By 2268, he had risen to become the science officer of the ISS Lexington. During a war game held in that year, the M-5 computer was put in command of the ISS Excalibur, where it faced off against the Lexington, ISS Enterprise, ISS Potemkin and ISS Hood. However, when it was supposed to use low-powered phasers, it instead fired on the other ships at full power, destroying the Potemkin and nearly destroying the Enterprise.
Clayton had become somewhat disoriented but was shaken and brought back into focus by his commanding officer, Commodore Robert Wesley, in order to work the controls of his console. (TOS - Mirror Universe novel: The Sorrows of Empire)