The USS Enterprise, commanded by Captain Kirk, undertakes an 8-day supply mission to Colony Beta VI. While traversing a "star desert," they encounter a hidden rogue planet. Suddenly, Lt. Sulu and Captain Kirk vanish from the bridge.

First, Officer Spock assumes they were taken to a planet with a lethal atmosphere. Then, after receiving a strange message, Spock sends Dr. McCoy, Lt. DeSalle, and geophysicist Karl Jaeger to investigate.

The landing party discovers a breathable environment and a medieval castle where Kirk and Sulu are immobilized. A humanoid, General Trelane, introduces himself and invites them to stay on Gothos, his world. He explains his interest in Earth's history, but McCoy's tricorder cannot detect him as a living being.

Spock locates the landing party and beams them back to the ship, excluding Trelane. However, Trelane appears on the Enterprise's bridge and transports the bridge crew to Gothos.

Kirk grows impatient with Trelane, who seems reliant on a wall mirror. Kirk provokes Trelane into a duel, destroys the mirror, and damages the machinery. The crew returns to the ship, but Gothos keeps blocking their escape. Finally, Kirk beams down to confront Trelane.

Kirk finds Trelane dressed as an English judge, accusing him of various crimes and sentencing him to death. Kirk suggests a more engaging alternative, leading to a royal hunt with Kirk as the prey. Cornered but defiant, Kirk slaps Trelane and breaks his sword. Two energy beings appear, scolding Trelane before taking him away and apologizing to Kirk, who then returns to the Enterprise.