The TARDIS has arrived on a far-distant and seemingly idyllic world. Yet the Doctor, Steven and Dodo learn it hides a terrible secret: the apparently civilised Elders maintain their advanced society by draining and transferring to themselves the life-force of the defenceless Savages.
Outraged at this exploitation, the Doctor is seemingly helpless to prevent it when some of his own life-force is tapped by the Elders' leader, Jano. In the process, however, Jano also acquires some of the Doctor's attitudes and conscience. Turning against his own people, he enlists the Savages to destroy the Elders' transference laboratory — a task with which the time travellers gladly assist.
Steven agrees to remain behind on the planet to become leader of the newly united Elders and Savages.