Starfleet informs Kirk that the Enterprise is to serve as test subject for the new M-5, a groundbreaking advancement in computer technology, designed to make command decisions faster than captains and reduce the number of people required to run a starship. An astute allegory for contemporary automation at the expense of "the little guy," this episode's first few acts are superb, as Kirk finds himself debating whether he's selfish for wanting to keep his job at the expense of technological progress, or if it's a matter of actual danger or principle.
A wonderfully acerbic debate between Spock and McCoy about the role of computers is also well conceived, ending in Spock's well-put notion to Kirk, "...but I have no desire to serve under them." Following the M-5's initial success, the scene where another captain calls Kirk "Captain Dunsel" is the episode's best-played and simultaneously funny and painful moment. (In a word, ouch.)
Once M-5 runs out of control and hijacks the Enterprise—resisting attempts to be shut down in acts of self-preservation (including murder and eventually full-fledged attacks on other Federation starships), the episode turns to an frightening analysis of M-5's creator, Dr. Richard Daystrom (William Marshall), a man obsessed with outdoing his prior successes, who has created a monster that he has come to regard as a child. Though it pushes a little hard toward the end (Shatner and Marshall going a bit overboard), the story is a compelling one.