The Doctor prescribes some augmented D-sleep for Nyssa and constructs a delta wave augmentor to ensure that she gets it. The TARDIS has arrived on the planet Deva Loka and, while Nyssa sleeps in the TARDIS, the Doctor, Adric and Tegan decide to look around.

They find, hung from some trees, a set of wind chimes which seem to have a strange effect on Tegan. The Doctor and Adric are distracted when they find an armoured suit – a Total Survival Suit (TSS) – elsewhere in the jungle. Tegan, feeling sleepy, settles down to rest by the chimes. 

The suit is activated when Adric slams shut its door, and it herds the Doctor and Adric to a dome-shaped base. There they meet Sanders (Richard Todd), Todd (Nerys Hughes) and Hindle (Simon Rouse), the remaining members of an expedition aiming to classify planet S14 – as they term it – as to its suitability for colonisation. Hindle has captured two of the native Kinda (Michael Mungarven, Barney Lawrence) for study and he discovers that, by using a mirror, he can hypnotise them and then telepathically order them to do his will. Already slightly unstable, Hindle slips into madness. When Sanders leaves in the TSS to look for Roberts – a missing crew member – Hindle places all the others under arrest. 

Meanwhile, the natives drape Tegan’s sleeping form with garlands and other offerings. In her dreams, the woman meets a strange man (Jeffrey Stewart) with a snake tattooed on his arm, who wants to take over her body. The stranger confuses her with multiple images of herself and then leaves her totally alone in her own mind. Terrified, she capitulates to his demand. She wakes up back in the forest, but is now possessed by an alien force. 

Sanders has met up with the blind wise woman Panna (Mary Morris) and her young telepathic acolyte Karuna (Sarah Prince). Panna gives Sanders the Box of Jhana, which he opens. Light floods his face. 

Back at the dome, Hindle is planning to raze a fifty mile area around the dome with fire as protection against the plants and trees. The Doctor and Todd have been locked up and Adric is pretending to be on Hindle’s side in order to get the electronic key to the cell. When Sanders returns he gives Hindle the Box but Hindle orders the Doctor to open it. The Time Lord does so and sees an image of Panna and Karuna beckoning him to them. 

The possessed Tegan meets another Kinda, Aris (Adrian Mills), and offers to help him. The snake mark moves from her arm to his and he is possessed. The creature within Tegan reveals itself to be a Mara. 

The Doctor and Todd escape from the dome only to come across a group of Kinda. Aris arrives and orders that the pair be restrained. Karuna suggests that Aris’s speech – he was previously mute, as are all Kinda males – fulfils a prophecy and that he must be obeyed. She takes the Doctor and Todd to see Panna. The old wise woman speaks of the cycle of life, of great wheels turning and civilisations rising and falling. She shows the Doctor and Todd a vision – time running out as the Kinda despair. When they return to reality, Panna appears to be dead, but Karuna is revealed to be her reincarnation. The Doctor and Todd find Tegan in the jungle and wake her. She seems fine, if puzzled by her bad dreams. 

Aris, meanwhile, is exhorting the Kinda to attack and destroy the dome. They have built a TSS-like structure out of sticks with which to battle the real TSS. This machine now attacks, and the Kinda are scattered. Adric is inside the machine and the Doctor releases him. Hindle has wired the dome with bombs and is planning to blow it up. The Doctor and Todd rush back to try to stop him. Hindle has reverted to a child-like state, and Todd tricks him into opening the Box of Jhana. The light floods his mind. 

The immediate danger over, the Doctor realises he must deal with the Mara and plans to use a circle of mirrors to trap it. Karuna acts as bait to draw Aris into the circle. Trapped, the Mara leaves Aris’s body and grows into a giant snake (Operator: Stephen Calcutt). Then it weakens and vanishes. 

Later, Todd recommends that the planet is unsuitable for colonisation. The Doctor bids the expedition team farewell. A recovered Nyssa greets the Doctor, Tegan and Adric and they leave in the TARDIS.

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}