Stardate 4842.6: Kirk, Spock and McCoy beam down to a planet to inform any inhabitants that they must evacuate the planet due to an approaching asteroid's imminent collision. A society similar to Native American Indians has arisen on the planet, but near their villages, the landing party finds a strange obelisk whose design and construction is far beyond the capabilities of the planet's natives. Kirk finds that the monolith can be opened by the combination of sounds found in the order "Kirk to Enterprise", but when he enters the obelisk, he is attacked by waves of energy that erase his mind. With no time to spare, Spock and McCoy have to return to the Enterprise without Kirk, and begin trying to use the ship's tractor beam to divert the asteroid. Meanwhile, Kirk becomes the tribal chief, takes a wife and even expects to become a father, but the Enterprise may not be able to save her former captain's future.

Trying to escape from a volcanic eruption, the Doctor activates the TARDIS' emergency circuits, taking his ship out of real time and space altogether. However, the void is inhabited by a hypnotic force which lures Jamie and Zoe outside, keeping the Doctor distracted until the strain on the TARDIS becomes too great and the ship explodes. The Doctor and his companions find themselves in a Land of Fiction, where wordplay can be deadly and fictional characters come to life. The travellers must make their way to the castle at the centre of the Land, while the Master of the Land of Fiction waits for them to make the wrong move and transform themselves into fictional characters and thus pawns under his control.