An experimental computer designed to run the ship goes rogue and takes aim at other Starfleet vessels.
Starfleet wants to test a new computer called the M-5, designed by Dr. Richard Daystrom, which can run a starship without a crew. The Enterprise is the test ship, but M-5 begins to act on its own and attacks other ships in a war game. Kirk must stop it before it destroys more lives.
The episode is a landmark study of automation, with Daystrom's pride and pain as the cause of the problem. It raises the question of whether humans can be replaced, which is still discussed. Fans admire its tension and the performance of William Marshall.