An unnatural black cloud descends upon a Himalayan expedition led by Lord Mortimer Davey. Most of his men are torn apart by the forces within the cloud, but Davey shelters in a nearby cave, only to find that the cave is an artificial structure, like a mirrored pyramid carved into the heart of the mountain. As Davey tries to understand what he’s seeing, another figure approaches and strikes him down, chanting in an alien language. The chamber fills with an unnatural whispering sound...

The TARDIS materialises in the livestock wagon of the “toy train,” part of the railway system connecting Siliguri to Darjeeling. The Doctor has an invitation to a cricket match at the Grand Imperial Hotel, and he intends to take the adventurous route rather than simply materialising on the pitch. The Doctor decides to take a nap while Erimem appreciates the variety of the landscape, but before going to sleep, he chastises Peri for using canisters of liquid nitrogen to freeze the TARDIS swimming pool into an ice-skating rink. As the Doctor naps, Peri settles down to listen to Erimem practice reading from The Hunting of the Snark.

General Alexander Bruce, who organised the Himalayan expeditions and the cricket match, takes some issue with the work of journalist John Matthews, the Royal Geographical Society’s expedition correspondent, feeling that Matthews’ prose doesn’t make him sound heroic enough. Bruce compares Matthews’ work to that of Arthur Conan Doyle, claiming that Sherlock Holmes is not a fictional character but a real man whose exploits have been fictionalised by Doyle. Matthews disagrees, but eventually gives up arguing and agrees to write a more heroic description of Bruce’s arrival at the hotel. Satisfied, Bruce then sits for a heroic portrait of himself while Matthews presents him with the first set of photos from the base camp. Bruce dismisses them as flawed and orders Matthews to dispose of them, as most of the photos bear a black stain that looks like a cloud descending down the mountain towards the camp.

The Doctor, Peri and Erimem travel from the Darjeeling station via a hand-drawn cart, leaving the TARDIS with the rest of the expedition’s luggage. On the way, they see lepers begging in the streets, and Erimem recalls how her father’s guards used to clear the streets before the royal family so their eyes would not be offended by the sight of beggars when they passed through the city. As the cart passes through the streets, it is being observed by Lord Davey’s killer, who has now taken the form of Lord Davey himself -- or rather, has had it thrust upon him by his masters, as he died with his task incomplete. When he sees Erimem, he understands what he is to do, but the streets are too crowded for him to make his move immediately. He thus introduces himself to the travellers as Davey, determines that they are heading to the Grand Imperial Hotel, and bids Erimem goodbye, kissing her hand. As the Doctor, Peri and Erimem resume their journey, Erimem becomes distracted, claiming that she can hear whispering voices. The Doctor dismisses this as the effect of her senses acclimatising to the thin air -- but in fact, Davey and his masters have made their first move...

The Doctor and his friends finally reach the hotel, only to find that they’re the first to arrive apart from Bruce, Matthews and the Sherpas. The Doctor catches a ball struck by Bruce, who is offended until the Doctor flatters him by praising his past military essays. The Doctor joins the match, and Matthews retreats to the veranda for refreshments with Peri and Erimem. Peri is puzzled by the poor turnout; the cricket match was meant to be a show of solidarity amongst the English, Swiss and French, but nobody has returned from their base camps for the match. Erimem is still distracted, and she tells Peri that she’s still hearing strange whispers -- and that she’s caught a glimpse of the man they saw in Darjeeling, even though he couldn’t possibly have made it to the hotel this quickly. Matthews offers to cheer Erimem up by showing her photographs of the mountain, but she flies into a panic upon seeing them. To her, the snow-capped mountain resembles a white pyramid, and when she was a child, her father told her of a white pyramid containing an evil that would destroy her soul. Peri dismisses this as a childhood fear, but the nightmares Erimem suffered from the story stay with her even now...

The black cloud has taken the lives of 16 Sherpas, 7 of Bruce’s men -- and Erimem. At the mass funeral, Bruce delivers a eulogy for the dead, and invites the Doctor to speak a few words in memory of his lost companion. The Doctor is sorrowful, Peri is grieving -- and Erimem is frightened and bewildered, unable to understand why her friends don’t seem to see or hear her though she’s standing right next to them. Only Lord Davey can see and speak to her, but this isn’t particularly reassuring, as nobody else can see or hear Lord Davey -- and he’s claiming that Erimem is dead. Erimem tries to reach out for the Doctor, but her hand passes right through him. As Peri accompanies Bruce to the clubhouse to grieve, Davey tells Erimem that she has transcended this level of reality and only perceives it and her friends through force of habit. She must learn to see things differently now.

Erimem hears the strange whispering again, and suddenly finds herself back in the temple where she spent her childhood. She recognises one of the priests, Pyran, who once helped her to find her lost kitten; however, he does not respond to her attempts to attract his attention, and she finally begins to accept that she’s dead. Distressed, she lashes out at Davey, insisting that these are illusions meant to ensnare her and that Pyran remains as loyal to her as he was loyal to her father -- but Davey transports her elsewhere, and to her horror, she sees her father, Pharaoh Amenhotep II, ordering Pyran to have Erimem bound and thrown to the leopards on her next birthday. Amenhotep speaks of his daughter with disgust, and vows to erase all memory of her existence; none shall ever know he had a daughter called Erimem. Erimem, stunned, can’t put up much of an argument when Davey reminds her that she said that the priests would carry out her father’s wishes without question. Obviously, her father died before his instructions could be carried out -- but when Erimem left Egypt with the Doctor, who’s to say that the priests didn’t carry out their orders and erase her completely from their history?

Erimem rallies, insisting that even if she’s lost the love of her father and her place in history, the Doctor and Peri will always remember her. But Davey transports her to the TARDIS, where the console is in flames and Peri lies dying on the floor, struggling to call for help. Davey reminds Erimem that she’s tried and failed to touch her friends before, and Erimem is forced to stand by helplessly and watch Peri die. The next thing she knows, the dead Peri is standing next to her, furious with her for freezing up and doing nothing when the TARDIS console exploded. Peri and the Doctor have been there for her whenever she needed them, and now she’s repaid their trust by failing them, destroying all the hope that the Doctor ever represented. The TARDIS is in flames, Peri is dead and no longer listening to Erimem, the Doctor is dead or dying somewhere, and Erimem is unable to do anything about it. Reduced to tears, she begs Davey to take her away from this place, and he does so, another of her anchors to reality shattered.

Erimem next finds herself on the mountainside, where Davey shows her the bodies of the real Lord Davey’s team-mates. He is not himself Davey, but the man who killed him, an explorer of dark and arcane mysteries. When he made his final discovery, his acolytes turned on him, fearing what he had found -- and he killed them, sacrificing their minds and bodies to the dark cloud. The cloud itself is only the guardian of the forces he now serves, and in order to continue serving them, he had to take on Davey’s form. Erimem accuses her tormentor putting himself on the level of the gods, and Davey realises that he’ll never convince her to do what must be done next. He thus transports her into the heart of the white pyramid, the cave in which the real Davey lost his life; there, she finds the Doctor waiting for her. He too was killed when the TARDIS exploded, but he’s been brought here to persuade Erimem to accept her fate. She may feel as though the gods have abandoned her, but the truth is, she possesses a power stronger than the gods, one that frightened even her father.

The Doctor transports Erimem back to her childhood home, where she sees her father at work, charting the course of the stars and analysing their astrological meaning. She remembers seeing him argue with the priests and scholars over their interpretation of the scrolls his traders had found, and now recalls a particularly vicious argument in which her father accused the priests of disposing of certain scrolls because they feared their implications. If the scrolls were to be believed, there are forces older and more powerful than the gods of Egypt, and one day, they will return. Erimem now puts together different events from her childhood and realises that she is the one who took the scrolls in question, seeing them as a child and thinking that the pictures meant they were for her. But the pictures were of horrific things with claws and talons and burning red eyes, shredding their way through life wherever they found it, and Erimem suffered terrible nightmares after reading them. She now understands that her father had learned that these creatures once existed -- and the Doctor transports her back to the white pyramid, telling her that she is the one destined to bring about their return.

Mere seconds have passed at the Grand Imperial Hotel, and when the black cloud sweeps back up the mountain, there is no sign of Erimem. The Doctor realises that she was its target all along; if she had hidden in the cellar, the storm would have torn the hotel apart to get at her. There are strange marks in the ground, and the Doctor has Peri make a cast of the marks using plaster of Paris from the expedition’s medical supplies. The results seem to indicate that the cloud was shrouding some sort of beast with articulated claws or talons, something like a dinosaur but much, much stranger. Bruce has Matthews fetch the photographs which he’d earlier dismissed as rubbish, and the Doctor, noting strange shapes at the base of the cloud, asks Matthews to blow them up in his private darkroom. On the Doctor’s instructions, Peri offers to help Matthews, intending to find out a bit more about Bruce along the way. Meanwhile, Bruce checks the expedition’s maps, and confirms that the black cloud was last seen hovering directly above Lord Davey’s base camp. Bruce agrees to help the Doctor mount a follow-up expedition to find out what’s happened to their friends, but it will take until morning to muster their resources.

As Peri and Matthews wait for the blow-ups to develop, Matthews warns Peri that Bruce is less of a war hero than he’d like people to believe. He was invited to organise this expedition because he fought with Lord Davey’s father in the Boer War, but in truth, it’s Matthews’ job to make Bruce sound more heroic than he really is, and Matthews fears that he may not return from the expedition. He then takes the finished photographs to the Doctor, who examines them and confirms to his satisfaction that the cloud and the creatures within are psychic projections given form by the natural elements enveloped by the cloud -- including the blood of those it has killed. The cloud is only an avatar of even worse creatures, which must be trapped elsewhere, unable to get out into the real world... for the moment.

Disturbed, Matthews leaves to get some rest, but hears Bruce apparently talking to himself and investigates to find Bruce writing his last will and testament. Caught out, Bruce admits that he knows his men regard him as a blowhard, and that he’s allowed their doubts to get to him. What if his own incompetence results in the deaths of civilians? This isn’t what Matthews wants to hear from him, and he insists that Bruce remember his past victories and all the work he put into making this expedition a success. Stirred by Matthews’ noble speech, Bruce tears up his will and vows to lead the expedition to victory -- but if it should cost him his life, he orders Matthews to report that his last words were suitably heroic.

Unable to sleep, Peri tries reading one of Bruce’s books about Mount Everest (or Chomolungma, as it’s known in Tibetan). She is surprised when the Doctor silently shows up in her room to discuss Erimem’s disappearance, and is shocked when he wonders aloud whether it’s worth risking so many other people’s lives to rescue her when she’s most likely already dead. Unnerved by this display of doubt, Peri asks the Doctor to change the subject -- and he thus asks her if she thinks it was right to let Erimem aboard the TARDIS. He seems to be implying that Erimem is unable to take care of herself, and Peri angrily takes him to task for this lack of trust -- until the real Doctor enters the room to find out why she’s shouting. The impostor, caught out, transforms back into Lord Davey, but the Doctor isn’t fooled by his appearance or by his show of confidence; he knows this is not the real Lord Davey, and that their enemy would not make such an effort to dissuade them from their rescue attempt unless it believed they posed a real threat. Irritated, Davey vanishes into thin air, and blasts of super-cooled psionic energy like balls of blue fire rain down from the sky, incinerating the expedition’s equipment and supplies and putting paid to their hopes of reaching the plateau by ordinary means.

Fortunately, the Doctor has an alternative means of travel. When Peri awakens the next morning, she finds that the Doctor has bribed the Sherpas with jelly babies to lend him their cart and yaks, and has descended the mountain to fetch the TARDIS from Darjeeling. The Doctor decides to take Bruce and Matthews along to face their enemy together, admitting to Peri that he rather enjoys seeing the looks on people’s faces when they see the interior of the TARDIS. While the astonished Bruce looks about the console room, offering to act as the Doctor’s theatrical agent should he wish to take his miraculous blue box on tour, Peri leads Matthews to the wardrobe to fetch close-fitting, all-terrain clothing for their arrival on the plateau. Matthews isn’t quite sure what to make of all this, but Peri assures him that the Doctor can be trusted.

The force now possessing Erimem does not respond to the Doctor’s questions, but when Peri speaks, Erimem herself seems to respond to her friend’s voice. The Doctor urges Peri to keep eye contact and try to lure Erimem into the TARDIS, where the Doctor can analyse the nature of the force possessing her body. Peri urges Erimem to remember the Lewis Carroll poetry she’s been reading, and, with great effort, Erimem recites the first lines of Father William, slowly moving towards Peri as Peri backs towards the TARDIS doors. Inside, the Doctor stands Bruce and Matthews on opposite sides of the control room with twin sensors keyed to feed data into the TARDIS console. Once Erimem is inside, despite Peri’s attempts to keep her focussed, the black cloud sweeps forward into the console room. The Doctor keeps the doors open until the very last moment, getting all of the data he can out of the black cloud -- but as he tries to close the doors, the black cloud envelops him and Erimem and sweeps them both out of the TARDIS, leaving Peri, Bruce and Matthews alone inside.

The Doctor is somewhat surprised to find himself sitting in the Belvedere Club off Pall Mall with Lord Davey, next to the famous grandfather clock donated by Lord Charles James Mandeville. He realises that Erimem was under the black cloud’s control all the time; while he was using Peri as bait to get information out of the black cloud, it was using Erimem as bait to get to the Doctor. A third figure joins the Doctor and Davey by the fireplace, taking the form of Pharaoh Amenhotep II, Erimem’s father. The Doctor tries to provoke him by being deliberately flippant, and though Amenhotep appears to remain in control, the psychic projection of the Belvedere Club fades to reveal that they are in the cavern, the white pyramid. The Doctor soon deduces that the creatures have taken the form of Amenhotep, not from Erimem’s mind, but from their own memories of him, and when the faux-Amenhotep admits that the real Pharaoh was their jailer, the Doctor deduces that the Pharaoh had his people carve this pyramid out of the ice in order to trap these creatures on Earth. Amused, the faux-Amenhotep agrees to reveal the rest of the story.

Stuck in the TARDIS but unwilling to give up, Peri tries to access the readings the Doctor took from the black cloud. Bruce wants to stage a full-out frontal attack on the cloud, but Peri understands that this would be suicidal without knowing what they’re up against. Fortunately, Matthews recognises some of the data as temperature readings, and concludes that the cloud is much warmer than it should be at this altitude. The cloud was moving more sluggishly than back at the hotel, and there was no sign of the monsters within. Peri thus deduces that the cloud, which is made up of physical matter, is vulnerable to the cold -- and recalls the canisters of liquid nitrogen which she used to freeze the pool into a skating rink. She fits the canisters with firing triggers from the TARDIS laboratory’s fire extinguishers, and she, Bruce and Matthews head out to confront the cloud. Peri quotes Lewis Carroll to give herself courage as she approaches the cloud, but fortunately, it seems to have been relying on Erimem to guide it earlier, and it does not attack. Once they are close enough to the cloud, they open fire with their canisters, spraying the cloud with liquid nitrogen and clearing a path through to the cave.

The creatures in the cave grant the Doctor a vision of the Pharaoh’s slaves hollowing out the white pyramid, a structure of infinite reflections in which the creatures’ evil would be trapped forever. The Doctor now identifies his enemies as the Great Old Ones, unimaginably ancient creatures preserved in humanity’s race memory as their worst nightmares; creatures that spread evil, chaos and destruction in their wake wherever they go. Amenhotep knew that he could never harness their power for himself, and therefore lured them into a trap to keep the world safe from their evil. The Doctor vows to defeat them, and Davey, amused by his arrogance, returns Erimem to him. She is shaken by her experiences, and is having trouble focussing due to the plasma that leaked from her eyes, but she is safe and well -- which, the Doctor realises, means that the Great Old Ones no longer need her. This can only mean that she has served their purpose and released them into the world.

As the Great Old Ones take on corporeal form, the Doctor dismisses them as parasites of no more significance than cosmic locusts, and urges Erimem not to believe their attempts to erode her self-esteem. At the Doctor’s urging, Erimem begins to recite Father William once more. As she repeats the poem, the thing that used to appear as Lord Davey confronts the Doctor, angrily demanding to know why he does not fear them. But the Doctor has seen real evil in his life, and he knows that as bad as these creatures may be, there will always be worse to come. They are the last of their kind, already weak and dying, feeding on others’ lives to maintain their shaky grasp on the glories of their past; one day, something even worse will come along to destroy them, and the Doctor can take comfort in that. As the enraged Davey lashes out at the Doctor, Peri, Bruce and Matthews burst into the cavern, and Bruce opens fire with his canister of liquid nitrogen, freezing the thing that has taken on the form of his old friend.