Eight days away from colony Beta 6 on a supply delivering mission, the Enterprise encounters an unknown planet in quadrant 904. Sulu and Kirk then vanish from the bridge, and are presumed to have been transported to the planet. However, geophysicist Karl Jaeger reports the planet has no soil or vegetation, but does possess an extremely hot and toxic atmosphere.
The Enterprise then receives a message ``Greetings and Felicitations,'' followed by ``Hip hip hoorah. Tallyho.'' McCoy, navigator DeSalle, and Jaeger beam down to find Sulu and Kirk, only to find that the planet's surface is lush and the atmosphere breathable. However, their communicators are inoperable. They discover a castle, and are introduced to its occupant, the playful harpsichord-playing alien calling himself General Trelane.
Trelane, who amuses himself using machines with the ability to transform energy and matter, has been making a study of Earth and believes it to be as it was 900 years previously (the light travel time from his planet). Trelane insists that they all stay, but Kirk and DeSalle try to stun him with a phaser. Trelane confiscates the phaser, and amuses himself by vaporizing his statuary. Bones' tricorder fails to register Trelane at all, and Jaeger notices that the fire is giving off no heat. Spock is able to beam them aboard the Enterprise by beaming up every life form in the area. Trelane is upset by this, puts in an appearance on the bridge, then quickly brings the landing party back to his planet Gothos, this time including Spock, Uhura, and Yeoman Teresa Ross. Spock tells Trelane ``I object to you. I object to intellect without discipline. I object to power without constructive purpose.''
Kirk notices that Trelane never strays far from a mirror, and guesses that the mirror hides the source of Trelane's power. Kirk therefore challenges Trelane to a duel, giving the impression that he is offended by Trelane's attentions to Ross. He instead uses the opportunity to destroy the mirror. Kirk's supposition is proved correct, and the landing party once again escapes to the Enterprise.
However, as the Enterprise attempts to warp away, Gothos keeps appearing directly in its path. Kirk is forced to re-enter orbit and beams down to have it out with the Squire. Trelane becomes upset and condemns Kirk to death by hanging in a ``trial'' for the crimes of treason against a superior authority, conspiracy, and the attempt to foment insurrection. Kirk convinces Trelane that there is no sport in a simple slaying, and manages to escape death in a deadly hide-and-seek game. In return, Trelane agrees to let the Enterprise go. Unfortunately, Trelane welshes on his promise when Kirk wins the game and refuses to release the Enterprise. However, the wayward Trelane is finally taken away by his green energy-form ``parents'' to be disciplined and Kirk and the Enterprise are released.