An Earth colonisation survey team is on the jungle planet Deva Loka. Members of the team are disappearing—four so far. Sanders (Richard Todd) is a martinet who follows the rules religiously, Hindle (Simon Rouse) is about ready to break. Only Todd (Nerys Hughes), the science officer, is keeping her head. She does not believe the native Kinda are a threat, despite their telepathic powers. Only the women of the tribe can speak. The humans are holding two silent male tribesmen hostage. Todd thinks they are not so primitive as assumed, because they wear necklaces based on the DNA double helix.
The TARDIS has landed in the jungle, the time-travellers worried that Nyssa has collapsed from exhaustion. While she rests in the TARDIS, the Doctor and Adric explore the jungle. They find an automated Total Survival Suit (TSS) which activates and marches them at gunpoint to the Dome. Sanders is gruff but welcoming, making Hindle even more nervous. Sanders heads off in the TSS. Hindle’s will is enforced by the Kinda hostages, who have a telepathic link with him. They believe he has imprisoned their souls in a mirror. The Doctor, Adric, and Todd as well are arrested by the half-crazy Hindle.
Tegan has fallen asleep near the wind-chimes, not knowing their power. She finds herself in a void, meets nightmare characters which are a manifestation of the Mara, an evil being that seeks corporeal reality. Her mind tortured, she agrees to become the Mara and is possessed. There is a snake symbol on her arm which she passes to a young Kinda male named Aris (Adrian Mills), the brother of one of the prisoners in the dome. He can now speak. Back at the Dome, Hindle plans to destroy the jungle. Adric plays along. Sanders returns from the jungle with a milder personality. Panna (a wonderful Mary Morris), the aged mystic of the tribe, presented him with the wooden Box of Jhana, which cleared his mind and enlightened him. He too plays along with Hindle’s madness. He shows the box to Hindle, who makes the Doctor open it.
The Doctor and Todd share a vision from Panna and her ward Karuna (Sarah Prince). They are invited into a cave and see visions of the coming of the Mara. Panna dies and her life experiences are transferred to Karuna. She urges Todd and the Doctor to return to the Dome before Aris (now the Mara) can attack it. In the Dome, Hindle is becoming quite insane. Adric escapes and tries to use the TSS, but Aris and the Kinda confront him. The machine responds to Adric’s panic and injures Aris. The Kinda scatter. The Doctor and Todd find an emotionally wrecked Tegan near the wind-chimes and conclude that she was the path of Mara into the world. They find Adric and return to the Dome, where Hindle has laid explosives. Hindle is tricked into opening the Box of Jhana and the visions restore his mental balance. The mirror is shattered and the two Kinda who believed themselves trapped in it are freed.
The Doctor figures that Evil cannot face itself, so he gets several mirrored solar panels and has the Kinda surround Aris with them. The snake on his arm grows to enormous size and then is banished to Dark Places. The Dome personnel are returned to sanity. The Doctor, Adric, and Tegan leave in the TARDIS with a recovered Nyssa. Sarah Sutton was supposed to have been let go. Peter Davison argued for keeping her, but Kinda was pretty much finished, so they had her get sick for a couple of episodes. These kinds of casting shenanigans happened all the time on Doctor Who. The Kinda story has been compared with Ursula K. Leguin’s Word for World is Forest; the native mysticism and the TSS remind me of Avatar. Literary reviewers have referred to Buddhism, mystical Christianity, and Jungian psychology. I’m not sure what they were smoking, but it may have been similar to what the Doctor Who writers were smoking.
Doctor Who fans in general were not terribly impressed, but the story has gained fans since. After the dreams and nightmares and madness, the giant rubber snake, like something out of a toy-box, was something of a letdown. Since then, in the Blu-Ray release, the Giant Serpent was re-made with 21st Century special effects technology and is much less of a shock. Tegan, at least, got to act instead of just complaining all the time. This story was a favorite of hers.