The U.S.S. Enterprise intercepts a stolen Federation shuttlecraft which contains a humanoid named Lokai, who is half black and half white, starkly separated down the middle of his body. Taken aboard the ship, Lokai tells the crew he is from the planet Cheron, and asks for asylum on the U.S.S. Enterprise. Another vessel intercepts the Enterprise and the ship's pilot beams aboard, Bele, also from the planet Cheron, except that his black and white skin is reversed from Lokai's. Their hatred is rooted in a deep racial prejudice which threatens to engulf not only them, but the Enterprise and Kirk's crew.

James Doohan (Lt. Cmdr. Montgomery Scott)
Frank Gorshin (Bele)
Lou Antonio (Lokai)
Walter Koenig (Ensign Pavel Chekov)
Nichelle Nichols (Lt. Uhura)
George Takei (Lt. Hikaru Sulu)
Majel Barrett (Nurse Christine Chapel)