The TARDIS lands on a nameless desolate planet where The Doctor, Jamie and Zoe learn the native Gonds are ruled by the mysterious and unseen Krotons, where The Krotons chooses their top students to join The Krotons in their machine as their companions. But The Doctor, Jamie and Zoe discover The Krotons are crystalline alien beings and are turning The Gonds mental power into energy and are feeding off it.