Ancient disembodied beings borrow the bodies of Kirk, Spock and Dr. Ann Mulhall for a deadly purpose.
The Enterprise answers a distress call on a dead planet and finds Sargon, an ancient being who survived as pure energy. Sargon and two other beings ask to borrow the bodies of Kirk, Spock and Dr. Ann Mulhall to build android bodies to live in. Kirk agrees, but one of the visitors has a different plan.
The episode is known for the opportunity it gives the actors to play different characters. Nimoy, in particular, plays a villain who takes over Spock's body. It also contains one of Kirk's most stirring speeches about risk, and fans like the philosophical aspects.