The Starship Enterprise is sent to escort the Klingon chancellor to peace negotiations with the Federation. When the chancellor is assassinated, the Klingons refuse to continue negotiations unless Captain Kirk (William Shatner) and Dr McCoy (De Forrest Kelley) are tried for the chancellor's murder in a Klingon court. Kirk and McCoy are sentenced to life imprisonment in a penal colony, leaving Spock in charge of the Enterprise and the only one who can solve the riddle of the murder.