In the TARDIS, Tegan sleeps as the Doctor and Nyssa discover that they are not where they expected to be – and that there are traces of anti-matter present. They have arrived on Planet G 139901 KB, otherwise known as Manussa – a type 314S planet in the Scrampus system and part of the Federation of Three Worlds. The Doctor realises that it was Tegan who set the co-ordinates to bring them here. 

In her room, Tegan dreams of a cave-mouth shaped like a snake. She awakens screaming at what she has seen there. The Doctor is interested and tries to recover Tegan’s dream by hypnotising her. This attempt fails, however, so he then constructs a device to limit the production of brain waves associated with dreaming, his aim being to try to protect Tegan from whatever is in her mind. The three friends leave the TARDIS to investigate Manussa. 

Also on Manussa is Lon (Martin Clunes), the Federator’s son, and his mother Tanha (Colette O’Neil). Lon is a bored young man and craves excitement, while his mother is concerned that the affairs of state should be seen to be carried out. Preparations are being made for a pageant which is held every ten years to celebrate the destruction of the Mara 500 years earlier. Tanha and Lon go with Ambril (John Carson), the planet’s Director of Historical Research, to see the nearby cave systems. There, the history of the Manussan empire and the earlier Sumaran empire is told in a series of pictograms on the walls. 

Tegan, confused and effectively deaf while wearing the Doctor’s device, becomes separated from her two friends and ends up in a barker’s hall of mirrors, wherein she is tempted by the Mara. She is possessed by the creature and a snake tattoo appears on her arm. She sends the barker, Dugdale (Brian Miller), to get Lon, who is then also possessed. 

The Doctor meanwhile tries to find out more about the Mara, first from the Director and then from looking at the pictograms in the caves. He realises that the legend revolves around the Great Crystal – the Great Mind’s Eye – which collected energy from living minds and focused it. Chela (Jonathon Morris), one of Ambril’s attendants, gives the Doctor a smaller version of the Crystal, a Little Mind’s Eye, and by experimenting with it he discovers that it has the power to turn thought into matter. The Doctor realises that the Mara is planning to recreate itself in the ceremony at the conclusion of the pageant, but he is locked up when he tries to warn Ambril. 

Ambril is tempted by Lon and Tegan, who reveal to him numerous ‘lost’ artefacts from the ancient Sumaran civilisation. They instruct him that the ‘real’ Great Crystal will be used in the ceremony. When Lon and Ambril leave to arrange this, the snake on Tegan’s arm starts to come alive and she destroys Dugdale’s mind with a look. 

Chela gives the imprisoned Doctor the diary of Dojjen, the Director before Ambril, who wrote of the Mara’s re-birth before he wandered off into the wilderness and disappeared. Through the diary, the Doctor realises that the Manussans brought the Mara into being themselves when they created the Crystal. With Chela’s help, he and Nyssa – who has been captured and imprisoned with him – escape and head off to try to contact Dojjen. 

The Doctor summons the old man via the small Crystal. Using the venom from a small snake, the pair telepathically commune. Dojjen (Preston Lockwood) states that fear is the only poison and that, in order to defeat the Mara, the Doctor must find the still point within himself. Recovering, the Doctor returns to the caves with Nyssa and Chela to try to stop the ceremony. 

In the cave, Lon plays out his role and ritually challenges the snake. There is a triple temptation: fear in a handful of dust delivered from a skull; despair in a withered branch; greed in the hidden depths of the Crystal. Lon suddenly breaks from the established litany and smashes the fake Crystal. He shows the real Crystal to the assembled Manussans and opens a hidden door to reveal Tegan clutching a snake. Lon places the Great Crystal in the allotted space in the carved design on the cave wall. Lines of power then radiate from it and, except for Lon, Tegan and the mindless Dugdale, everyone present collapses and writhes in mental agony. Tegan drops the snake she is holding and it starts to grow. The Doctor, Chela and Nyssa rush in, but they too are assaulted by the power from the Crystal. The Doctor warns them not to look – not to believe – as the Mara is creating itself from their fear. The Doctor concentrates on his smaller Crystal, trying to find the still point. Tegan merges with the giant snake, but the Doctor manages to hold off the final becoming. The real Tegan then starts to reassert herself as the Doctor concentrates on Dojjen and the Crystal. The Mara orders Lon and Dugdale to destroy the small Crystal but they are repelled. The Doctor snatches the Great Crystal from the wall and the power drain stops. The Mara and Tegan split apart once more and, robbed of power, the snake dies. 

As the Manussans recover, the Doctor tells a tearful Tegan that she is now free of the Mara forever.

_{Synopsis from Doctor Who: The Fifth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}