Much to the Doctor's surprise, the TARDIS has materialized in the wrong place -- not on Earth, but on the planet Manussa. Nyssa looks up Manussa in a planetary gazetteer, and finds that it is one of three planets in a Federation, and the former homeworld of the Manussan and the Sumaran Empires. The Doctor realizes the significance of what she's found and recalls that Tegan set the co-ordinates while he was trying to teach her how to programme the console. They then hear Tegan screaming; she has been asleep in her room, and is just waking from a terrible nightmare about a cave with the mouth of a snake. The images are fading, but she's had the dream before -- and the Doctor knows why. He takes Tegan into the console room and asks her where they are, and after a moment's thought, Tegan guesses that they are on Manussa, a planet she's never heard of before. It is the former homeworld of the Sumaran Empire; it is the world of the Mara. Where, somewhere in the desert sands, an old man wearing a small blue crystal on a pendant around his neck sits meditating, and waiting.

Lon, the bored young son of the Federator, is on Manussa, deputising for his father in his absence, and his mother Tanha has arranged an outing to the local caves. The Director of research into the Sumaran era, Ambril, is on his way, but Tanha admits that she preferred his more colourful predecessor -- a man who believed that the legends of the Snakedancers were true, and that one day the Mara would return. Lon is not interested in the second-hand reported ravings of a madman; the Mara was destroyed by the Federation over 500 years ago and will not return, not in a dream or anywhere else. Ambril arrives, enthusiastic about the planned outing, and Lon takes delight in puncturing his enthusiasm and challenging the authenticity of the artefacts which Ambril has unearthed himself. Nevertheless, he concedes to his mother's wishes, gets dressed and accompanies them out into the streets. A festival atmosphere is prevalent as the people of the planet prepare for the ceremony to celebrate the destruction of the Mara, and Lon again delights in mocking Dugdale, the owner of a hall of mirrors who tries luring him in without realizing at first who Lon is. Tanha delicately ushers Lon away from the unfortunate Dugdale, and their party finally arrives at the caves of the Mara -- vast natural formations, where the entrance has been carved into the mouth of a snake...

Tegan is horrified by the thought that she might still be possessed by the Mara at an unconscious level, and that the battle for control of her mind is taking place in her dreams. The Doctor uses a sonic device with an earpiece to hypnotise her and tries sending her back to the dream, and Tegan regresses to six years old and then enters the dream -- only to be cast out immediately, an alien voice shouting through her mouth at them to go away. The Doctor, realizing that the Mara has almost gained control of her -- that it was in a moment of weakness that it set the co-ordinates for its homeworld through her -- tells her to keep wearing the device, which will inhibit further dreaming, and takes her and Nyssa out into the carnival streets of Manussa to find the cave from her dream, which he is certain is a real place. But Tegan is already falling into a trance and is not entirely in control of herself, and when they arrive at the real cave from her dream she is too terrified to enter. The Doctor, unable to convince her that there is no danger, leaves Nyssa with her while he enters the cave himself to find out what he can learn. But when a hawker selling children's toys like wriggling snakes approaches them and waves his wares in Tegan's face, one look at the toy snakes sends Tegan screaming into the crowd -- where Nyssa loses her.

Lon is still bored and even Ambril's delightful enthusiasm about the pictograms and artefacts in the caves cannot spark his interest. He asks Ambril about the legend of the return, but this is the one area of Sumaran study where Ambril is less than enthusiastic; as far as he's concerned the legend is superstitious nonsense invented by mothers to frighten their children, and he's not at all impressed by his predecessor Dojjen's apparent credulousness; it is only under Ambril that the vital work of scientific study and cataloguing of the artefacts has continued. The party continues on to the cave of the Mara itself, where a carving of a snake is embedded directly within the wall, looping around itself and concluding in an empty mouth socket where the Great Crystal was once embedded. The Crystal itself has historically been the property of the Director, and Ambril is its current guardian.

The Doctor arrives just as Ambril assures Tanha once again that the legend is nothing more than that, but when he interrupts and tries to tell them that it's true, the furious Ambril orders their bodyguard to throw the raving lunatic out. Lon, however, senses a potential distraction and urges the Doctor to tell his story, and the Doctor does so -- but realizes that Lon is merely humouring him and that Ambril doesn't believe a word of it. He urges them to follow and meet his companion, who has been possessed, and despite Ambril's objections Lon agrees to do so... but when they return to the cave mouth the Doctor finds only Nyssa, out of breath, having lost Tegan in the crowds. Lon, disappointed, wants the Doctor punished, but the Doctor ignores him and sets off with Nyssa in search of Tegan. Lon decides to let them go; it's not important, none of it is important. The Doctor and Nyssa are unable to locate Tegan, and return to the TARDIS in the hope that she was making her way there; unfortunately, either she wasn't or she never got there. If Tegan takes off the dream inhibitor for whatever reason all may be lost, and the Doctor still doesn't understand why the Mara has brought itself back to its homeworld after so long...

The Doctor realizes that he can't do anything without further information, and thus sends Nyssa in search of Tegan while he tries to get Ambril to see sense. Nyssa eventually spots a commotion in the marketplace; the fortune teller, terrified out of her wits, is being led away to rest by a couple of men, and on the outskirts of the crowd, the delighted Tegan is watching and barely containing her laughter. Nyssa tries to speak with Tegan, but Tegan's mood changes abruptly from gleeful to terrified and then to angry, and she flees into the crowd. Once again Nyssa loses her, and is forced to return to the TARDIS -- taking with her the dream inhibitor, which she has found abandoned in the fortune teller's booth. Meanwhile, Tegan hides inside Dugdale's booth, where she laughs at the distorted versions of herself she sees in the funhouse mirrors -- until one of the reflections alters itself, and she sees her face replaced by the skull of a snake...

Ambril refuses to see the Doctor at first, but the Doctor pushes past Ambril's hesitant assistant Chela and demands an audience. Trying to convince Ambril of the danger posed by the Mara, he learns, much to his surprise, that tomorrow the people of Manussa celebrate the anniversary of the Mara's destruction. He insists that the ceremony be postponed until he can locate Tegan, and Ambril, humouring him, agrees to do so and invites him once again to leave. The study of the Sumaran era has always attracted more than its share of cranks, and Ambril has no reason to believe that the Doctor is not yet another of them. Chela, however, seems unnerved by the Doctor's claim that the Mara had returned to his companion in the form of a dream, and to Ambril's despair the apparently credulous Chela tells the Doctor what he wants to know about the legend of the Return -- that the Mara will return to gain power over men when minds meet in the Great Mind's Eye. Ambril points out a headdress with five faces, referred to in the legend as the Six Faces of Delusion, and questions the validity of a legend which can't even count properly. The Doctor suggests that Ambril wear the headdress -- and points out that the wearer's face counts as the sixth. Humiliated, Ambril throws the Doctor out of his office, but Chela follows and gives him a small crystal called a Little Mind's Eye, one of those used by the Snakedancers in their ceremonies.

Tegan speaks to the snake skull in the mirror, which replies with a distorted, low-pitched version of her own voice. She fears what it represents but is tired of fighting it, and it tells her to borrow its strength -- and to stop fighting herself. Just who does she think she is? The snake skull disappears, and Tegan finds that she now has a snake tattoo on her arm. Dugdale has been listening to her conversation, and, assuming that she is a ventriloquist, offers her a partnership. But this is of no interest to her, and she turns on the suddenly terrified showman and demands that he bring the one man of importance to her. That person is Lon, who is sulking at home, being ostentatiously bored while his mother attends a terribly dull dinner function hosted by Ambril. Lon is more than a little surprised when the nervous Dugdale enters his rooms and tells him that he is summoned. Someone summons *him*? Intrigued, Lon follows Dugdale to his booth, where Tegan is waiting for him, and when she holds out her arm to him, he takes it, assuming that this is intended to be a romantic rendezvous... but what passes from her to him has nothing to do with romance. Now Lon too has a snake tattoo on his arm, and the terrified Dugdale accompanies them both out of his booth and back to the cave of the snake.

The Doctor and Nyssa have spent the rest of the day in the caves, studying the pictograms and the carved snake in the chamber of the Mara, and the Doctor has come to some unpleasant conclusions. Amongst the pictograms is a depiction of a circle of humanoids surrounding a large blue crystal; lines of energy seem to be coming from the crystal and entering their heads, but the Doctor realizes that in fact it's the other way around. Mental force is being projected from the beings into the crystal, and focussed into a final product... which has been scratched out of the pictogram. The Doctor takes Nyssa back to the TARDIS, where he experiments with the Little Mind's Eye, trying to focus his thoughts upon it. At first nothing happens, until he uses the dream inhibitor to block out all external distractions... at which point a blue light appears in the crystal. The Little Mind's Eye has the power to transform thoughts into energy, and the Great Mind's Eye may be able to do much more -- it may be able to transform thoughts into matter. And the Mara exists, for the moment, only as a thought inside Tegan's head... The Doctor, now understanding the nature of the threat, rushes off to try to convince Ambril once again -- but bursting into his dinner party and insisting that the Mara is back isn't the way to go about it, and he is dragged away by Tanha's bodyguards, still protesting that the ceremony will herald not the destruction of the Mara but its triumphant return...

Tegan sends Lon back to the palace while Dugdale remains with her, terrified into a trance by what he has seen. When Lon returns he finds his mother waiting up for him; she noticed that he wasn't there but didn't raise the alarm, believing he was out having fun by himself while she was stuck at the dreary dinner. He refuses to tell her what he was doing or to answer the questions about the goblet he's brought back with him; instead, he dresses in long gloves to mask the tattoo on his arm and heads off to see Ambril. Chela is speaking with Ambril about the Doctor, who has been locked up in the palace cells and will be kept there until after the ceremony; Ambril dismisses him as a harmless crank and refuses to accept that he has any kind of doctoral qualification whatsoever. He shows Chela Dojjen's journal, and points out the mad scribblings towards the end -- where greed and hatred hold sway, here in the human heart lies the Mara. Lon arrives and asks to speak privately with Ambril, and Chela departs, taking the journal with him. Lon then questions Ambril about the Great Crystal and requests that it be used in the ceremony, but Ambril refuses; such an act is strictly forbidden by his oath of office. Lon claims to understand -- and pulls out the goblet which he claims just to have stumbled across. The delighted and amazed Ambril begs Lon to tell him where he found it, but Lon has a better idea -- he'll show him.

Nyssa manages to get into the palace, avoiding servants and guards, and even finds her way to the Doctor's cell. Unfortunately, without the key, there's not much she can do. Chela arrives with Dojjen's journal for the Doctor, who manages to trick Chela into admitting that Ambril has the key in his office and then takes the journal to read while Nyssa slips out to collect the key. To his surprise, he learns that Dojjen deserted his post to join the Snakedancers -- people who undergo a ritual of purification in preparation for the return. The Snakedance has been banned by the Federation, whose official position is that the Mara has not been banished but destroyed, and who fear the mental powers used by the Snakedances in their ceremonies. Before the Doctor can learn more, however, Nyssa arrives -- accompanied by Tanha and her bodyguard, who caught her attempting to steal the key from Ambril's office...

Lon leads Ambril through the streets of Manussa, and while he pauses to collect luminescent candles to light their way, the impatient director watches a Punch and Judy show in which Punch, after beating his wife, is attacked and devoured by a giant snake. Ambril is surprised when Lon takes him to the caves of the Mara, which have been thoroughly explored -- and where Lon insists upon blindfolding Ambril, telling him that if he agrees to accept the blindfold Lon will allow him to take all the credit for the discovery. Lon leads the blinded Ambril through the caves to the cavern of the Mara, where he uses his tattoo to open the secret panel and lead Ambril through to the cavern beyond. There, Ambril is stunned by the sheer number and variety of artefacts lying abandoned on the ground, a dream come true... but before he can do anything he is confronted by the entranced Dugdale and the coldly evil Tegan, whose skin is reddening as her possession takes hold. Lon demands that Ambril disobey his oath of office and allow the Great Crystal to be placed back in position for the ceremony -- otherwise he will never see these artefacts again. Ambril, torn, finally agrees to do so when Lon begins smashing the artefacts before his eyes. Lon blindfolds him again to lead him out of the caves, and Tegan and Dugdale remain behind. As Dugdale watches in horror, unable to look away, the snake tattoo on Tegan's arm swells up, separates itself from her arm and wraps itself around the chortling Tegan, feeding on the fear and belief in Dugdale's mind...

The Doctor and Nyssa read Dojjen's journal and realizes that it describes his spiritual journey from sceptic to committed believer -- so committed that he left behind everything he had to join the Snakedancers and prepare for the battle to come. It is Nyssa who has the insight into the crystals that provides the missing piece of the puzzle -- the crystals are so perfectly formed that they can only be artificial in origin, and to be as perfect as they are, the Doctor realizes, they must have been created by an advanced people who had mastered the art of molecular engineering in a zero-gravity environment. And yet very little remains of the Manussans' civilisation and technology -- and the Doctor finally understands why. When they created the Great Mind's Eye they failed to realize its full potential, and it drew the fear, hatred, and evil from their minds, amplified it and fed it back to them. And in the reign of terror that followed they forgot that the Mara was something they themselves had called into existence.

Tanha arrives and stops the guards, refusing to believe that these harmless people wish to hurt her son. They are taken back to Ambril's office to explain why they wish to stop the Great Crystal from being used in the ceremony. Chela reminds them that tradition dating back to the destruction of the Mara forbids it, but Tanha sees nothing wrong with indulging her son's whim. When the Doctor learns that Lon suggested it, he realises just why Lon is wearing gloves, but Lon knows that nobody will believe the Doctor's claims. Even when the Doctor demands to know what Lon has done with Tegan and warns him he'll never win, this only serves to convince Tanha that Lon was right all along; these people do wish her son harm. Lon, in his triumph, offers to show the Doctor one glimpse of the Great Crystal -- but as Ambril starts to open the box, the Doctor takes advantage of the distraction to overpower his guard, and he, Nyssa and Chela escape and flee from the palace into the streets outside. The preparations are underway for the festival, and the streets are filled with "attendant demons" who touch people with sticks and demand payment. Only a few hours remain until the ceremony, and with no way to find Tegan in that time, the Doctor decides on an alternate plan -- he will use his Little Mind's Eye to contact Dojjen and ask for help.

Back in the palace, Tanha overhears Ambril and Lon discussing their arrangements for after the ceremony and is puzzled. She presses Lon with questions, asking him what's really happening and why he is wearing gloves, but he claims that he scratched himself and angrily rebuffs her when she presses for an answer. Hurt and confused, she does not enthuse when Ambril arrives with the traditional costume for Lon, the sky cloak his ancestor wore when the Mara was destroyed -- but when Lon arrives clad in the traditional garments she is so delighted by his appearance that all is forgiven. Tanha, Ambril and Lon proceed through the streets outside to the cave, as the mass of celebrators cheer them on. Elsewhere, the plastic snake and the glass "crystal" in its mouth are paraded through the streets as a man with a megaphone challenges the crowd; who will be the first to look into the Mara's gaze and save them all?

The Doctor, Nyssa and Chela find an isolated spot on the edge of the city, where the Doctor sends a call for help into the Little Mind's Eye; hopefully this will resonate in the other crystals worn by the Snakedancers and Dojjen will answer. Perhaps then they will learn why he did not destroy the Great Crystal when it was in his care. Dojjen arrives, but to learn the answers he needs the Doctor must allow himself to be bitten by a poisonous snake, as Dojjen has done. Despite Nyssa's protests the Doctor allows himself to be bitten, but his fear nearly costs him his life. He hears Dojjen's voice in his mind, soothing, seeing him through the crisis, urging him to be calm and to find the still point within himself. Only then will he be able to defeat the Mara. The Doctor recovers, realizing that he has survived, and he, Nyssa and Chela rush off to stop the ceremony, hoping that they will be in time.

The pantomime Mara is carried into the cave of the Great Crystal, where Tanha watches proudly as her son steps forward to take his place in the ceremony. He is playing the part of his ancestor, facing down the temptations of the Mara and rejecting them. He rejects fear in a handful of dust, and despair in a withered branch; while he lives his hand is clean, and he knows the tree will flourish and live again one day. But when the time comes for him to gaze into the hidden depths of the Great Crystal, the ceremony goes horrifically wrong. To everyone's horror, Lon snatches the crystal from the mouth of the Mara and dashes it to the floor, smashing it to pieces. It's fake, he claims, a trinket created by a soft civilisation which has forgotten the truth of the Mara. He strips off his gloves to reveal the mark of the snake on his arm, and the horrified Ambril and Tanha are too confused to stop him from removing the real Great Crystal from its box and marching up to the snake carving on the wall of the cavern. He uses the mark of the snake on his arm to open the secret panel, and Tegan and Dugdale emerge. All this while, the Mara has been feeding on the terror in Dugdale's mind, and has grown, wrapping itself about Tegan's arm. And now everyone assembled for the celebration can see it for what it is. After five centuries the Mara has returned.