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. The Krotons are in suspended animation, in a crystalline slurry form, awaiting a time when they can be reconstituted by absorption of mental energy. Periodically, the two most brilliant Gond students are received into the Dynatrope, nominally to become "companions of the Krotons", but in truth to have their mental energy drained, after which they are killed.
Patrick Troughton
as The Doctor
Frazer Hines
as Jamie McCrimmon
Wendy Padbury
as Zoe Heriot
James Copeland
as Selris
James Cairncross
as Beta
Gilbert Wynne
as Thara
Philip Madoc
as Eelek
Terence Brown
as Abu
Madeleine Mills
as Vana
Richard Ireson
as Axus
Maurice Selwyn
as Custodian
Bronson Shaw
as Student
