The Enterprise comes across a shuttlecraft taken from Starbase 4 on their way to the planet Arianis. The alien piloting the shuttlecraft is completely white on one side of his body and completely black on the other. The alien claims that he is a political refugee and wishes asylum. Shortly thereafter, another alien much like the first (except he is white and black on opposite sides) arrives and demands that first alien be handed over to him.

For centuries the alien Krotons have hidden inside their Machine in the Hall of Learning, educating a small settlement of Gonds and selecting the two best students of each graduating class to become their Companions. This is seen as a great honour - until the Doctor and his friends arrive and discover that the Companions are in fact being killed. This revelation rocks Gond society to its foundations, and while the ambitious councillor Eelek seizes the opportunity to stage a coup and grasp power for himself, the Doctor and his friends investigate the Krotons' true agenda. Unfortunately, while doing so the Doctor and Zoe inadvertently wake the Krotons, crystalline conquerors who have been dormant for centuries waiting for the Gonds to become intelligent enough to provide them with the mental energy they need to power their spacecraft. And now that the Doctor and Zoe have arrived, they'll do nicely.