It is night in the Himalayas. A man sleeps in his tent until he hears a scream of terror from outside. He calls to his companion, but the man is not there and his bunk is empty. Realising that the scream belonged to his friend, the man grabs his rifle and goes outside. The screams increase and he goes toward them in the dark. He finds his friend dead, his body crushed. Beside the body is a creature, something large and covered in fur. It grabs his rifle and breaks it in two, then knocks the man unconscious.

The TARDIS materialises and the Doctor checks the scanner quickly. He is overjoyed when he sees where they are - the Himalayas. After all this time! He quickly checks the time indicator to find out if they have landed in the proper period. Jamie thinks it looks just like Telos, but Victoria patiently explains that the Himalayas are mountains on Earth. The Doctor finishes checking the date and is very excited. They are in Tibet.

Suddenly, he dives into piles and trunks full of junk. He says he is looking for "the ghanta", exhorting their help but not answering when they ask what it is. The Doctor and Jamie both find useless junk before Victoria finally gets through to the Doctor that they don't know what a ghanta is. He explains that it is a Tibetan bell, a holy relic decorated with a dragon and that it has an "interesting history". However, the Doctor's search is curtailed when he finds an enormous fur coat which will be perfect for the cold outside. He decides on a scout round outside while the other two carry on searching. The Doctor tells them that the ghanta will guarantee them the welcome of a lifetime "down there". And then he is gone.

Jamie sets to work to find the ghanta, but Victoria is curious to know where the Doctor has gone and adjusts the scanner control to follow his movements. Jamie finds a sword which he intends to keep. Turning to the scanner, he sees what he thinks is "a great hairy beastie"...which turns out to be the Doctor in his enormous coat. The Doctor moves off down the mountainside.

Victoria is feeling left out as she watches the Doctor explore. She moves the scanner again and sees a group of buildings down below them - a monastery. She rejoins the search, soon finding an object wrapped in cloth with the words "Det-sen monastery" on it. It is the ghanta.

The Doctor comes across a giant footprint on the mountainside and is quite concerned. He heads back to the ship.

Victoria has found warm clothing to wear outside and tells Jamie that he should do the same, but he says he is unaffected by the cold. The Doctor returns, and is distractedly happy when Victoria shows him the ghanta. The Doctor reveals that the monastery is down below them on the mountain but says that only he is going and that they must stay here. Both Jamie and Victoria realize something is wrong, but the Doctor insists everything is fine...even when he leaves again, almost forgetting the bell!

Some time later, Victoria is becoming bored and wants to go out. Jamie is sure they should stay put, but when she determines to go, he knows he cannot let her go alone.

The Doctor comes across an abandoned campsite on his way down the mountain. Near the tent, the Doctor finds the man's dead body with more of the giant footprints. He picks up a rucksack lying near the body and takes it with him to the monastery.

Victoria marvels at the view, but Jamie is too cold to enjoy it. She locates the giant footprints the Doctor saw and is intrigued. It looks to her like some wild animal has been walking all round the TARDIS. She goads Jamie into following the footprints, which lead further up the mountain. Jamie grabs his new sword and they are off.

The Doctor, meanwhile, has arrived at the door of the monastery. He knocks and shouts, but there is no answer. The door is open and he enters. Inside is a large, silent, and nearly empty room. He wanders in and calls out again, watching a large statue of the Buddha. Behind him, the front door closes and a group of monks appear, some armed and one in particular looking quite threatening.

The lead monk, Khrisong, begins to question the Doctor, but he is interrupted by an Englishman who accuses the Doctor of murder! The man, Travers, is the same one whose companion was killed up on the mountain earlier and he now accuses the Doctor of that murder. Khrisong points out that he swore it was an animal who killed his friend, but Travers is adamant that he saw only a glimpse of the murderer and that the Doctor in his giant fur coat could look like an animal in the dark. He also carries the dead man's rucksack. This is enough for Khrisong, an impetuous warrior, and he has the Doctor imprisoned to await a trial. Travers may have accused him of one murder, but many others have been killed. If the Doctor is guilty, he will be punished. The Doctor is lead off, protesting all the way. He is locked into a small chamber with only a straw pallet for furniture. This is not the welcome he had expected.

Up on the mountain, Jamie and Victoria have followed the footprints upward and have found a cave entrance. Jamie initially wants to go back, for fear of the arrival of the creatures, but Victoria's curiosity will not let her turn back. However, when Jamie sees manmade wooden supports inside the cave, he forges inside. Soon, Victoria is the one begging to go back, but Jamie presses on. The wooden beams and pillars which support the roof are definitely man made, however something definitely not human approaches the cave mouth from the outside and seals it over with a boulder.

In his cell, the Doctor tries to open the sealed shutters on the window, looking for escape, but there is a 100-foot drop outside. Travers opens the window in the cell door and smirkingly tells the Doctor that there is no escape.

Travers believes the Doctor to be a reporter trying to "steal his discovery" from him. For 20 years he has been searching these mountains trying to find the "Abominable Snowmen" - the Yeti - and always the reporters and the scientists have laughed at him. Now that he is getting closer, rivals are trying to beat him to it. He assures the Doctor that he will not succeed.

Jamie and Victoria cannot shift the boulder. They are well and truly trapped. Jamie decides to investigate further into the cave.

Meanwhile, the Doctor presses Travers, suggesting that the Yeti could have killed his companion, but Travers assures him that the Yeti are timid creatures. He repeats his claim that the Doctor is a reporter out to take his glory away, stopping at nothing, not even murder. The Doctor calmly explains that his friend was killed by someone much more powerful than himself, but Travers simply will not listen. His only concern is for his expedition. The rest is up to Khrisong. He leaves the Doctor to cool his heels.

Khrisong is indeed trying to find out the truth, as he consults with fellow monks, non-warriors Rinchen and Sapan. For Khrisong, the word of Travers is enough, but the other two are sceptical. Khrisong presses on, noting that four monks have also been killed and all the monks believed it was the Yeti. But they have always known the Yeti to be timid, elusive creatures. Why did they suddenly turn savage? If they didn't, the Doctor could be responsible for the murders. If they did, the Doctor may be the cause that turned them savage. Khrisong wants to put him to the test now, but the other monks are hesitant. The trial Khrisong plans will condemn the Doctor to certain death.

Khrisong argues that he is the chief warrior, charged with protecting the monastery and its inhabitants. The only way he can do that is to administer the test, but Sapan reasons that they do not have the right to condemn a man to death, especially with so little evidence. Only the Abbot Songsten can do such a thing. A gong rings out gently, calling the monks to prayer, and for Sapan and Rinchen, this is the end of the matter. They will wait until after meditation to consult the Abbot. They head off for prayer, leaving Khrisong to stew. He is furious, knowing that he must act now, before others die. He orders a guard to bring the prisoner to him.

Jamie runs back into the cave, warning Victoria to keep back as the unstoppable Yeti approaches. He disturbs the wooden support holding up the ceiling and it gives a little under his weight. He grabs a nearby piece of wood and heaves it at the support. It gives way completely and a large section of the cave roof falls in on top of the creature, burying it. They are safe.

Jamie heads again to the back of the cave to grab one of the glowing spheres to show the Doctor. He and Victoria head out past the pile of the debris. However, just as they get past it, the Yeti comes to life again, pulling itself easily out from under the rocks. To their horror, it is soon after them again.

In his cell, the Doctor bides his time and tootles on his recorder. A young monk called Thonmi comes to fetch him. However, the Doctor is not interested in following just yet. He wants some information. Thonmi says the monastery is besieged. The Yeti, once friends of the monks, have turned on them and begun killing them. No one knows why, but they now think that the Doctor is responsible. The Doctor comments that the monastery always seems to be in some kind of trouble when he visits, piquing Thonmi's curiosity. The Doctor asks about the outcome of the attack on the monastery in 1630, the last time he was there. Thonmi explains that this too was a dark period for them, for it was then that the holy ghanta was taken from them. The Doctor wants to reveal that he has brought back the ghanta, but they are interrupted when Khrisong bursts into the cell, angry that his orders were not carried out. The Doctor is unceremoniously hauled out of the cell, but he manages to indicate to Thonmi that he should check the straw pallet and take what he finds to the Abbot.

The young monk is slow to leave the cell and he is soon alone. He searches the pallet and to his astonishment finds the bell. Realising what he holds in his hands, he quickly kneels in homage.

Jamie and Victoria continue down the mountain, pursued by the Yeti. Victoria is confused when Jamie heads in the opposite direction from the TARDIS, toward the monastery. He wishes to warn the Doctor about the creature. Victoria has little time to argue as the Yeti bears down on them, so she follows him.

The Doctor is taken to the main hall of the monastery in preparation for his trial. Rinchen and Sapan attempt to stop Khrisong, but he will not wait for the Abbot. Travers passes at this point, heading back out to search for the Yeti, and the Doctor appeals to him. He knows that Travers now has doubts of the Doctor's guilt, but the single-minded man is only interested in finding the Yeti and will do nothing to help.

Khrisong reveals his plan: The Doctor will be tied to the monastery gate, used as bait for the Yeti. If they come to rescue him, the monks will know he is their controller. If not, hopefully Khrisong's men can rescue him before the creatures kill him. Khrisong cuts short the protests of the other monks and ushers the Doctor outside.

Deep in the heart of the monastery, Thonmi carries the holy ghanta, looking for the Abbot Songsten. He finds him coming out of the inner sanctum and prostrates himself, apologizing for entering the sacred area. The Abbot bids him to rise and looks at what he has brought. Neither of them are sure it is their holy ghanta but this is confirmed by a disembodied voice which echoes through the halls. It is the unseen lama, Padmasambhava, whose sibilant voice surrounds them from the inner sanctum. Both men drop to their knees at the sound. Thonmi, frightened and hesitant, explains about the stranger who brought the bell back. The lama knows it is the Doctor and seems to have anticipated his return. He bids the two to enter the sanctum.

At the monastery gate, the Doctor has been securely bound with his hands above his head. He argues that no one will rescue him and that this trial will not determine his guilt. His protests go unheeded.

Jamie and Victoria slow their descent, seeing that the Yeti is no longer following them. Stopping to catch their breath, they hear something approaching and hide. Travers spots them and keeps his rifle levelled at them. When they tell him they are heading for the monastery, he realizes that they are friends of the Doctor. He purposely neglects to tell them of the Doctor's trial.

Jamie and Victoria warn him of the "great hairy beastie" they ran into in the cave. Travers reacts with shock and surprise and orders them to show him where the Yeti's cave is, but Jamie refuses to take Victoria back there. He tries to force them at gunpoint, but the two bravely refuse, claiming ignorance of the Yeti. Travers begins to believe that he has indeed misjudged the Doctor and his friends, but he still wants to find his Yeti. Jamie says he'll show Travers where the cave is... after he has shown them the way to the monastery. The three set off down the mountain.

In the sanctum, Thonmi and Songsten kneel and listen to the voice of Padmasambhava as if in a trance. The lama says that the Doctor is a friend to the monastery and that he should be treated with respect and kindness. He orders the young monk to go to Khrisong and tell him to release the Doctor. He stresses that Thonmi will only remember speaking to the Abbot and not hearing the lama's voice. The young monk goes off.

Padmasambhava's voice changes and takes on a more menacing characteristic as he speaks to Songsten alone. He says that the Doctor is a man of great intellect but that he must learn nothing of "the powers that guide us". He must not be allowed to imperil the Great Plan. The voice returns to normal and suggests that the Doctor departs as soon as possible.

At the gate, the Doctor hangs helplessly. Shortly, though, the lookout spots something approaching. Khrisong is sure he has his proof, but it is only Travers, Jamie, and Victoria who appear. Khrisong and his men surround the newcomers. Travers tells Khrisong that his suspicions about the Doctor were all a mistake, but the warrior is not about to be dissuaded so easily.

However, Thonmi rushes out and delivers the words of "the Abbot", that the Doctor should be treated with kindness. He tells Khrisong of the holy ghanta. The Doctor confirms that he brought the bell back but says he had not mentioned it before because he had been accused of murder. Khrisong was not in the right frame of mind to listen to his story. As he is freed, the Doctor also takes the time to chastise Travers for jumping to conclusions so quickly.

Back in the inner sanctum, Songsten receives his final instructions. Nothing must hinder "the preparations". The Doctor has been a good friend in the past, and hopes that he will leave the monastery peacefully. The lama says that it would be "ill" if his life would become imperilled...

The Doctor examines the sphere Jamie found in the cave while Jamie describes the creature that attacked him. The Doctor is sure it is the abominable snowman, but Travers is adamant that the Yeti are shy, retiring creatures who wouldn't attack people. The Doctor puts the sphere aside, left with more questions than answers, and he wishes he could see one of these creatures up close.

Suddenly, a monk on watch calls out for Khrisong. Three Yeti stand on the hill just above the monastery. Khrisong mobilizes his warriors to protect the inhabitants. Travers is stunned, having finally achieved his 20-year goal, but the Doctor is sceptical. Jamie is pressed into service as a defender and gets an idea on how to capture one for the Doctor.

In another part of the monastery, Rinchen and Sapan talk to the Abbot about the situation. Songsten believes they will be safe now that the ghanta is back in their possession, but Rinchen thinks that Khrisong'srashness will be their undoing. He was so certain that the Doctor was a threat and that has been proven wrong.

Outside, Jamie and the warriors set up a trap for the Yeti. When one of the creatures attacks, several men beat it with their staffs. They manoeuvre the creature over the net and Jamie calls for the monks up on the wall to pull on their ropes. The net closes and the creature is hauled up. However, it has stopped moving altogether, seeming almost dead. The other two do not attack. In fact, they turn and leave. As the men work to get their captive inside, they do not see another of the strange metallic spheres embedded in the soft ground nearby. As they go in, the sphere comes to life, beeping mechanically to itself and moving slowly along the ground.

Inside, the sphere that Jamie brought in sits silently at the feet of the Buddha statue.

Later that evening, the Doctor, Travers, Khrisong, and all the others are gathered around the body of the creature, examining it. It is made of metal, not flesh and blood, and it isn't Travers' abominable snowman. The Doctor believes it is a robot, but cannot figure out why it has stopped functioning. As he examines it, he finds a semi-circular hollow in its chest, covered by a fur flap. The hollow is empty, but something has been there.

The sphere at the feet of the Buddha continues to beep and move, moving purposefully off of the statue's base and into the monastery.

The Doctor realizes that the robot Yeti's control unit must have been in the hollow in its chest and that the struggle outside must have dislodged it. He and Jamie wish to go outside to find it, but Khrisong stops them and says that no one will leave the monastery. Someone has created and controlled these false Yetis and sent them after the monks of Det-sen. He will trust no one until he knows who and why. He puts Thonmi on guard over them all. Uncharacteristically, Travers appears to accept this and give up, heading to his room to rest for the night.

At the main entrance, Khrisong puts a monk called Ralpachan on guard at the doors. Travers, who has not gone to his room at all, arrives to talk to Khrisong alone. He pleads to be allowed out to continue his search for the real Yeti. Even the possibility that the robots may kill him will not dissuade the obstinate scientist. He even fears that the real Yeti may be killed by the robots, but Khrisong is resolute.

The Doctor and Jamie try to reason with Thonmi, but he will not disobey Khrisong, even though he himself trusts them. Victoria points out that the hollow in the robot's chest is round, and wonders if the sphere Jamie found would fit inside. The Doctor is sceptical as the sphere was so light - nothing was inside it - but he is still curious. He looks around for the sphere but it is not there. They all remember that it was left at the feet of the Buddha statue and go to the entryway to retrieve it. However, when they reach the statue, the sphere is gone. Someone must have picked it up. They go to talk to Ralpachan, inadvertently passing the sphere, which has hidden itself in the shadows as they pass. Then it begins its journey again, heading the way the Doctor's group had come.

Ralpachan has seen no one and nothing. Only Khrisong and Travers have passed through the entryway. Travers only spoke to Khrisong and then went outside. He has tricked Ralpachan into letting him leave the monastery, and he must have taken the sphere with him. Could Travers be the one controlling the Yeti? Jamie heads to Travers' room to search for the sphere, while the others go to find Khrisong and tell him what has happened.

Meanwhile, the sphere moves slowly down a corridor.

Sapan and Rinchen have also been busy, constructing a framework of wood and coloured threads around the Yeti robot where it lays. It is a "ghost trap" to keep the evil spirits from reanimating the creature. Sapan suggests the addition of chains as a more practical method to subdue it, but Rinchen is not convinced. This creature is not metal, but simply wearing armour beneath its skin, as the devil would "to protect his evil heart". Khrisong and Songsten arrive and see the ghost trap, the Abbot praising the two men for their wise precaution. But as the other two leave, Rinchen takes the opportunity to criticize Khrisong for allowing the creature inside in the first place.

Khrisong laments to the Abbot his difficult task. It is his job as a warrior to protect the monastery and its monks, but his adversary is formidable. He must be allowed to use force to defeat the mechanical creatures. Songsten disagrees, saying they are men of peace and only peaceful means will prevail. He sits down on the floor and enters a trance to consult with the master, Padmasambhava.

Immediately, the Doctor bursts in, telling Khrisong that Travers has gotten out of the monastery. Khrisong heads for the entryway with the Doctor fast behind him. Thonmi and Victoria stay behind, watching in fascination as the Abbot remains in his trance. Shortly, the Abbot responds to an unheard voice and gets up to leave. Thonmi tells Victoria he is going to the holy sanctum, to where Padmasambhava is. Victoria wants to follow and Thonmi is unable to stop her. Khrisong is furious at Ralpachan for being duped. Jamie returns, having found no trace of the sphere in Travers' room. The Doctor turns his attention again to the sphere that must be outside. He must be allowed to go out and get it. Khrisong forbids this, but after consideration, decides to go himself.

Elsewhere, a table has been set up with a map of the area. Det-sen is in the centre of the map. A wizened hand with long fingernails picks up small models of two Yeti, moving them closer to the location of the monastery. Out on the mountain, two lifeless Yeti activate and begin moving toward the monastery, watched from hiding by Travers.

The Abbot arrives at the inner sanctum, still deep in his trance, and enters. As the doors close behind him, Victoria arrives with Thonmi right behind her. She starts to open the doors, but a horrified Thonmi stops her. He will not allow her to go into the forbidden area. She agrees for now, but assures him that she will get in there eventually.

Outside, Khrisong searches for the sphere, watched anxiously by the Doctor and Jamie.

Songsten has told his master the whole story and Padmasambhava chastises Songsten for allowing the Yeti robot inside. The Abbot suggests that the Doctor might assist them with the Great Plan, but Padmasambhava believes he will likely oppose their work. He decides he must ensure that their Plan is no longer imperilled, and uses his wizened hand to move the two Yeti models right to the gates of the monastery.

Outside, Khrisong has found the sphere embedded in the ground. Immediately, it begins signalling. The Doctor shouts out a warning as two Yeti attack Khrisong. They wrench the sphere from his hands but do not kill Khrisong. Jamie and the monks burst out and engage the creatures, succeeding in forcing them off. However, the victory seems a bit too easy, and the Doctor surmises they simply came to "get their ball back", and nothing more.

Padmasambhava is pleased with the Yeti's success. He points to three more Yeti models and tells Songsten that they are waiting for him. He is to take the glowing pyramid that has been prepared and take it to the cave. Then, he says, the Great Intelligence will focus on this planet. It will begin to grow and take on physical form. Its wanderings in space will finally be ended, and Padmasambhava's work will be done.

As Khrisong nurses his wounds, he too reflects on how easily the "battle" was won. The Doctor assures him that the creatures only wanted the sphere. They are like brains for the robot Yeti and someone or something doesn't want him examining one. Jamie reasons that the Yeti they captured had come to find the sphere he'd stolen from the cave. The Doctor agrees, wondering what has happened to that one.

At that moment, that sphere is continuing along the corridor, seeming to have an overriding destination in mind.

The Doctor asks Khrisong if he may leave the monastery. He believes that the beeping of the spheres is a signal to each other and that he can track it with equipment in the TARDIS to find out where they are getting their instructions. Khrisong immediately agrees. He knows that his warriors are powerless and he feels that he has failed. He must trust the Doctor.

Victoria and Thonmi have returned to the room where the Yeti rests. She is trying hard to rid herself of her "escort" and almost gets her wish when Sapan and Rinchen send him to Khrisong in the entryway. Thonmi puts Victoria into Sapan's care before he goes, much to Victoria's dismay. However, after giving her misinformation about the Doctor and Jamie, both Rinchen and Sapan leave her on her own as they have duties to perform. She heads directly for the inner sanctum.

Khrisong hopes to destroy the robot Yeti in the monastery, but he has no time to outline his plan. Rinchen arrives, leading an angry Victoria. He caught her near the sanctum, and he returns her to Thonmi's guard.

Up on the mountain, the Doctor and Jamie pick their way in the dark toward the TARDIS.

Songsten, on the orders of Padmasambhava, uses the power of his mind to hypnotize Ralpachan and leave with the pyramid. Once he is gone, Ralpachan awakes with no memory of what he has done.

Victoria is tired of being guarded, but Thonmi assures her they are just concerned for her safety. She will not be locked up, but she must promise to behave. Thonmi tells her the Doctor and Jamie are gone, believing she already knew, and she is furious. She settles down into a funk, at first refusing Thonmi's offer of food. But, getting an idea, she sends the trusting monk off to fetch some. Victoria wastes not a second in leaving the room as soon as he is gone.

Jamie and the Doctor, meanwhile, have come across three inactive Yeti. The Doctor determines that they are switched off and wonders what they are waiting for, but he decides on caution and cuts short his investigation to head for the TARDIS. Songsten arrives right behind them. Immediately, the Yeti come to life and fall into protective formation around him, leading him upward toward the cave.

Thonmi returns with Victoria's food and finds that she is gone. He determines that she has not gone outside and so picks up search inside.

Victoria has made it to the inner sanctum, starting to feel more scared than brave. The hypnotic voice of Padmasambhava comes to her from inside, warning her away. In a daze, she complies and returns to the room where the Yeti rests.

The control sphere has already arrived there. It levitates up to the table, heading for its socket in the creature's chest. Victoria arrives just as it reaches the socket and settles in.

Thonmi attacks the creature, trying to hold it back, while Victoria runs for help. She finds Sapan in the corridor and the two of them search for Khrisong and his warriors. The robot is too strong for Thonmi. It brushes him aside and moves off down the corridor.

Victoria and Sapan reach the entryway but only have a moment to warn Khrisong and his warriors before the Yeti arrives. The warriors engage it, trying to destroy it. Victoria, seeing the hopelessness of the situation and fearing for the lives of the monks, urges Thonmi to open the doors and let the creature escape. Seeing the men losing their battle, Thonmi finally does so, against Khrisong's orders. The doors open, the Yeti ceases fighting with the warriors and heads outside and up the mountain.

The Doctor stops on his way to the TARDIS. He confides to Jamie that he is very worried about what is happening here. Even though they haven't come across any Yetis yet, he knows it will happen. The Doctor's fear disturbs Jamie, but both try to shake off their foreboding and continue.

Further up the mountain, Travers continues his search for the real Yeti. He has found the entrance to the cave, but has not entered. He crouches nearby behind a bush, waiting. Soon two Yeti robots arrive, carrying the control sphere stolen from Khrisong earlier. They enter the cave and deposit the sphere, then exit and disappear back down the mountain. Travers watches as they go.

The Doctor and Jamie finally reach the TARDIS, only to find a Yeti standing guard outside it. They take cover behind some rocks and try to decide what to do. Jamie wonders what the creature is waiting for and how it could have known they were heading for the TARDIS.

In the inner sanctum, Padmasambhava communes with the Great Intelligence. He wonders if the "great experiment" will be over soon and he can rest. He gets a response that only he can hear and seems pleased. He tells his master that Songsten is making the final preparations now.

At that moment, the Abbot has reached the cave with his Yeti escort. Travers watches as they all enter cave, the Abbot carrying the glowing pyramid. When they all emerge a moment later, the pyramid is gone, left inside. The group head back to the monastery, leaving Travers alone.

Lacking any cleverer plan, the Doctor tosses a rock at the Yeti guarding the TARDIS. When the robot does not react, he knows it is inactive, like the others they saw earlier. Even more boldly, the Doctor reaches inside the chest unit to remove the control sphere. Jamie holds up the fur flap and the Doctor prizes out the sphere with a screwdriver. Quickly and easily, the job is done. Perhaps too easily. Jamie wonders why there wasn't some sort of protective device to keep them from removing the control sphere, but the Doctor doesn't have an answer. He leaves the sphere with Jamie and goes into the ship to retrieve his tracking equipment.

At the cave, Travers finally decides to go inside and explore. Inside, he is drawn to the back of the cave by sounds and lights. He finds a collection of spheres there, arranged into a complex pattern, with the pyramid at its centre. As he watches, the humming and the glow increase and the pyramid seems no longer transparent. Something is inside! He watches, horrified, as the pyramid cracks open and a viscous, cobwebby substance oozes out. It seems directed, heading straight for him. He screams in terror and runs out of the cave.

The Doctor emerges from the TARDIS with a small tracking device, but just as he does so, the sphere in Jamie's hand comes to life, signalling loudly. It beings moving of its own accord, trying to leap out of Jamie's hands and back into the Yeti. Jamie fights but cannot hold it. The Doctor blocks its path with his body and continues the struggle. He instructs Jamie to wedge a rock into the hole in the robot's chest unit. Jamie does so and the sphere becomes dormant again. The Doctor realises that the spheres are programmed to return to their Yeti's if removed and also realises that the sphere in the monastery wasn't taken; it moved on its own and the robot there could already have come back to life. They hurry down the mountain back toward the monastery.

At the monastery, calm has returned after the Yeti's escape, but Khrisong's anger has risen again. He is furious at Thonmi and Victoria for opening the doors. He begins to wonder if Victoria was responsible for to bring the creature back to life, as she was the only one in the room when it did so and can only offer no excuse as to why she was there. Even Rinchen starts to believe. Thonmi tries to defend Victoria, only to be accused of plotting with her. Both are hauled away to be locked up.

As Jamie and the Doctor head down the mountain, the sphere begins again to signal, but this is a different sort of signal. The Doctor activates his tracker.

Further down the mountain, the Yeti escorting Songsten stop and turn back, heading up the mountain. The Abbot continues downward on his own.

The Doctor is excited as his tracker has picked up the other signals, but Jamie fears that means more Yeti on the way. Soon enough, there are three below them and two above. They are trapped.

Thonmi and Victoria pass the time chatting. Thonmi wonders about the Doctor and how he came to have the holy ghanta in his possession. It was lost 300 years ago when it was given to a stranger for safe keeping. Victoria is sure that this stranger was the Doctor, and she tries to explain that the Doctor can travel in time and space. Thonmi understands completely, saying that the master of the monastery, Padmasambhava, can do the same through astral travel.

Khrisong cannot locate Songsten. At the main entryway, Sapan and Rinchen suggest that he is consulting with the master, but Khrisong figures he must simply be hiding from his responsibilities. He tells Ralpachan to let him know when the Doctor returns and stalks off, leaving Sapan and Rinchen to comment on his rash words and actions. Shortly after they depart, there is a knock on the door and Ralpachan admits Songsten. The Abbot hypnotises him again and then he disappears into the monastery. The guard wakes up as he leaves.

Songsten goes immediately to the inner sanctum and reports to Padmasambhava. The lama is pleased, reporting that the Great Intelligence has already begun to take on physical form. But that form must expand and grow. For that reason, all of the monks must leave the monastery. There is some sadness in the master's voice over this.

Out on the mountain, the Doctor devises a means of escape. The Doctor herds Jamie closer to the oncoming Yeti and tells him to make ready to run. They advance and close the gap. At the last second, the Doctor rolls the sphere the other robots and he and Jamie run around the advancing Yeti. As he expected, the robots ignore them and head for the sphere.

Victoria suggests contacting Padmasambhava to plead their case, but Thonmi rejects this. Ralpachan arrives with food and water. Victoria takes one drink of the water, complains of a funny taste, and collapses with a scream. Horrified, Ralpachan runs off to find help while Thonmi tends to her. As soon as Ralpachan is out of sight, she is up and heading for the door. She has tricked him. She apologizes, but does not stop.

Elsewhere in the monastery, Songsten tells the monks that they must all leave as Padmasambhava has commanded. Khrisong is furious, sure that he can stop the Yeti, but Songsten is adamant that nothing can be done. A guard informs Khrisong that the Doctor has arrived with the needed equipment. He vows to fight, then heads off to meet the Doctor. Songsten tells the rest of the monks that Khrisong has forgotten his vow of obedience, but he will eventually comply. Ralpachan reports Victoria's escape. The monks scatter in order to track her down.

At the entryway, Khrisong fills the Doctor in on events at the monastery. He trusts that the Doctor will find a way to defeat the Yeti, but they must work quickly or the Abbot will force them to leave. At that moment, Travers pounds on the doors and is let in. He is raving about a pyramid and a cave, very frightened, but he makes no sense before he passes out.

Rinchen reports Victoria's escape and orders all the strangers arrested. Khrisong pleads for them as only they can help Det-sen against the Yeti. But Songsten lends his weight to Rinchen's orders and the Doctor, Jamie, and Travers are taken away.

Khrisong continues to argue with Songsten, saying that he cannot give up and retreat, but the Abbot orders him to obey and sends him to find Victoria. Sapan is sent with him. The others, too, continue the search, leaving the Abbot in the entryway. Songsten communes with Padmasambhava, who decides that if the monks will not be lead from the monastery, they must be driven from it. He opens wide the doors of the monastery, leaving no defence against the Yeti.

The master identifies himself as Padmasambhava and is quick to offer Victoria the help she seeks. But before she can ask her favor, he stops her, his manor changing abruptly. In his mind's eye, he can see the entryway. It is empty and the door is wide open, now he must do what he is compelled to do. He reaches over to the model Yeti. Victoria is horrified, realizing that he is the one who controls the creatures, but he puts her into a trance and wipes her mind. Then he places three Yeti models right inside the monastery.

Out on the mountain, the Yeti obey his instructions and head for the monastery.

In their locked room, Jamie and Thonmi worry about Victoria, but the Doctor is more interested in Travers, who appears to be waking from his sleep. The Doctor questions him about where he went and what he saw that frightened him so. As he remembers, he becomes more agitated, remembering only a bright light and a piercing sound.

Thonmi asks about he pyramid he mentioned earlier, but Travers cannot remember. His mind is now a blank, except for a feeling of evil and a "shadow on his mind." He is traumatised and cannot remember any more and the Doctor allows him to fall back into sleep. Suddenly a deep rumbling shakes the monastery. The Doctor reaches for his tracking device, knowing the answer before the screams of "Yeti!" race through the monastery.

Khrisong and Rinchen burst in and tell them that the Yeti have broken in to the monastery. Khrisong leaves Rinchen with the others and goes off to try and fight them. Jamie is concerned about the absent Victoria. Rinchen, still believing that she is responsible for controlling the Yeti, leaves his charges and goes in search of her.

Khrisong finds Songsten and the rest of the monks gathered in a hall, and he is abject in his failure to defend them. But Songsten is stoic, saying that this was destined to happen and that no one could stop it. Khrisong pledges obedience and his life for those of his brothers. Shortly, Sapan arrives, concerned for Rinchen's safety as he has gone off alone into the monastery to try and stop Victoria. Ralpachan enters, saying that the Yeti appear to have turned back. He and Khrisong go to see this for themselves.

In the entryway, Rinchen calls out at the top of his voice for Victoria, the "she-devil", to call off the creatures. However, one of the Yeti finds him and kills him where he stands. Khrisong and Ralpachan follow the screams and find him dead.

The Yeti leave the monastery and head back up the mountain. In the inner sanctum, Padmasambhava is pleased with the fear he has placed in the hearts of the monks. By nightfall the monastery will be empty. The lama wonders if this will be the end, but he will continue to serve nonetheless. His demeanour changes abruptly and he realises that the Doctor will not be frightened easily and so other means must be used to make sure he leaves quickly. Victoria will be the means to this end.

Thonmi has gotten a map of the area and the Doctor studies it. He needs a second bearing to triangulate where the signals are originating. This will be easy, but the question of why the Yeti are being controlled will still be unanswered..

Travers awakes once again, this time much calmer than before. But now his memory is even blanker than before. He can't even remember why they are locked in the room.

The death of Rinchen has hit the brothers hard, especially Khrisong, who blames himself. Defeated, he will allow the monks to leave Det-sen after meditations as the Abbot has commanded. The strangers will accompany them, especially Victoria as soon as she is found. The Abbot says that the master, Padmasambhava, will stay, as "his powers are great".

Before Khrisong can leave to continue the search, Victoria arrives, bearing the holy ghanta. It is clear that she has been in the sanctum. She is in a trance, and when she speaks, it is in the voice of Padmasambhava. He tells his monks that Victoria and the other strangers are innocent of any malice and they only want to help, but there is no help against the Yeti and the monastery must be abandoned. The strangers and the ghanta must be borne away to safety.

Songsten orders all the strangers released and for the last preparations for departure made. Then a gong sounds and it is time for meditation, the last ever at Det-sen.

Ralpachan releases the Doctor, Jamie, and Travers, telling them that they must get ready to leave at once. Travers heads off to speak to the Abbot. Khrisong brings Victoria to them, and Jamie and the Doctor are overjoyed. When Victoria hears the Doctor's voice, she becomes hysterical, screaming that there is great danger and that he must take her away. Then she falls silent. When the Doctor speaks again, she repeats her hysterical assertion word-for-word.

Khrisong reasons that she is still in a trance, and relates to the Doctor that she had earlier spoken in the voice of the master. Thonmi explains that she was sure Padmasambhava would recognize the Doctor as he had been here before. The Doctor thinks this is impossible as that was over 300 years ago, but wonders what it would mean if the lama was the same man.

Khrisong and Thonmi, unconcerned by this, head off to join the meditations. The Doctor tells Jamie (in a whisper) that he is going to visit "an old friend" in order to gain some more information. He leaves Victoria in his care.

In the inner sanctum, Padmasambhava communes with the Great Intelligence He feels that he has done all the Intelligence has commanded, clearing the monastery so the Intelligence can expand. Why, then, has it not let him rest? The answer, heard only by Padmasambhava, chills him. Has he in his obedience to the Intelligence brought about the end of the world?

In the cave on the mountain, the oozing, weblike mass which is the physical form of the Great Intelligence continues to expand.

The Doctor enters the sanctum and immediately recognizes the wizened face of his old friend, still alive after all these years. The Doctor demands to know why this has been done. Padmasambhava says that he was astral travelling long ago when he made contact with a formless Intelligence in space. It used his mind and controlled his body to pursue an "experiment". He says he did not know what the experiment was, and now he begs the Doctor for help. There is great evil behind this Intelligence.

The Doctor peppers him with questions, but the master's body has gone limp, as if dead. The Doctor leaves sadly. Then the body of Padmasambhava comes suddenly back to life, the evil light of the Great Intelligence behind his eyes.

The Doctor returns to Jamie and Victoria. Victoria is still entranced. She reacts only to the Doctor's voice, repeating her hysterical demand to be taken away from here. The Doctor says he needs more information to help Victoria properly, but he has no way of getting it. With nothing else for it, he attempts to break Victoria's trance.

He tells her that she is no longer in the monastery but in the TARDIS, safe and away. He succeeds in putting her to sleep...and Jamie too! He is tempted to ask her for information, but he is afraid that if he doesn't remove this trance-induced fear and panic she'll go out of her mind. He erases her memory of recent events, saying she went to sleep and is just now waking up. The effort works and she awakes. Pleased, the Doctor leaves with urgent work to do.

He heads to the entryway and finds Travers just returning from outside. Travers says the robots are still up on the mountain, just standing still. The Doctor presses him for more information about what he saw in the cave, but Travers insists his mind is blank. The Doctor then presses him into another trip up the mountain.

Shortly, they are up on the mountain and heading upward. They come across three immobile Yeti and Travers begins to get jumpy. The Doctor takes the second reading he needs and the readings worry him. He doesn't stop to explain before heading back down the mountain.

In the cave, the Intelligence has grown even further. It covers the cave floor and is expanding outward.

At the monastery, the monks are ready to leave. Khrisong learns that there is no sign of the Doctor or Travers inside. Songsten heads off into the monastery to ask a final blessing of Padmasambhava.

Suddenly the Doctor and Travers burst in. The Doctor has found the control area and he turns to Khrisong for help, but the warrior has been demoralised by his failures and feels powerless. The Doctor explains that the control point is in the monastery, provoking derision from Sapan. The monastery is empty and everyone is here in the entryway. Thonmi realises the answer: Padmasambhava! Khrisong, fearing for the life of his Abbot, dashes off after him.

Travers, taking it all in, suddenly finds his memory block lifted. He remembers seeing Songsten on the mountain with the Yeti. The cave... He starts to tell his story.

Travers continues his tale, telling the Doctor about the pyramid Songsten took into the cave. The Doctor realizes that Songsten is the link between the monastery and the creatures. They hurry off to help Khrisong.

Meanwhile, Khrisong has caught up with the Abbot outside the sanctum, and warns him of the danger he believes to emanate from Padmasambhava. Before Songsten can protest, the voice of the master booms out, denouncing the warrior's words. But Khrisong persists.

The master must now act. He puts Songsten into a trance and orders him to disarm Khrisong, then the Abbot is forced to run him through. At this, the real voice of Padmasambhava comes through, reacting in horror to what they have been forced to do by the Great Intelligence. But just as quickly the Intelligence takes over and instructs Songsten to leave the monastery with the other monks.

The Doctor and his friends reach the sanctum and find Khrisong lying outside the sanctum doors, the Abbot standing over him with the bloody sword. Songsten snaps out of his trance and immediately accuses the strangers of the murder. Impatiently, the voice of the Intelligence booms out to kill them all, and Songsten attacks. But Jamie, Thonmi, and Travers overpower him, wrest the sword away, and drag him away.

The Doctor tends to Khrisong, who is mortally wounded. The loyal warrior uses his last breath to tell the Doctor that Songsten is not to blame in this, as he was in a trance and could not control his actions. The Doctor agrees with this and Khrisong dies peacefully. Saddened by the loss, the Doctor leaves to finish his work. As he goes, the evil laughter of the Great Intelligence fills the sanctum.

Thonmi, Jamie, and Travers return to the gathered monks, dragging Songsten, and tell the news. The monks do not believe that their Abbot could have killed Khrisong, but when the entranced Songsten lashes out at the group, chanting that he must kill them, they change their minds. Jamie and the monks struggle to hold him.

Thonmi argues for Songsten, saying that he was a helpless puppet, but even Travers is not convinced as he saw the Abbot leading the Yeti on the mountain. The Doctor arrives and confirms that he was acting under command. Some of the monks then accuse the Doctor and his friends of controlling him, but Thonmi knows and speaks the truth: it was Padmasambhava.

The Doctor tells them that neither Songsten nor Padmasambhava were responsible for their actions. They are both being controlled by a great evil. Everyone must leave the monastery so that he can fight it. Sapan, now in charge of the monks, decides to follow the Doctor's instructions. Thonmi, Victoria and Jamie all agree to stay and help the Doctor. Jamie, in charge of Songsten, reports that he has fallen unconscious. The Doctor uses this opportunity to try and get answers from him using hypnosis.

Meanwhile, Travers, unconvinced of the Doctor's belief that he evil is in the monastery, draws the warrior Ralpachan aside. Travers tells him that the source of the evil is the spreading ooze in the cave. It controls Padmasambhava and the Yeti. The Doctor hasn't seen it so he doesn't know. He talks Ralpachan into going to the cave to destroy the pyramid and both men sneak out of the room.

Songsten tells the Doctor everything. The Yeti were designed to serve the Intelligence; the Intelligence has lied to them all in order to gain corporeal form and has now expanded beyond the cave. It now demands the whole mountain for itself. He also says the Yeti are controlled via Padmasambhava, which intrigues the Doctor. He asks about the construction of the Yeti and their control units, and how the model Yetis influence the movements of the real ones. He discovers that there is a machine which relays the movement orders which is located in a hidden room behind the master's throne.

Travers and Ralpachan head up the mountain and nearly collide with a group of Yeti heading down toward the monastery. They hide as the creatures pass, then continue on toward the cave.

The Doctor gives Songsten into the care of Sapan. It is time for the monks to leave. After they have gone, the Doctor and his friends will head for the sanctum. The Doctor will keep Padmasambhava occupied and Jamie and Thonmi will smash the relay equipment. Victoria will be there to help any of them in case the lama tries to use mind control against them. He tells Thonmi to teach her the Jewel of the Lotus prayer so that she can fight Padmasambhava's influence.

Jamie notices that Travers is gone.

Travers has reached the cave by this point, and he and Ralpachan see the ooze emerging from the cave and spreading down the mountain, lit by its own eerie light. But it is quickly clear that they cannot get to the pyramid inside and destroy it. It is far too late. They must head back to try and save the monastery.

Inside the monastery, the monks are gathered at the main door. Sapan offers one last time to stay, but the Doctor presses them to leave. Brute strength and numbers will not win this battle. He hopes that they will be able to return soon. With that, the monks file out of Det-sen. The Doctor gives last instructions to his friends, assuring them that everything will be all right, and they head off for the inner sanctum.

Travers and Ralpachan approach the monastery and see the Yeti waiting outside the gates. Below them, the torches of the monks can be seen descending. Travers wonders if the Doctor is still inside but with the Yeti on guard, both men are too scared to attempt to enter.

The Doctor and his party arrive outside the sanctum. Padmasambhava challenges them for not heeding his warning to leave. The Doctor knows this is not Padmasambhava speaking and he asks where the Intelligence has come from and what its purpose is. As they talk, a torch detaches itself from the wall and hurls itself toward the Doctor, but he ducks in time.

The Doctor is unfazed by the "childish trick" and orders the doors to be open. They do so and he enters alone. Suddenly, he cries out in pain as he is mentally attacked by the Intelligence. Jamie starts to follow and help, but Victoria holds him back. They must wait, as the Doctor has instructed.

The Intelligence laughs evilly as it attacks the Doctor's mind. But he is holding his own and occupying the Intelligence. At last he orders his companions into action and Jamie and Thonmi burst into the sanctum and race to the control room behind the throne. They begin smashing everything in sight. The battle between the Doctor and the Intelligence continues, but it still has enough energy to manipulate the lama.

Padmasambhava reaches out for the Yeti models to bring them inside to his aid. Victoria sees this and warns the Doctor. Under tremendous mental strain, the Doctor can do nothing. He tells Victoria she must go and knock over the models, but she is transfixed by the power of the Great Intelligence. The Doctor encourages her to repeat the Jewel of the Lotus prayer. She does so, but it is no use. She still cannot overcome the power bearing on her mind.

Outside, the three Yeti move as commanded, entering the monastery. Travers and Ralpachan see their chance and head in as well.

The Doctor sees the Yeti enter the sanctum and shouts out a warning to Jamie and Thonmi. They are perplexed as they have destroyed all the machinery in the control room. The Yeti advance on the Doctor. Jamie recalls the spheres which control the Yeti. Nearby is a larger version which they have not smashed. When they do so, the spheres inside the Yeti robots also explode and the robots crash to the floor.

However, the Intelligence is not defeated yet and renews its assault on the Doctor. Travers reaches the sanctum and quickly fires his rifle into the body of Padmasambhava. It has no effect - his corporeal body is animated only by the Intelligence. The Intelligence gloats over his impending victory, asking Travers if he did not feel enough of his power in the cave. But this comment triggers something in both Jamie and the Doctor. They suddenly remember the pyramid that Travers mentioned, and the Doctor shouts to Jamie. Jamie sees another pyramid in the control room and smashes it too.

The Intelligence shrieks in agony as it leaves Padmasambhava's body and its only material existence. The old man slumps backward in his chair. In the cave, the other pyramid explodes as well, and a huge explosion rocks the mountainside, destroying the weblike ooze.

The Doctor goes to his dying friend. Padmasambhava has at last found peace and he is finally allowed to die. The Doctor sadly announces to his friends that it is over and they have won. He sends Thonmi to signal the monks to return.

Shortly afterward, the Doctor is ready to go. Despite Thonmi's entreaties, the Doctor does not want to wait for the monks and so says goodbye. Travers offers to see them safely up the mountain. Thonmi goes to await the return of his brothers.

As they head upward, the group runs across a dormant Yeti robot lying on the ground, its control sphere destroyed. The Doctor laments the destruction of such a clever machine and suggests that Travers take it home with him as a trophy. Another Yeti appears further up the mountain, but this creature runs from them quickly and hides. It is a real Yeti, timid and shy. Travers is elated and hurries off after it with a short goodbye.