Written by Robert Holmes
Directed by David Maloney
The TARDIS arrives on the planet of the Gonds, a simple people overseen and educated by the Krotons. The Krotons are crystalline aliens whose ship, the Dynatrope, crashed on the planet thousands of years ago.
The Krotons exist in slurry-like form, requiring the absorption of enough mental energy to reform themselves fully. Periodically, the two most brilliant Gond students are received into the Dynatrope. As far as the Gonds are concerned these two are taken to serve the Krotons, but in truth their mental energy is drained, and their mindless bodies are then taken outside the Dynatrop to be killed.
The Doctor and Zoe take the Krotons' test, and their brains are advanced enough to reanimate the Krotons. However, the Doctor also discovers that make up of the Kortons is based on tellurium, which allows him to destroy them and their ship using a form of sulphuric acid.