Poor Nyssa is put to sleep and left behind in the TARDIS as the Doctor, Adric and Tegan explore the beautiful paradise planet Deva Loka. They quickly find some musical crystals hanging from the trees that seem to have a mesmerizing affect on Tegan. As the Doctor chases after a curious Adric, Tegan is left behind beneath the crystals and falls into a trance.

The Doctor and Adric come upon a mechanical device and realize that the planet has other occupants. The empty machine suddenly activates, and holds the Doctor and Adric at gunpoint as it directs them to its base. Once there, the Doctor meets what remains of a colonization survey team; a by the book commander, Sanders; an unstable security officer, Hindle; and a scientist, Todd. The other team members have been mysteriously disappearing, and only these three remain.

The Doctor learns from Todd that the planet is inhabited by a seemingly primitive people known as the Kinda. However, first impressions are not always accurate, and Todd has come to believe the Kinda are not that primitive. Todd has been studying a pair of captive Kinda being held at the base and has been noticing a few discrepancies. At first they seem unable to speak, but she begins to suspect they use some form of telepathy to communicate

Meanwhile beneath the crystals, Tegan is having an internal struggle. An entity calling itself the Mara is trying to take control of her body. The Mara torments her mind relentlessly, making her question her own sanity to the point where she gives into the creatures demands and submits to its control. This possession is evident by the sign of the snake that appears on Tegan's arm.

Back at the base, Sanders has decided to take control of the mechanical device and take it out to explore more of the surrounding forest. He leaves Hindle in charge. Unfortunately, Hindle has become quite mad and quickly begins abusing his new position of authority. Telepathically controlling the Kinda captives, he uses them to imprison the Doctor, Todd and Adric. In hs madness, Hindle believes the plants are attacking the base, and his plan is decimate a hundred mile radius around the base with explosives.

The blind Kinda shaman, Panna tells her apprentice Karuna to approach Sanders in his mechanical device, and present him with a mysterious box. Sanders opens the box and is mesmerized by its contents.

Sanders returns to the base, but appears to be drastically changed. No longer the gruff commander, he is now friendly and playful like a child. Sanders has brought the box back with him and he tries to get Hindle to open it. Hindle is too fearful to open it himself. He forces the Doctor and Todd to open it instead to test it out.

In a flash of light Todd and the Doctor are shown a vision of the shaman, Panna, beckoning them. They use the distraction to escape from the base and into the forest. It isn't long before the Kinda approach them.

Meanwhile, the Mara has been busy transferring itself from Tegan to one of the Kinda men, Aris. Tegan is left unconscious again in the forest, while the Mara in his new stronger body finds the rest of the Kinda who now surround the Doctor and Todd. The Mara orders the Kinda to grab the Doctor and Todd. The Kinda are shocked by Aris's voice. One of their prophecies says the Kinda must obey the "One with voice".

Before the Kinda can respond, Karuna shows up and spirits the Doctor and Todd away and takes them to Panna. Panna tries to explain to the pair what is going on. She shows them a vision of the future, a future of utter destruction caused by the Mara. They emerge from their trance to find Panna has died. The strain of providing the vision has proven to be too much. Fortunately, her knowledge and wisdom has now been transferred to Karuna.

The Mara mentally subjugates the Kinda and forces them to join his attack on the colonizer's base in order to drive them from his planet. At the same time, Hindle has been busy planting explosives everywhere, all it will take is a push of a button. Thinking to help in some way, Adric climbs into the mechanical survey device. However, he has trouble controlling the machine and begins flailing about firing its weapons randomly. Oddly enough, this scares the Kinda and the Mara off.

Meanwhile, the Doctor and Todd attempt to distract Hindle so they can disarm the bombs he's planted.Todd tricks him into looking into the Kinda box. Suddenly Hindle is cured of his insanity. It seems the box is actually a Kinda medicinal device. The Doctor is now able to disarm the bombs, but the Mara is still out there.

The Doctor believes the Mara's one weakness is its inability to face its own reflection. With that in mind, he tricks the Mara in Aris' body into a circle of mirrors. The Mara is forced from Aris' body and grows into an enormous snake. It's attempts to break out of the circle are futile, and the Mara is eventually destroyed.

With everything restored to normal, the colonizers decide to recommend not colonizing Deva Loka. The Doctor, Tegan and Adric return to the TARDIS just in time to greet Nyssa waking from her power nap.