The Enterprise responds to a distress call from the Triacus Science Colony. When they arrive, they find that all but one of the adults have committed suicide. The lone survivor dies shortly thereafter in Kirk's arms. The children seem to be fine and of good health, and are transported to the Enterprise to be taken to the nearest starbase. Once aboard, they begin to act oddly and start to refuse to answer any of Kirk's questions.

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.