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 earlier after being damaged in a space battle. When the Second 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. With help from the Gond scientist Beta, he destroys them and their ship with an impure form of sulphuric acid.