The crew of the Starship Enterprise come out of retirement when Earth is threatened by a massive energy field. Captain Kirk (William Shatner), now an Admiral, Spock (Leonard Nimoy), Bones (DeForest Kelley), and the rest of the television regulars are all on board for what proved to be the first of a series of movie adventures.

For their fourth big-screen outing the crew of the USS Enterprise abandon their usual 23rd century time frame for Earth in the year 1986. In order to save the planet from a rogue space probe in the future, Kirk (William Shatner), Spock (Leonard Nimoy), Bones (DeForest Kelley) and co must travel back to the twentieth century to find the only creature capable of communicating with it: a whale. The story is given comic treatment - Spock hides his Vulcan ears with a headband and poses as a burnt-out hippy, while the normally intrepid Kirk gets lost on a bus.

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.