An experimental computer built by Richard Daystrom to run a starship without a crew, which took control of the USS Enterprise during war games in 2268 and killed hundreds.
The M-5 appears in the Original Series episode The Ultimate Computer. Dr Richard Daystrom, inventor of the duotronic computer, installs his new multitronic unit aboard the Enterprise for a trial, and all but twenty of the crew are sent ashore. Commodore Wesley teases Kirk as Captain Dunsel, a part that serves no purpose.
The machine performs well at first, then destroys the unmanned ore freighter Woden and kills a crewman who tries to disconnect it. Daystrom reveals that he impressed his own memory engrams on it, and with them his instability.
During war games M-5 treats the exercise as real, cripples the USS Excalibur and fires on the USS Lexington. Kirk argues that it has committed murder and must pay the penalty, and M-5 shuts itself down. Daystrom suffers a breakdown.