The TARDIS materializes on a new planet.  They meet the Gonds, a human looking race who are unknowingly ruled and used by the Krotons.  Their purpose is to use human brains that are powerful enough to power the Dynatrope (their ship).  The humans that are not powerful enough metally are disposed of.

The Doctor and Zoe take the test and their mental powers reanimate the sleeping Krotons.  When the Doctor discovers that the Krotons are based on tellurium, he and a Gond scientist are able to destroy them and their ship with sulphuric acid.