The TARDIS arrives on the planet Exxilon, where all electrical energy is drained off by an unknown force. The Doctor meets a Marine Space Corps expedition from Earth who tell him that a plague is sweeping the galaxy and that the antidote, parrinium, can be found only on Exxilon. Their ship has been disabled by the energy drain, so they are unable to leave with the mineral.

Sarah meanwhile has seen a magnificent white edifice with a flashing beacon on top. She is captured by a group of savage Exxilons and taken to their cave to be sacrificed for defiling their city. The Doctor and the humans enter into an uneasy alliance with a group of Daleks who also land on the planet and whose weapons are rendered inactive by the energy drain. They too are taken prisoner by the Exxilons.

The Doctor interrupts Sarah's execution but is then sentenced to death himself. The two travellers escape into some tunnels as the cave is attacked by a second force of Daleks armed with mechanical guns. There they meet Bellal, an enlightened Exxilon, who tells them that the city was created by his own race but then brought about their downfall. The Doctor, realising that the city's beacon is causing the energy drain, resolves to put it out of action.

He and Bellal enter the city and, by passing a series of potentially deadly tests, reach its centre. There the Time Lord uses his sonic screwdriver to give the controlling computer a brainstorm.

The Daleks plan to take all the parrinium but their ship is destroyed with a bomb triggered by one of the humans who has stowed away on board. Sarah then reveals that she and another of the humans had already smuggled off all the parrinium and transferred it to the Earth ship.