shares of Common Stock and share limitations of such outstanding
Options. The Board shall make such adjustments, and its determination shall be final, binding and conclusive. (The conversion of any convertible securities of the Company shall not be treated as a transaction that does not involve the
receipt of consideration by the Company.) 
 (b) Without limitation on the preceding provisions, in the event of any corporate transaction,
the Board may make such adjustment it deems appropriate to prevent dilution or enlargement of rights in the number and class of the Company’s Common Stock which may be delivered under the Plan, in the number, class of or Option Price of the
Company’s Common Stock available for purchase under the Plan and in the number of the Company’s Common Stock which an employee is entitled to purchase and any other adjustments it deems appropriate. Without limiting the Board’s
authority under this Plan, in the event of any transaction, the Board may elect to have the Options hereunder assumed or such Options substituted by a successor entity, to terminate all outstanding Options, either prior to their expiration or upon
completion of the purchase of the Company’s Common Stock on the next Exercise Date, to shorten the Offering by setting a new Exercise Date or to take such other action deemed appropriate by the Board. 

18. Amendment of the Plan. The Board may at any time and from time to time amend the Plan in any respect, except that without the
approval within 12 months of such Board action by the stockholders, no amendment shall be made increasing the number of shares approved for the Plan or making any other change that would require stockholder approval in order for the 423 Component of
the Plan, as amended, to qualify as an “employee stock purchase plan” under Section 423(b) of the Code. 
 19. Insufficient
Shares. If the total number of shares of Common Stock that would otherwise be purchased on any Exercise Date plus the number of shares purchased under previous Offerings under the Plan exceeds the maximum number of shares issuable under the
Plan, the shares then available shall be apportioned among Participants in proportion to the amount of payroll deductions accumulated on behalf of each Participant that would otherwise be used to purchase Common Stock on such Exercise Date. 

20. Termination of the Plan. The Plan may be terminated at any time by the Board. Upon termination of the Plan, all amounts in the
accounts of Participants shall be promptly refunded. The Plan shall automatically terminate on the ten year anniversary of the Registration Date. 

  
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 21. Governmental Regulations. The Company’s obligation to sell and deliver
Common Stock under the Plan is subject to the completion of any registration or qualification of the Common Stock under any U.S. or non-U.S. local, state or federal securities or exchange control law, or under
rulings or regulations of the SEC or of any other governmental regulatory body, and to obtaining any approval or other clearance from any U.S. and non-U.S. local, state or federal governmental agency, which
registration, qualification or approval the Company shall, in its absolute discretion, deem necessary or advisable. The Company is under no obligation to register or qualify the Common Stock with the SEC or any other U.S. or non-U.S. securities commission or to seek approval or clearance from any governmental authority for the issuance or sale of such stock. 

22. Governing Law. This Plan and all Options and actions taken thereunder shall be governed by, and construed in accordance with the
General Corporation Law of the State of Delaware as to matters within the scope thereof, and as to all other matters shall be governed by and construed in accordance with the internal laws of the State of Delaware applied without regard to conflict
of law principles. 
 23. Issuance of Shares. Shares may be issued upon exercise of an Option from authorized but unissued Common
Stock, from shares held in the treasury of the Company, or from any other proper source. 
 24. Tax Withholding. Participation in the
Plan is subject to any applicable U.S. and non-U.S. federal, state or local tax withholding requirements on income the Participant realizes in connection with the Plan. Each Participant agrees, by entering the
Plan, that the Company or any Subsidiary or Affiliate may, but will not be obligated to, withhold from Participant’s wages, salary or other compensation at any time the amount necessary for the Company or any Subsidiary or Affiliate to meet
applicable withholding obligations, including any withholding required to make available to the Company or any Subsidiary or Affiliate any tax deductions or benefits attributable to the sale or disposition of Common Stock by such Participant. In
addition, the Company or any Subsidiary or Affiliate may, but will not be obligated to, withhold from the proceeds of the sale of Common Stock or any other method of withholding that the Company or any Subsidiary or Affiliate deems appropriate to
the extent permitted by U.S. Treasury Regulation Section 1.423-2(f) with respect to the 423 Component. The Company will not be required to issue any Common Stock under the Plan until such obligations are
satisfied. 
 25. Notification Upon Sale of Shares Under the 423 Component. Each Participant agrees, by entering the 423 Component of
the Plan, to provide if requested by the Company prompt notice of any disposition of shares purchased under the Plan where such disposition occurs within two years after the date of grant of the Option pursuant to which such shares were purchased or
within one year after the date such shares were purchased. 
 26. Effective Date and Approval of Shareholders. The Plan shall take
effect on the date immediately preceding the Registration Date, subject to prior approval by the holders of a majority of the votes cast at a meeting of stockholders at which a quorum is present or by written consent of the stockholders. 

  
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