An Earth colonisation survey expedition to the beautiful jungle planet Deva Loka is being depleted as members of the survey disappear one by one. Four have now gone, leaving the remainder in state of deep stress. The leader, Sanders, relies on bombast and rules; while his deputy, Hindle, is evidently close to breaking point. Only the scientific officer, Todd, seems at ease with the stresses of the situation. She does not see the native people, the Kinda, as a threat, but rather respects their culture and is intrigued by their power of telepathy. The social structure is also curious in that women seem dominant and are the only ones with the power of voice. The humans are holding two silent males hostage for "observation". Todd believes they are more advanced than they first appear, as they possess icons of the double helix of DNA about their necks, indicating a more advanced civilisation.
Elsewhere in the jungle the TARDIS crew are also under stress, especially Nyssa of Traken, who has collapsed from exhaustion. The Fifth Doctor constructs a delta wave augmenter to enable her to rest in the TARDIS while he and Adric venture deeper into the jungle. They soon find an automated total survival suit (TSS) system which comes to life and marches them to the Dome where the human colonists have established their base. Sanders is a welcoming but firm presence, further undermining Hindle at regular opportunities, and it is therefore somewhat unnerving when Sanders decides to venture out into the jungle in the TSS, leaving the highly strung Hindle in charge. His will is enforced by means of the two Kinda hostages, who have forged a telepathic link with him believing their souls to have been captured in his mirror. The Doctor, Todd and Adric are immediately placed under arrest as Hindle now becomes completely mad.
Tegan Jovanka is in even deeper danger. She has fallen asleep near the peaceful Windchimes, unaware of the danger of the dreaming of an unshared or non-telepathic mind. Her mind opens in a black void where she undergoes provocation and terror from a series of ghostly white visages, including the malevolent Dukkha, who taunts her that: “You will agree to being me, sooner or later, this side of madness or the other". The spectres are a manifestation of the Mara, an evil being of the subconscious that longs for corporeal reality once more. Mentally tortured, she eventually agrees to become the Mara and a snake symbol passes from Dukkha to her own arm. When her mind returns to her body she is possessed by the Mara and passes the snake symbol to the first Kinda she finds, a young man named Aris, who is the brother of one of the Kinda in the Dome. He too is transformed by evil and now finds the power of voice.
Back at the Dome Hindle has conceived a crazed plan to destroy the jungle, whom he views as a threat. Adric plays along with this delusion. Hindle’s world soon starts to fall apart when first Adric betrays him and then Sanders defies expectation and returns from the jungle. Sanders has been changed by his experience in the jungle. An elderly wise woman, Panna, presented him with a strange wooden box which when opened has regressed his mind back to childhood. Sanders still has the box and shows it to Hindle, who makes the Doctor open it.
The Doctor and Todd see beyond the toy inside and instead share a vision from Panna and her young ward, Karuna, which invites them to cave. The shock of the situation allows the Doctor and Todd to slip away into the jungle where they encounter Aris dominating a group of Kinda and seemingly fulfilling a tribal prophecy that “When the Not-We come, one will arise from among We, a male with Voice who must be obeyed.” Karuna soon finds the Doctor and Todd and takes them to meet Panna in the cave from the vision, with the wise woman realising the danger of the situation now Aris has voice. She places them in a trance like state and reveals that the Mara has gained dominion on Deva Loka. The Great Wheel which turns as civilisations rise and fall has turned again and the hour is near when chaos will reign. The vision she shares is Panna’s last act: when it is finished, she is dead.
In a further curiosity, the old woman seems to have been reborn in Karuna. She urges Todd and the Doctor to return to the Dome to prevent Aris leading an attack on it which will increase the chaos and hasten the collapse of the Kinda civilisation.
Back at the Dome Hindle, Sanders and Adric remain in a state of unreality, with the former becoming ever more demented and unbalanced, lapsing back into an unhappy childhood at moments of crisis. Adric slips away during confusion and dons the TSS but is soon confronted by Aris and the Kinda. He fires, hitting Aris and scattering the Kinda.
The Doctor and Todd find an emotionally wrecked Tegan near the Windchimes and conclude that she was the path of the Mara back into this world. They then find Adric and the party heads back to the Dome where Hindle has now completed the laying of explosives which will incinerate the jungle and the Dome itself: the ultimate self-defence. Todd persuades Hindle now to open the wooden box, the Box of Jhana, and the visions therein restore both his balance and then Sanders’ own. The two servile Kinda are freed when the mirror entrapping them is shattered. This gives the Doctor an idea. He realises the one thing evil cannot face is itself and so engineers the creation of a large circle of mirrors in a jungle clearing. Aris is trapped within it and the snake on his arm breaks free. The Mara swells to giant proportions (as shown in the picture above) but then is banished back from the corporeal world to the Dark Places of the Inside.
With the threat of the Mara dissipated, and the personnel of the Dome back to more balanced selves, the Doctor, Adric and an exhausted Tegan decide to leave. When they reach the TARDIS, Nyssa greets them, fully recovered too.
Doctor Peter Davison (Fifth Doctor)
Companions Matthew Waterhouse (Adric)
Sarah Sutton (Nyssa)
Janet Fielding (Tegan Jovanka)
Richard Todd — Sanders
Nerys Hughes — Todd
Simon Rouse — Hindle
Mary Morris — Panna
Sarah Prince — Karuna
Adrian Mills — Aris
Lee Cornes — Trickster
Jeff Stewart — Dukkha
Anna Wing — Anatta
Roger Milner — Annica
Writer Christopher Bailey
Director Peter Grimwade
Script editor Eric Saward
Producer John Nathan-Turner
Executive producer(s) None
Production code 5Y
Series Season 19
Length 4 episodes, 25 minutes each
Originally broadcast February 1–February 9, 1982