Another adventure for everyone's favourite Time Lord. The Doctor (Peter Davison), Tegan, Adric and Nyssa land on paradisical Deva Loka, for rest and recuperation. However, the military expediton on the planet has lost several crew members, and the Doctor and Adric are taken hostage by the near hysterical Hindle. Meanwhile, Tegan's dreams have provided the gateway to an ancient evil, the snake-like Mara. The Doctor must prevent the Mara from taking over the Kinda and destroying the expedition, as the wheel of creation begins to turn.
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.