Dreams are important... never underestimate them.Uh-oh. Looks like Tegan isn't quite over her encounter with the Mara, the mind-controlling snake god from "Kinda": not long after they land on the planet of Manussa, she starts having sinister snake-themed dreams. The Doctor that Mara might still be alive, not in a physical form but one that exist in a mind.On the planet Manussa, Tanha wants Lon to be prepared for the ceremony to celebrate the end of the Mara. He isn't really bothered, but is interested in a jewel kept between the teeth of a model snake. It is Ambril's - an artefact kept for many years, as a symbol of the Mara. They decide to set off to the cave, where Ambril will explain the ceremony's proceedings.Under the influence of the Mara, Tegan has set the TARDIS controls to Manussa, a planet known well to the Doctor. She also talks of a dream she's been having, of a cave, shaped in the jaws of a snake. Upon their arrival on the planet, they decide to investigate.On a local street, a man is shouting to the crowds about a hall of mirrors. Lon finds it hilarious and is pulled along by his mother, so that they can reach the cave. There, Ambril explains the legend of the snake and how they will add to the ceremony using the stories. They enter the cave and travel far through the tunnels inside.The Doctor is searching for the cave, which Tegan had a vision of in her dreams. They finally find it and the Doctor enters, telling Nyssa to look after Tegan outside. As a consequence of her dreams, Tegan is frightened by wriggling toy snakes, bought by children on Manussa. She runs away and Nyssa is forced to follow her, but loses her in the crowds.Tegan enters a fortune teller's tent, eventually controlled by the Mara. The fortune teller explains that her stories aren't real, but then becomes aware of a snake's skull appearing in her crystal ball. It explodes violently and the fortune teller screams.Tegan (the Mara) escapes from the tent. The fortune teller is helped out of the tent by locals, completely overwhelmed by the situation.The Doctor has appeared in the cave where Lon is having a lesson on the Mara legend. Ambril is surprised by the Time Lord's appearance. The Doctor is keen to note that the legend is very real. Lon finds this curious. Ambril believes that the Doctor is crazy.Nyssa finally finds Tegan, who is laughing about the fortune teller. Nyssa soon realises that she is under the influence of the Mara again, her emotions different to what Tegan would express. Tegan then runs again and this time Nyssa loses her for real. Tegan has secretly entered the hall of mirrors.Nyssa arrives just outside the cave where Lon, Tanha and Ambril have just emerged to return to the palace. The Doctor is behind them. Nyssa explains about Tegan's disappearance. The Doctor looks worried. He wants to return to the TARDIS though to monitor the wavelengths of an interesting blue jewel he has obtained.The Mara in Tegan is interested in the mirrors, remembering that she was trapped by mirrors before on Kinda. Dugdale finds her talking to herself in the mirror, and is also influenced by the Mara when he looks into the mirror. Tegan orders him to bring Lon.The Doctor and Nyssa are now inside the TARDIS where the Doctor is creating a circle where he can project thoughts into the jewel. When he tries along with Nyssa, the jewel lights up, but only when they concentrate.Lon arrives at the hall of mirrors and is quickly taken by the Mara. They then proceed to the cave and behind the symbolic wall where they use Dugdale as a servant.The Doctor returns to the palace to try to persuade Ambril to believe him. Ambril is unimpressed and orders the Doctor to be jailed. Nyssa overhears everything and tries to work by herself.Lon arrives in Ambril's office to gather the jewel, which opens the symbolic wall and will eventually bring back the Mara. Ambril doesn't know that Lon is under an influence. The Doctor tries to persuade Chela to get him out, but with no luck.Lon lures Ambril to the cave with the promise of discovering priceless historical artefacts. Once inside, Ambril is cornered by the possessed Lon, Dugdale and Tegan, who pressure him to return the Great Crystal during the ceremony. After Ambril agrees and Lon leads him away, the mark of the serpent on Tegan's arm manifests as a live snake.Chela brings the Doctor a diary written by Dojjen, Ambril's predecessor as Director of Historical Research. Dojjen believed that the Mara had not been destroyed and would someday return. He eventually left his post to study the forbidden teachings of the Snake Dancers.Nyssa searches Ambril's office for the key to the jail cell, but Tanha catches her in the act and has her locked up alongside the Doctor. The Doctor and Nyssa read through Dojjen's diary and realise that the ancient Manussans manufactured crystals that could conduct mental energy. The unanticipated result was that the crystals absorbed and reflected the Manussans' own negative thoughts and emotions, creating the Mara. As centuries passed, the Manussans forgot that they themselves had brought the Mara into being. The Doctor deduces that Dojjen learned this truth from the Snake Dancers, the only people who kept the knowledge alive.Lon and Ambril return to the palace, where a dazed Ambril informs Tanha and Chela that at the impending ceremony, the Great Crystal will be restored to its original place. Alarmed, Chela frees the Doctor and Nyssa in hopes that they can intervene. They attempt to flee the palace, but are surrounded by guards. Lon claims that the three are plotting his death, and orders the guards to kill them.Tanha overrides Lon's order and allows the Doctor a chance to speak. The Doctor realises that Lon has fallen under control of the Mara. Ambril offers to reveal the Great Crystal, and as he, Tanha and Lon are distracted, the Doctor seizes the opportunity to escape with Nyssa and Chela.Determined to destroy the Mara completely, the Doctor uses his crystal to summon Dojjen. Dojjen and the Doctor enact the Snake Dance ritual, in which live snakes bite their wrists, allowing them to communicate telepathically. Guilt-ridden over what has befallen Tegan, the Doctor asks how he can save her and defeat the Mara once and for all. Dojjen urges him to find the "still point" within himself.Tanha questions Lon's strange behaviour, but he brushes aside her suspicions. In the cave, the ceremony proceeds according to custom until Lon stuns the audience by announcing that the Mara has returned. The Doctor, Nyssa and Chela burst onto the scene, but are too late to stop Lon from placing the Great Crystal in the mouth of the snake on the cave wall. Tegan appears holding the Mara in the form of a snake wrapped around her arm. The Mara feeds on the crowd's fear and panic, growing larger and stronger.The Doctor alone is unaffected. He focuses mental energy into his crystal in a battle of wills against the Mara. Although his resolve is tested when the Mara speaks to him with Tegan's voice, he refuses to submit. Dojjen adds his own mental energy to the effort, helping to subdue the Mara long enough for the Doctor to pull the Great Crystal from the wall. The Mara's influence is broken, and the snake falls to the ground, dead.Restored to herself, Tegan weeps with horror at having felt the rage and hatred of the Mara inside of her. The Doctor comforts her, assuring her that the Mara has been destroyed.
— The Doctor