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 to deal with the situation with equanimity. 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 necklaces representative of the double helix of DNA, 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 activates and marches them to the Dome, the colonists' base. Sanders is a welcoming but gruff presence, further undermining Hindle at regular intervals. At this point 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 evinces megalomania.