The Enterprise, commanded by Captain Pike, discovers survivors from a ship that crashed on the planet Talos IV twenty years ago. Among them is the young Vina. Pike is abducted by the indigenous Talosians and subjected to illusions, in which Vina appears too. Meanwhile on the surface, the camp of survivors has vanished and the crew attempts to break into the underground city of the Talosians with a big laser cannon to free their captain, apparently in vain. The Talosians' goal is to make Pike mate with Vina to breed slaves, because after many centuries in which they focused on their mental development they can't maintain their machines any longer. Pike and his crew finally prevail when they refuse to believe in the Talosian illusions any longer. Vina, however, is really a woman who was found alive by the Talosians. Since they didn't know anything about human physiology, they couldn't help her. She never recovered from her injuries and is kept alive only through the illusions.