The Doctor arrives on the paradise planet of Deva Loka to find a colonial mission on the verge of collapse. Several of its members have vanished into the jungle without a trace, leaving the survivors suspicious and paranoid. The mystery deepens as it becomes clear that the planet's native inhabitants, the Kinda, possess hitherto unsuspected powers that challenge human understanding. Meanwhile the Doctor's companion Tegan becomes possessed by the Mara, a force of pure evil that lives in dreams and preys on fear.