Sanders, Todd and Hindle

After witnessing Nyssa collapsing onto the Console Room’s floor, at the end of the previous story (see "Four to Doomsday"), The Doctor prescribes some augmented D-sleep for his exhausted companion and constructs a delta wave augmenter to ensure that she gets it.

Meanwhile the TARDIS has materialised on the beautiful jungle planet Deva Loka and, while Nyssa remains behind in the TARDIS to sleep, The Doctor, Adric and Tegan Jovanka decide to look around. Nearby 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 meanwhile has been left alone, by the wind chimes, starts to feel sleepy. She settles down to rest and soon falls asleep.

While investigating the TSS it becomes activated when Adric slams shut its door, and it herds The Doctor and Adric to a dome-shaped base. There they meet Sanders, Todd and Hindle, the remaining members of a survey team aiming to classify planet S14 - as they term it - as to its suitability for colonisation. Hindle has captured two of the native Kinda 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. This becomes evident when Sanders leaves in the TSS to look for Roberts - a missing crew member – and Hindle places all the others under arrest.

The Doctor

Meanwhile, the natives drape Tegan’s sleeping form with garlands and other offerings. In her dreams, Tegan meets a strange man 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. Tegan wakes up back in the forest, but she is now possessed by an alien force.

Elswhere in the forest Sanders has met up with the blind wise woman Panna and her young telepathic acolyte Karuna. Panna gives Sanders the Box of Jhana, which when he opens it light floods his face.

Tegan sleeping

Back at the dome, Hindle is planning to set fire to a fifty mile area around the dome as protection against the plants and trees which he is convinced are a threat. 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 of Jhana 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.

Lost in the jungle the possessed Tegan meets another Kinda, Aris, and she offers to help him. As they hold hands the snake mark moves from Tegan's arm to Aris' and becomes possessed. The creature within Tegan then reveals itself to be a Mara.

After escaping from the dome The Doctor and Todd manage to leave the dome only to come across a group of Kinda. But when Aris arrives he 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 Discovers Tegan

The Doctor and Todd then find an emotionally wrecked Tegan in the jungle near the Windchimes. When they wake her she seems fine, if puzzled by her bad dreams. But The Doctor is convinced that she was the path of the Mara back into this world.

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. It is then that The Doctor discovers that 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 and the light floods his mind.

With the immediate danger over, The Doctor realises he must now 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. Then it weakens and vanishes.

With the threat of the Mara dissipated, and the personnel of the Dome back to more balanced selves, Todd tells The Doctor that Sanders intends to stay on Deva Loka but that it is a bit too green for her. As the Kinda arrive and look on, The Doctor bids the expedition team farewell. When The Doctor, Adric and Tegan reach the TARDIS a recovered Nyssa greets them and they all leave in the TARDIS.