"The Ultimate Computer" is a season two episode of the original science fiction television series, Star Trek, first broadcast on March 8, 1968, and repeated June 28, 1968. It is episode No. 53, production No. 53, written by D.C. Fontana, based on a story by Laurence N. Wolfe, and directed by John Meredyth Lucas.

In this episode, a skeleton Enterprise crew are assigned to test a revolutionary computer system that is given total control of the ship.

On stardate 4729.4, the Federation starship Enterprise is summoned to a space station without explanation. Commodore Wesley, commanding a group of starships from his flagship USS Lexington, explains that the Enterprise will be a test vessel for a revolutionary tactical and control computer called the "M-5 Multitronic System", designed by the brilliant Dr. Richard Daystrom (who had also invented the TOS era computer systems after developing duotronics when he was 24 years old). The M-5 will handle all ship functions without human assistance. While Captain Kirk and Chief Medical Officer Dr. McCoy are unhappy about the test, Science Officer Spock is impressed with M-5. However, Kirk learns that four earlier prototypes were unsuccessful, giving him further doubts.

At first M-5 works well, performing ship functions more quickly and efficiently than a living crew. Later, M-5 exhibits quirks such as turning off power and life support to unoccupied parts of the ship. It draws increased power for unknown reasons. But Daystrom maintains M-5 is working properly.

In a drill, M-5 defends the Enterprise against mock attacks from starships Excalibur and Lexington. The Enterprise is declared the victor, prompting Commodore Wesley to call Kirk "Captain Dunsail" (pronounced "dunsel"). Spock explains the term is used by midshipmen at Starfleet Academy to describe a part serving no useful purpose. Kirk is visibly shaken by this.

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