The TARDIS arrives on the unnamed planet of the Gonds, who are ruled and taught in a form of self-perpetuating slavery by the alien Krotons - crystalline beings whose ship, the Dynatrope, crash-landed there thousands of years ago after being damaged in a space battle.

The Krotons are at present in suspended animation, in a crystalline slurry form, awaiting a time when they can be reconstituted by absorbtion of mental energy. Periodically, the two most brilliant Gond students are received into the Dynatrope, apparently to become 'companions of the Krotons' but in truth to have their mental energy drained, after which they are killed.

When the Doctor and Zoe take the students' test, their mental power is sufficient to reanimate the Krotons. The Doctor discovers that their life system is based on tellurium and, with help from the Gond scientist Beta, he is then able to destroy them and their ship using an impure form of sulphuric acid