The U.S.S. Enterprise must cross an empty sector of space on their way to deliver supplies to colony Beta VI. In this space, they find an uncharted planet whose presence can't be explained. After Kirk and Sulu suddenly disappear, Spock orders McCoy and geophysicist Lt. Karl Jaeger to the planet's surface to begin a search. They find Trelane, a humanoid with tremendous psionic powers and a passion for Earth's 18th-century military history, and it was he who kidnapped Kirk and Sulu, wanting to add them to his arena of the Napoleonic era that he has created.

William Campbell (Trelane)
Richard Carlyle (Lt. Karl Jaeger)
DeForest Kelley (Dr. Leonard H. McCoy)
James Doohan (Lt. Cmdr. Montgomery Scott)
Nichelle Nichols (Lt. Uhura)
George Takei (Lt. Hikaru Sulu)
Michael Barrier (Lt. Vincent DeSalle)
Venita Wolf (Yeoman Teresa Ross)