The TARDIS materializes in mid-air over Alaska, and the console detects the presence of an odd form of energy it is unable to analyse. The scanner shows the Doctor and Nyssa a plume of smoke rising from the ice fields, but before they can investigate the TARDIS nearly collides with a light aircraft and is forced to dematerialize. To the Doctor's surprise, it rematerializes in the same place, but three decades later, in 1994 -- apparently on the trail of the energy readings, which had decayed to trace levels shortly after their departure but which are once again increasing in intensity. He and Nyssa set off to investigate, but despite the bleak isolation of their surroundings Nyssa finds that she has a sense of being watched. Their detector leads them to a cliff face against which an impressive but incomplete building is under construction. As they approach, night falls and the Doctor's torch batteries begin to fail -- and they are attacked by a monstrosity with a roar that sounds like a herd of animals and a flock of shrieking birds all together at once. As they flee for cover in the building, a second monster joins the chase, and what little light the Doctor is able to shed on them convinces him that his pursuers are like nothing on Earth.

Inside the building, interior designer Monica Lewis is feeling the stress of three years' hard work. She has given up everything in her life to see this through, and refuses to take a break until Shaun Brett's monument is complete. But with mere weeks to go, they have begun to experience excruciating delays. Tulung, Brett's half-American, half-Inuit assistant, believes that Brett's employee Gaborik has been spreading tales about Brett to ensure further delays to the work and thus guarantee his continued employment. Gaborik in turn feels only contempt for Tulung's half-hearted attempts to keep one foot in both his worlds -- and even Brett knows that the bond that links Tulung to him is stronger than any ties of blood. Monica doesn't understand why Tulung remains with the patronising and abusive Brett, who has grown even colder and more obsessed in recent months, and Tulung is unable to explain to her what it is that holds them together -- only that he cannot leave until the business between them is concluded.

The Doctor and Nyssa arrive, seeking shelter, and Tulung stops the misanthropic and suspicious Gaborik from throwing them out of the house. Gaborik reluctantly goes to tell Brett of the strangers' arrival, while the Doctor takes in his surroundings and realizes that the foyer is more like the entrance hall of a museum than that of a private dwelling. Nyssa is feeling somewhat unwell, no doubt due to the exertion of the chase and the sudden change of temperature as she entered the building, but she nevertheless checks the detector and finds that the energy readings are stronger than ever; whatever their source, it must be within the house. Tulung turns on the floodlights and looks around the grounds with the Doctor, but the light seems to have frightened off whatever attacked them. The Doctor decides that, if Brett is willing, he will stay the night and resume his investigations in the morning.

Tulung takes the Doctor and Nyssa to the drawing room to warm themselves before the fire, and tells them more about the house; the Bretts made their fortune from Alaskan oil, and Brett's father came to love the land which had made him rich. Tulung's father was a professional archaeologist who spent his life chasing funding about the state, until Brett Senior hired him to work on a nearby dig... which is how they both died. Brett himself arrives, and is only too happy to show the Doctor around the house, a monument to his dead father. Nyssa is still feeling unwell, and remains in the drawing room with Tulung. She senses the tension between Brett and Tulung, and sympathises with Tulung's feelings of loss -- feelings she knows only too well herself. Apparently Brett Senior and Tulung's father had made a tremendous discovery at the dig, but it caved in and Brett Senior was the only one to walk away alive. And this took place three decades ago...

As the Doctor tours the main floor of the monument, he is disturbed to learn that each room is built from a single natural element carved out of the very land which Brett claims to honour. Brett admits that he hired Tulung and Gaborik to assure the Koyakun that he honours their traditions, even as he tears apart their land. Beyond the stone room, the timber room, the ice room, the earth room and the bone room lies the cliff wall and Brett and Monica's masterpiece -- the sea room, a stretch of genuine shoreline complete with 500,000 gallons of Arctic seawater, a holographic moon and virtual tides. It has even been stocked with native wildlife, although the sea lions and walruses appear to be in hiding at the moment. As the Doctor and Brett depart, they encounter Monica in the bone room, a place which she hopes will provoke a reaction from the public -- but which she admits gives even her nightmares.

Brett sends Gaborik to fetch lights with which to illuminate the fissure, and on his way, Gaborik finds the Doctor struggling to open the sea room door and helps him to do so. In the light from the open door Nyssa catches a glimpse of the things attacking her, and when she emerges she tells the Doctor that they appeared to be composed of several living creatures which had somehow become fused together. Since there is no other way in or out of the sea room, Gaborik concludes that Nyssa has seen the familiars of the sea spirit Sedna, from whose flesh all animal life sprang. Betrayed by her father, Sedna sank to the bottom of the sea and now rules over Adlivum, the land of the dead. The spirits of the animals used to decorate the monument are angered at the desecration, and are seeking their revenge...

The Doctor, Nyssa and Gaborik return to the stone room, where the Doctor shines a torch on the fossil in the rock -- and sees it twitching, apparently trying to pull itself free. He takes a sample with a laser scalpel, and finds that the fossil is burning hot in his hand; it is undoubtedly the source of the energy readings. The Doctor and Nyssa theorize that the fossil creature is trying to regenerate itself, and has created the hybrids in the sea room as a side-effect. The terrified Gaborik plames the Doctor and Nyssa for bringing this evil to the house and refuses to remain, and as he sets off to pack up his belongings, Brett collapses, suffering the same symptoms from which Nyssa has just recovered. The Doctor theorizes that Brett too is being affected by the energy field generated by the fossil creature, and asks Nyssa to analyse the fossil fragment; as an expert in bioelectrics, she may be able to determine how such a creature can exist. Tulung offers to take her to a nearby camp where they were analysing the relics unearthed while the earth room was being made. He too is terrified by what is happening, and is trying to convince himself that the legends of his people have no bearing on the situation; Gaborik has been stealing the pelts intended for the fur room and selling them in Anchorage, telling himself that Brett had already defiled them, and Tulung tries to convince himself that this is the only reason Gaborik fears the revenge of the animal spirits. After taking the delirious Brett to rest, Tulung and Nyssa set off for the camp -- but Nyssa notices that Tulung also appears unwell...

The Doctor studies the fossil as it tries to work its way free of the rock, and realizes that the skeleton has no joints; the "bones" are attached directly together, suggesting that the bone itself is the flesh and muscle of the creature. He and Monica seal up the stone room and retreat to the earth room to ponder what they have learned over a cup of tea. The Doctor suggests that Monica's recent nightmares may have been caused by the energy field generated as the creature returned to life, but this does not explain why she and Nyssa have not been affected as badly as Brett. Too late, the Doctor recalls Gaborik's claim that the meat course tasted off; it too must have been infused by the same energy that created the hybrids, and by eating it Brett and Tulung may have allowed the creature's influence to gain a hold on their conscious minds. Monica warns the Doctor that Brett is not the most stable man at the best of times; he had idolised his father as a child, and was devastated when Brett Senior was driven to his grave by guilt over the collapse of the dig which killed Tulung's father. Like Nyssa, the Doctor realizes the significance of the fact that the disaster occurred three decades ago -- but how could a simple cave-in explain the amount of smoke they saw on the scanner?

As Tulung drives Nyssa to the survey camp, he tells her of his people's belief that the spirits of the dead walk amongst the living -- a story which no longer comforts him. He and Brett are bound by the deaths of their fathers; Brett believes that his father went to fetch help, while Tulung believes that Brett Senior fled to save his own skin and abandoned the rest of his party to die. As children Tulung and Brett each blamed the other's father for the death of their own, and they will be bound together forever until they can prove the other wrong. Nyssa understands that Tulung is trying to reconcile his spiritual and rational sides, just as her own people learned to do long ago, and, trying to give him some peace, she tells him that she saw a yellow plane with the number 7 on its side fleeing the dig the very day of the disaster. The delighted Tulung fails to notice until too late that a hybrid is approaching the vehicle, and when he swerves to avoid it the transport skids off the track and stalls. Horrified by what he has seen in the headlights, he now believes that Gaborik was right all along, and that the angry spirits of the animal dead have risen from Adlivum to seek revenge...

As the Doctor and Monica wait for word from Nyssa, the sound of the fossil's digging grows louder -- and the Doctor realizes too late that it is not digging its way out into the stone room, but burrowing through the wall into the earth room. Before he or Monica can react the fossil creature bursts through the wall, blocking the exit. It is unsteady on its feet and apparently confused by its surroundings, but judging by its skeletal structure it is carnivorous, fast, and no doubt hungry. It soon sizes up the Doctor and Monica and prepares to attack -- but Gaborik, who has just left the delirious Brett crying out for his father, hears the noise from within the earth room and opens the door at the worst possible moment. The fossil creature chases him out into the corridor, tears the clothes from his body and consumes him, using the energy field it generates to strip the flesh from his bones. Unable to save him, the Doctor and Monica retreat upstairs while the fossil creature -- apparently confused and disoriented by its first human meal -- staggers away to recuperate.

Tulung manages to restart the stalled transport, and he and Nyssa escape, running over one of the attacking hybrids as they flee. Shaken and terrified, Tulung recalls Gaborik's claim that Nyssa had communed with Sedna -- and now he knows she can see into the past. Her attempted act of kindness has backfired; Tulung now believes her to be a guardian spirit who can protect him from the spirits of the dead and tell him all he needs to know about his father's death. They arrive at the camp, where Tulung starts the generator and asks Nyssa for further wisdom as she tries to test the fossil fragments. The state-of-the-art equipment is quite primitive by Nyssa's standards, but she still learns enough to worry her deeply; as she knows only too well, Earth's dinosaurs died out 65 million years ago, but this bone fragment is over 260 million years old. She tries to contact the Doctor, but as she does so Tulung hears something moving outside the camp...

The Doctor flings the phone at the fossil creature and flees for the stairs, and when the creature attempts to follow him it weakens and falls dormant. The Doctor realizes that the rooms on the main floor have all been constructed from natural elements, but that the living quarters are made of synthetic materials which disrupt the creature's bioelectric field. As he studies the creature he realizes that it has incorporated into itself the genetic characteristics of Gaborik; if it consumes any more human beings, it may even develop intelligence. The Doctor and Monica slip past the dormant creature and check the stone room, where they confirm that another of the creatures is indeed returning to life; by bringing the two fossils into close proximity, Brett and Monica may be unwittingly responsible for their awakening. The Doctor sends Monica to fetch synthetic throw rugs, with which he hopes to disable the dormant creature before it regains its strength, but while she is doing so Brett stumbles out of his room, takes one look at the fossil creature and flees terrified into the night, all of his worst nightmares confirmed.

The hybrids have found the survey camp, smashed down the phone lines and attacked Nyssa and Tulung. Tulung is in a state of shock, believing that the spirits of the dead have risen in anger, and Nyssa realizes that his mind has been affected by the energy field from the creatures. Deducing that the hybrids, created by the same energy fields, are now sensitive to any other sources of power, she shuts down the generator, and the angry hybrids calm down. But once again she has unwittingly demonstrated to Tulung that she can control the spirits of the dead; she is his torngak, a guardian spirit who will keep him safe from harm and lead him to the truths he has sought all his life. Nyssa manages to get him moving again and back into the transport, and as he tries to start the engine the hybrids move in for the kill -- but a storm petrel alights on the bonnet of the vehicle just as the engine finally catches. As they return to Brett's monument, Nyssa tries to explain to Tulung that the petrel's uncharacteristic behaviour was due to the fossil creatures' energy fields affecting its mind -- but there is no reasoning with him, and he firmly believes she summoned it to bring them luck.

The dormant fossil creature regains its strength as Monica returns, and it tears its way free of the blankets which she and the Doctor have thrown over it and retreats to familiar surroundings, battering down the door to the sea room and entering to feed on the hybrids. As the Doctor and Monica try to jam the door shut and barricade it with Monica's paint tins, Tulung and Nyssa arrive, and the Doctor enlists their help to seal the doors to the stone and earth rooms. They must keep the fossil creatures confined; if they escape before the Doctor can summon reinforcements, they may manage to establish a colony out on the ice and begin breeding, and then there will be no stopping them. Tulung, however, still believes that they are the spirits of the dead, here to avenge Brett's lack of respect for the land. The Doctor believes that the dormant creatures' energy fields may have affected the racial unconsciousness of the Inuit and influenced the development of the legends which seem to be fulfilled here tonight, but Tulung is too far gone to listen to him and leaves to confront Brett about the truths he has learned from Nyssa. Nyssa admits that she told him about the plane in the hope that it would help him, but the combination of his fear and the influence of the creatures' energy field has driven him out of his wits...

Outside, Tulung finds Brett sitting before a fire on which he is burning Gaborik's favourite foods to comfort the dead man's spirit. Brett's father used to tell him all of the stories of the land, including the story of how the Raven Father grew tired with the first men and thus introduced the concept of mortality. Brett's father was struck down by a bored god who dredged up a special death from the depths of Adlivum for him, but he held the god at bay; it was his own fear, guilt and memories of that terrible day that took his life. Now that same death has returned for Brett, but he knows the truth at last, and if Tulung accompanies him back to the dig he will reveal all. Tulung is shocked to learn that Gaborik had secretly arranged to reopen the dig, and that Brett has already been there, but although he is desperate to know what it was that Brett saw he fears that they will be attacked by the animal spirits again if they venture out into the darkness. But he knows that Nyssa can keep him safe, and as he will go nowhere without her, Brett insists that they take her with them. Whether she wants to go or not...

As the Doctor and Nyssa barricade the door to the earth room, the Doctor explains to Nyssa that the death of the dinosaurs was not the only mass extinction to strike the Earth; 260 million years ago, at the end of the Permian era, 96 percent of all life on the planet was wiped out. And now it appears that they have found the reason -- a hitherto unsuspected life form which processes energy so fiercely that they even devoured their own flesh, leaving only bone and cartilege animated by a bioelectric field powerful enough to induce fear in the creature's prey. When these Permians revived after such a long period of dormancy, their bioelectric fields were still unfocussed, and the flesh of the nearby sea creatures -- the direct genetic descendents of the Permians' earliest prey -- was scrambled together, creating monstrous hybrids which lash out in pain and fear, not understanding what has happened to them.

Brett and Tulung force Nyssa into the transport and set off for the dig, to face what the past has brought to the surface. Nyssa tries to talk sense into them, insisting that they warn the world about the Permians and reminding Tulung that he's leaving their friends to die -- just as he's accused Brett's father of doing for all these years. For a moment it seems that she's getting through to him, but the pull of the past is too strong; he must see this through. At the dig, Brett has Tulung clear away the camouflage which Gaborik had set up to ensure that the Inuit did not realize they were trespassing on forbidden lands. Brett ties Nyssa outside, telling her to scream loudly if danger approaches, and he and Tulung enter the dig, leaving Nyssa to struggle against her bonds...

The Doctor and Monica struggle through the flooded corridor, surrounded by the wreckage of the barricade, including Monica's tins of expensive polymer paint. They are attacked by the second Permian, which has finally escaped from the stone room, but the Doctor flings paint at it, coating its entire body in synthetic material and forcing it to retreat. The Doctor now accepts that the creatures are too powerful for them to contain; their only hope is to escape and summon reinforcements. Which means walking miles through the cold pitch darkness of the Alaskan night while soaking wet and surrounded by murderous hybrids, and then trying to talk sense into a man whose lifelong torment over his father's death has been converted to insanity by the influence of the Permians. Monica notices that the first Permian has not yet emerged from the sea room, and when the Doctor investigates he finds that the tsunami has brought down the entire rock face, exposing the tunnels and mines that lie beyond. The second Permian recovers from their earlier attack and approaches, and they have no option but to flee into the exposed tunnels -- although well aware that the first Permian may be lying in wait ahead of them.

According to the official inquest into the dig's collapse, the bodies of the archaeologists were swept out to sea by the Koyakuk river... but when Tulung enters the remains of the dig, he finds the floor littered with bones. When Brett first found these skeletons in his father's closet, he realized that his father had lied and paid to cover up the truth -- but until tonight he did not understand why. Amongst the human remains are the bones of the Permians, and Brett now knows that his father deliberately sealed off the dig, sacrificing the rest of his party to keep the land of the dead away from the living. He now intends to put these bones on display in his monument, as a testament to the heroism of his father. But Tulung sees an all too familiar ring and backpack amongst the remains on the floor -- and, realizing that Brett was going to put the bones of his father on display to the public, the enraged Tulung attacks Brett, beating him senseless. In his fury he is unable to hear Nyssa's desperate cries for help. The hybrids are coming...

The Doctor and Monica struggle through the narrowing, water-filled passageways towards the sounds of struggle, and while the Doctor intervenes between Tulung and Brett, Monica goes to fetch Nyssa. Nyssa has just managed to fray her ropes and break free, and the approaching hybrids ignore her and Monica, drawn towards the dig by the influence of the Permians. Monica and Nyssa follow the hybrids inside, only to find that the Permians have arrived -- and as more and more hybrids arrive, further Permians emerge from the tunnels, freed by the collapse of the rock face. The increasingly desperate Doctor tries to throw the dead Permian bones at the living ones to make them think they are under attack from their own kind, but it's of no use; the first has already developed enough intelligence to force the others into a pack. And this is after eating only one human; if the pack attacks the nearby village, they may be able to decimate the entire country and even develop space travel within months. And if they consume the Doctor, a Time Lord, with his TARDIS in the vicinity, the result does not bear consideration...

Tulung fetches his father's rucksack, wanting it close to him at the moment of his death. But it suddenly occurs to the Doctor to wonder what killed the Permian remains they have found in the dig, and when he and Nyssa realize that the walls are covered in soot, he recalls the plume of smoke on the TARDIS scanner and realizes that someone blew up the dig using the archaeologists' blasting dynamite. The Permians were killed by fire, an ancient force too volatile for even them to control. Inside the rucksack Tulung finds a stick of dynamite, apparent proof that it was indeed his father who sacrificed himself to entomb the Permians while Brett's father fled to safety. Brett refuses to accept this, and he grabs the dynamite from Tulung, flings himself at the Permians and detonates it, bringing the roof down upon them. The Doctor, Nyssa, Monica and Tulung shelter from the blast, but when the smoke clears they see that the Permian leader and one other have survived. They flee back through the tunnels to the house while the Permians feed upon the hybrids and what remains of Brett, and on the way Tulung finally manages to shake off the Permian influence; now that he has as much of the truth as he ever will, he is finally free of his lifetime obsession.