The TARDIS lands on Manusa, ancient home world of both the Manussan Empire and the Samaran Empire. Tegan begins to dream of a cave shaped like a serpent’s mouth. The Doctor thinks the Mara, which seized control of her during their visit to the Kinda of the planet Deva Loka, is trying to possess her again. He tries to find the cave with her and Nyssa, as a kind of therapy, but she is too frightened to enter and runs into the city crowd. This makes her even more vulnerable to the Mara and she revels in horror and destruction, as the emblem of the serpent once more appears on her arm.
The city is in the midst of a festival celebrating the banishment of the Mara five hundred years before. The son of the former Federator, an indolent lad named Lon (Martin Clunes in just about his first role) is leading the celebration, aided by his mother the Lady Tanha (Colette O’Neil) and the archaeologist Ambril (John Carson) an expert in the Sumaran Period. Lon is intrigued by the thought that the Mara might reappear one day, but Ambril believes it is nothing but a myth. When the Doctor tries to convince Ambril that this could actually happen, he is dismissed. But a young Deputy named Chela (Jonathon Morris) gives the Doctor a blue crystal called Little Mind’s Eye, which is used by the cult of snake dancers to repel the Mara.
The Doctor understands that the small charm and the Great Mind’s Eye are focal points for energy and can turn thought into matter. This is how the Mara will transfer from Tegan’s mind to reality. This happened once before and marked the conquest of the Manusan Empire by the evil Sumarans.
Tegan meets Lon and passes the snake mark to him. They visit the cave from Tegan’s dream, in which there is a pattern on the wall that could hold the Great Mind’s Eye. Lon is sent back to the palace, but Tegan wreaks havoc and takes control of a showman named Dugdale (Brian Miller). Lon covers his arm and tries to persuade Ambril to use the Great Mind’s Eye in the ceremony. He shows Ambril a cave full of Sumaran treasures and warns that they will be destroyed if he does not place the gem in the wall carving.
The Doctor and Nyssa are shown the journal of Dojjen by Chela. This was a snake dancer who was Ambril’s predecessor. The Doctor, Nyssa, and Chela try to warn the palace, but Lon is in the grip of the Mara and creating a dangerous situation. The Doctor uses the Little Mind’s Eye to contact Dojjen himself, who lives in the dunes beyond the city. They go there, the doctor is bitten by a poisonous snake, and is able to commune with Dojjen (Preston Lockwood). He is told that the Mara may be defeated by finding a still point in his mind.
All three head back to the city and find the festivities at a peak, a procession marching to a celebration in the cave. Lon plays the role of his ancestor, the Federator, rejecting the Mara. After a series of verbal challenges, he seizes the Great Mind’s Eye and places it in the appropriate place. Tegan and Dugdale arrive and she displays the serpent on her arm. The Mara is now capable of creating itself in the cave as a huge serpent. The Doctor arrives and concentrates on the still place in his mind, communing with Dojjen with the Little Mind’s Eye. The manifestation is interrupted, the Mara’s mind-prisoners are freed, and it dies and rots as they watch. The Doctor assures a distraught Tegan that the Mara has been destroyed.
Brian Miller, who played Dugdale, was the husband of Elizabeth Sladen (Sarah Jane Smith). He provided the voice of several Daleks. Martin Clunes (Lon) is best known for the British TV series Doc Martin, for the movie Men Behaving Badly, and as Richard Burbage in Shakespeare in Love, but clips of him in the outrageous costumes he wore as Lon are always shown during his talk-show appearances. The idea of a third Mara adventure was abandoned. Nyssa gets to wear something other than the Traken outfit in which she came aboard the TARDIS, but the Doctor does not notice. Janet Fielding gets to act again and is rather chilling when she is possessed by the Mara. The story was praised for its Buddhist and American Native trappings. It is compared favorably with the largely unloved Kinda, of which it is a sequel. It might remind you of the Third Doctor’s final story Planet of the Spiders because it basically has the same plot, but with snakes instead of spiders.