The TARDIS visits the planet Deva Loka, where Nyssa remains behind in the ship to recover from a mild mental disorientation while the Doctor, Tegan and Adric explore. Tegan falls asleep under some wind chimes and becomes possessed by an evil force, a Mara.

Also on Deva Loka is a survey team assessing the planet for colonisation, but three of their number have disappeared and the remainder - Sanders, his deputy Hindle and the scientist Todd - are encountering difficulties in their dealings with the outwardly primitive but telepathically gifted native people, the Kinda.

Hindle becomes mentally unstable, but his mind is eventually cleared by a Kinda device called the Box of Jhana. Aided by the Kinda, the Doctor ultimately succeeds in banishing the Mara - which manifests itself in the form of a giant snake - by trapping it in a circle of mirrors.