A planet-killing weapon has destroyed a starship and now bears down on the Enterprise.
The Enterprise finds the wrecked starship Constellation, whose commander, Commodore Matt Decker, is the only survivor. A huge machine is devouring planets, and Decker, driven to avenge his crew, takes control of the ship in order to destroy it, while Kirk searches for a way to stop it.
The episode is a fan favourite for the tense action and for William Windom's intense portrayal of Decker, a Captain Ahab figure. It has often been compared to Moby Dick. The planet killer was later a model for threats in other series.