A U.S.S. Enterprise landing party beams to a human-colonized planet in answer to a distress call. A Klingon ship, apparently damaged, is detected and a group of Klingons accuse Kirk of having damaged their ship. Kang, their leader, claims the U.S.S. Enterprise as a prize and forces Kirk to beam the Klingons on board. A malevolent entity has entered the Enterprise computer and excites both sides to aggressive behavior, pitting Klingons against the Enterprise crew.

James Doohan (Lt. Cmdr. Montgomery Scott)
Michael Ansara (Commander Kang)
Susan Howard (Mara)
Walter Koenig (Ensign Pavel Chekov)
George Takei (Lt. Hikaru Sulu)
Nichelle Nichols (Lt. Uhura)
David L. Ross (Lt. Johnson)
Mark Tobin (Klingon)
Bajel Barrett (Computer Voice)

• Michael Ansara later reprised the role of Kang in the Deep Space Nine episode "Blood Oath" and the Voyager episode "Flashback".
  
• Mara is the first appearance of a female Klingon, the only appearance in The Original Series.