The
Federation starship Enterprise is summoned to a space station without explanation.
Commodore Wesley (Barry Russo) explains that the
Enterprise will be a test vessel for the M-5 Multitronic System, a revolutionary tactical and control computer designed by Dr. Richard Daystrom (
William Marshall). The M-5 is to handle all ship functions without human assistance. While Science Officer
Spock is impressed with M-5, Captain
Kirk and Chief Medical Officer
Dr. McCoy have doubts. The M-5 succeeds at its first tasks, performing ship functions more quickly and efficiently than a living crew. However, M-5 also exhibits unexpected behavior, such as turning off power and life support to unoccupied parts of the ship, and drawing increased power for unknown reasons; Daystrom maintains M-5 is working properly. Spock and Kirk engage in a discussion about whether Spock would prefer to serve under a computer or Kirk. In its first tactical drill, M-5 defends the
Enterprise against mock attacks from Starships
Excalibur and
Lexington. The
Enterprise is declared the victor, and Wesley jokingly refers to Kirk as "Captain Dunsel", employing a
Starfleet Academy slang term for a part serving no useful purpose. Kirk is troubled by this. Some time later, M-5 detects the
Woden, an unmanned freighter, and attacks with live torpedoes, destroying it. Kirk orders M-5 taken offline, but on attempting to do so, finds it protected by a powerful force field. Chief Engineer
Scott orders Ensign Harper to disconnect its power source, but the M-5 creates a direct connection to the ship's
warp engines, vaporizing Harper in the process. Spock and Scott attempt a manual override, but discover that the M-5 has rerouted all controls. Spock questions Daystrom on his computer design, and Daystrom reveals that he has imprinted human
engrams onto M-5's circuits, creating what amounts to a human mind operating at the speed of a computer. Meanwhile, four of
Enterprise's
sister ships,
Lexington,
Potemkin,
Excalibur, and
Hood, approach to begin a new tactical drill. Since M-5 has disabled communications, Kirk is unable to warn M-5's targets. M-5 detects the ships, and attacks them with its weapons at full power. The crew watches helplessly as the
Enterprise fires on the
Lexington, killing 53, and then turns to the
Excalibur, killing all aboard and leaving her adrift. Commodore Wesley assumes that Kirk himself is responsible for the attacks, and requests permission from
Starfleet Command to destroy the
Enterprise. Daystrom, having indicated that the engrams he used were his own, believes he can reason with the M-5, but his conversation with the unit quickly degenerates into a self-pitying lament over his own career disappointments. McCoy warns Kirk that he sees a
nervous breakdown coming, and as Daystrom begins loudly to proclaim his and his creation's invincibility, Spock subdues him with a
Vulcan nerve pinch. Kirk then tries to persuade the M-5 to stop its attacks. The M-5 acknowledges Kirk, who asks M-5 what its purpose is. M-5 responds that its purpose is to protect lives. Kirk argues that it acted contrary to its purpose by murdering people. M-5 acknowledges that it has committed murder and must therefore die, and shuts itself down. In so doing, it also cripples the
Enterprise. Having received permission to destroy
Enterprise, the other Federation ships close in. Since Scott is unable to restore communications immediately, Kirk decides to allow the ship to drift with shields down, hoping that Commodore Wesley will realize that the threat has passed. The gamble pays off as the Commodore orders his ships to stand down at the last moment. Spock reminds Kirk and McCoy that while computers are more efficient, they are not any better. ==Production and reception==