When his companion Tegan (Janet Fielding) begins exhibiting signs of possession by the Mara upon the TARDIS's arrival on Manussa, the Doctor (Peter Davison) must take her and Nyssa (Sarah Sutton) to find the cave mouth that's shaped like a snake that she keeps seeing in her nightmares. But when Tegan runs off under mind control, it's up to the Doctor to make sure that the Mara can't find the means for a physical return.