As the TARDIS door opens onto a barren planet, the Doctor steps straight into a struggle to destroy the merciless Krotons, the unseen rulers of the Gond people. Conditioned to obey by teaching machines, the Gonds put up no resistance until, with the Doctor's help, they discover exactly what has been happening to their best students.Battling against the Krotons' vast scientific knowledge the Doctor and Zoe must stop them turning mental power into pure energy. With a brave band of Gonds and Jamie, prepared to fight to the end, it may be defeat for the Krotons, but ultimately it will be a test of knowledge that only the Doctor can match...