Crystalline warriors, four of whom crashed on a primitive planet, populated by the humanoid Gonds. Two of the Krotons died, but the other two, living inside their tellurium-based ship, began instructing the Gonds in new sciences and skills with the aid of sophisticated Teaching Machines. For thousands of years the Krotons would name the two highest achievers amongst the Gonds every month, the High Brains, and invite them to become companions of the Krotons. The High Brains would enter the Krotons' base, but then had their minds drained. Their mental energy was used to keep the Krotons alive.

The bodies of the Gonds were then destroyed. The Doctor arrived on the planet and was selected, together with Zoe Heriot, to become the latest High Brains, and thereby meet the Krotons. He realised that their Dynatrope machine not only drained the Gonds' mental energy, but re-grew tellurium bodies for the Krotons.

He added sulphuric acid to the mixture, which exhausted the Krotons, destroying them, and left the Gonds to use the knowledge that they had gained from the Krotons to develop, but at their own pace.