Missouri Constitution
Article IV - Executive Department
Section 11(a) Order of succession to governorship, when.

If the governor-elect dies before taking office, the lieutenant governor-elect shall take the term of the governor-elect. On the death, conviction or impeachment, or resignation of the governor, the lieutenant governor shall become governor for the remainder of the term. If there be no lieutenant governor the president pro tempore of the senate, the speaker of the house, the secretary of state, the state auditor, the state treasurer or the attorney general in succession shall become governor. On the failure to qualify, absence from the state or other disability of the governor, the powers, duties and emoluments of the governor shall devolve upon the lieutenant governor for the remainder of the term or until the disability is removed. If there be no lieutenant governor, or for any of said causes the lieutenant governor is incapable of acting, the president pro tempore of the senate, the speaker of the house, the secretary of state, the state auditor, the state treasurer, and the attorney general in succession shall act as governor until the disability is removed.

Source: Const. of 1875, Art. IV, Secs. 16, 17. (Adopted August 6, 1968)