Del McAllen is a British Museum employee working in tunnels bored beneath the ice. He is investigating curious carvings in the rock, of humanoid stick figures, that predate the earliest civilizations. There are two other workers with him. One is Keely, who is a large, brusque man supervising the tunneling, and the other is Rwm’dek, his assistant. She is one of the Flisk; alien visitors to Earth who had arrived twenty years earlier after a stellar cataclysm had almost wiped out their race. At first they were welcomed, but when they let it be known that they were slightly telepathic the human population had taken against them. Del, however, has no such reservations and he and Rwm’dek arrange to go on a date. Then he notices that the ice around him is melting. A puddle is forming around his feet and the ice walls between the rocks are cracking. Despite the help of six large robots deployed to prop it up, the roof begins to collapse. In the ice Del sees a dark shape moving. Its whip-like arms drag Keely into a fissure in the ice. The Flisk girl orders the robots to drop the roof. At first they refuse, saying that will endanger the lives of the two workers, but they both reply that they are already dead. The tunnel collapses.

The Doctor has promised Martha a beach holiday and landed in Saudi Arabia at the end of the twenty-first century, where Bartholomew’s Planetary Gazetteer and Time Traveller’s Guide tells him the best beach ever can be found. Unfortunately they emerge into a world of ice and snow and have to dash back to put on more suitable clothes. However, after blundering about in a blizzard for a while they come to a glass wall, on the other side of which they see a woman in bikini. Realising that they are inside some sort of massive structure with artificial weather but very near the beach, they search for an exit. Martha soon becomes aware that she is being tracked by a dark shape. In fact there are several of these shapes which the Doctor holds back with his screwdriver as the pair of them run for safety.

Meanwhile, in SnowGlobe 6 Director Beth Cowley is meeting with Ahmed Jaffa. He is the doctor attempting to control a viral outbreak that has afflicted hundreds of people. He wants to send for the assistance of the World Medical Council but Cowley fears that this will lead to a quarantine order that will cripple the businesses in the SnowGlobe. He also has a discovery of egg sacs of some unknown creature to deal with but knows that Cowley is delaying him until she has completed a deal with a man called O’Keefe.

Cowley’s main project was SnowGlobe 7 but she took over as Director of SnowGlobe 6 four months earlier when the previous Director had to be replaced after some alarmingly poor decisions. The extra work is hampering her plans for SnowGlobe 7 but has allowed her to make the acquaintance of Maxwell O’Keefe whose money may see her plans come to fruition. As she makes her way to meet him she reflects that she only needs to keep the outbreak secret for a few more hours until her deal is completed.

Insurance company operatives are at work in the entrance hallway of SnowGlobe 7, trying to discover the cause of the previous week’s accident. There are three of them, Ku’ra Debrekseny (a Flisk), Brian Williams and Laurence Harrison. They are writing a report for the British Museum and also for Maxwell O’Keefe, both of whom are concerned about the deaths of the three workers in the conservation project regarding the subterranean carvings. An internal alarm alerts them to the presence of intruders as two figures burst through the airlocks into the foyer from inside the globe. Harrison tells them that they are trespassing but when one of the intruders introduces himself as ‘the Doctor’ Harrison expresses his delight and relief that a doctor has finally arrived.

While the two humans are so relieved to see them that they don’t even ask to see any credentials, Ku’ra is confused as to why they were already in the SnowGlobe and tries to probe Martha telepathically before the Doctor intervenes, warning Martha of the psychic abilities of the Flisk. They are taken by coach to SnowGlobe 6. On the way Martha reads with amazement the story of the climate change that has seen the ice caps almost disappear, the creation of the ten SnowGlobe to preserve the Arctic environments and their steady transfer from public to private businesses so that seven are now holiday resorts for the very rich. Meanwhile, the Doctor impresses Ku’ra when he tells him that he was once part of the Flisk armada as it fled across the Opius Expanse. Ku’ra then goes on to tell him how the conversion of SnowGlobe 7 from a scientific project was slowed with the discovery of prehistoric carvings even before the tunnel collapse and the outbreak of a mysterious infection.

Director Cowley’s meeting with O’Keefe is not going well even before she is forced to leave to deal with the new arrivals. O’Keefe keeps the contract she had wanted him to sign, readying himself to make some amendments while she is gone.

In SnowGlobe 7 three security personnel have been left with Brian Williams. They are disturbed in the foyer by a hammering on the door of the airlock. When they open the door a creature steps out: two metres tall with a furry body, eight legs and a long barbed tail. It kills one security guard before it is shot down by the others, but two more emerge and wipe out the humans.

The Doctor is taken to the Robotics Division of SnowGlobe 6, at his request. There he manages to piece together one of the robots destroyed by the collapsing tunnel (using the insides of a computer, two coffee vending machines and a door control panel) much to the amazement of the watching technicians. He accesses the robot’s memory and hears a tape of the last moments in the lives of the Flisk and two humans as they ordered the robot to drop the tunnel to kill something alien and alive in the ice.

Martha is taken to the medical rooms and soon comes to terms with the high-tech equipment. The ward is full of patients who came into contact with three eggs found in the collapsed tunnel. Ku’ra, however, passes out as soon as he enters the ward. The nurse, Marisha, is not confident with Flisk physiology, and Martha tells her that the alien is, in fact, half Polish. He, like all the others, appears to be suffering from accelerated brain activity. Beth Cowley arrives and is less than pleased to find Martha already at work but the Doctor sweeps in and takes control of the situation. When he reveals what he has learned from the robot and how Martha and he were stalked by something alive in the SnowGlobe Cowley seems to collapse inwardly and allows the Doctor to get on with things.

Ku’ra wakes up and says he collapsed after sensing the death of another Flisk nearby. As there are no Flisk on the ward the Doctor investigates the ‘eggs’ and finds that they are the metamorphosed and decaying skulls of the three victims from the tunnel. The Doctor and Ahmed Jaffa run a series of tests on the skulls but only come up with the conclusion that they are clues towards a creature with an unbelievably complicated life cycle.

O’Keefe and his hired man, Curtis Rabley, drive across to SnowGlobe 7 to check on the state of his investment before he signs the contract. They find it open and deserted. After O’Keefe makes the gruesome discover of a severed head and a dismembered body a large alien emerges from the interior of the SnowGlobe. Rabley shoots it a number of times to slow it down but only actually escapes when it follows him into the sunlight and recoils in pain. O’Keefe is not so lucky and is brought down by a second creature as he flees. Rabley disposes of his gun inside the SnowGlobe, intending to make it look like he was killed, too. He makes his way back to the hotel suite where O’Keefe has three million dollars in a safe. Realising that he cannot steal it all he takes enough for a comfortable life before resetting the timer on the safe so that it appears not to have been opened since the previous day.

After failing to contact Brian Williams or the three security men, and the discovery of O’Keefe’s car in SnowGlobe 7’s otherwise deserted car park, Captain Hassan and more heavily armed security men descend on the Doctor to find out what he knows. He tells them that a shoot-out is the worst thing that could happen. When he says he needs to confront the creatures to find out what they are and where they come from Hassan agrees to escort him.

The Doctor calls in on technician Roberts in the robot workshop. Roberts has retrieved some video footage from Service Robot Twelve, showing the alien creature underground. The Doctor is interested in the part-insect/part-primate appearance but is also keen to take the robot with him into the tunnel to find the exact point where the incident occurred. The robot expresses a desire to return, too, to find out why its co-workers died.

In the cavern beneath the ice the alien creature begins to reach out with its mind. Beth Cowley, drifting in and out of sleep disrupted by her failed plans, feels the creatures call. Taking the ‘eggs’ from the mortuary she plans to leave the medical rooms. The other patients revive simultaneously from their comatose condition. They attack Doctor Jaffa and breathe grey dust into his face, knocking him out. Martha, Marisha and Ku’ra barely escape and lock the doors. Cowley assaults Harrison and escapes with the eggs. As the patients smash their way out of their confinement Martha leads the other three into the safety of the mortuary. Here she surmises that all of the patients, and Cowley too, are under the influence of the aliens. Their only thought is to get to the other SnowGlobe to be eaten. Martha is worried that Cowley intends to use the skulls to infect everyone else in the resort so that they will meet a similar fate.

The Doctor and Robot Twelve enter SnowGlobe 7. As promised Hassan permits them to enter but promptly orders both SnowGlobes to be sealed. The Doctor was half expecting this and uses his screwdriver to lock himself in and the security staff out. Robot Twelve barely shuts the internal airlock before the aliens come out of the cold to attack. Using the emergency stairs they make their way to the lower levels and find the excavation where the tunnel was being cut. Robot Twelve detects a concentration of life forms within the area, one of which is human. The Doctor leaves the robot behind and makes his way down the tunnel. In the ice he sees the carvings that interested the conservationists. Before he can look more closely he is confronted be a newborn alien, out hunting. The Doctor has used information from the robot to work out that the aliens use sound to detect their prey. He has adjusted his screwdriver to emit a sound that makes him invisible, and in this way passes on to the central chamber. A wave of mental energy alerts him to the relationship between the alien and the patients in the hospital. He relaxes his telepathic barriers enough to let the creature’s consciousness touch his for a second, unfortunately coinciding with a burst of mental force that renders him unconscious.

Rabley, now disguised as a holiday maker, tries to leave SnowGlobe 6 with the stolen cash but finds the exit closed and a mob of angry tourists blocking him. He briefly sees Beth Cowley and expects her to blow the whistle on him but she seems not to recognize him. Instead of behaving like a Director she is also rather furtive. He follows her into service corridor, hoping to reach an alternative exit. She leads him down a corridor that joins the two SnowGlobes before heading into a ventilator shaft.

Martha puts on a biohazard suit, intending to leave the safety of the mortuary to stop Cowley. As she leaves through the door, intending to run across the medical rooms to the exit, Harrison panics and tries to barge the door closed before she has passed through it, knocking her to the floor and winding her. The angry patients pounce on her and she is engulfed by the mob. As she is dragged down she sees Harrison pulled out of the doorway and enveloped in a cloud of gas that the patents are breathing out. Other patients are forcing their way in to get to Marisha and Ku’ra. The Doctor finds that he has fallen into a pit. He survived the drop by landing on the bloated body of a dead alien which was killed by gunfire but has been partly cannibalized by others. There are four human corpses with it, dismembered for food and beheaded to act as hosts to new aliens. Under a snowdrift he finds Maxwell O’Keefe, still alive. In a panic O’Keefe wrestles with the Doctor, knocking the sonic screwdriver to the frozen floor where it blinks out. The other aliens are immediately aware of his presence and arrive at the lip of the pit with their teeth bared.

Just as the creatures pounce Robot Twelve bursts through the ice wall and starts to fight them off, killing one. The Doctor grabs O’Keefe and pulls him to his feet. The adult monster attacks the robot and as the two struggle the robot manages to fling its assailant away and tell the Doctor that he must leave. The Doctor refuses, saying he needs the robot’s help to get O’Keefe out of there. The robot picks up O’Keefe and climbs out of the cavern, pursued by the Doctor and the recovering alien monsters. The Robot brings down a fall of rock and ice to block their assailants but this is only temporary. The Doctor uses the time this gives him to connect Robot Twelve to an anti-grav sled. Looking at O’Keefe he realises that the man has an incubating alien in his head and needs to get him back to a sickbay to get some answers.

The patients attacking Martha suddenly stop as if their motivation has been cut off. Ku’ra announces that the telepathic signal has been halted. The patients are sedated and returned to their beds, and then Martha decides to get after Cowley. Her exit from the ward is blocked by two security guards but Marisha overcomes them with a hypo full of tranquiliser. She suggests to Martha that the guards’ uniforms will come in useful as a disguise.

Rabley follows Cowley up through a ventilator shaft until she reaches the huge fans. She appears to be sleep-walking but at last she turns to tell Rabley that she can not only hear the creatures but talk to them, too. She steps through a doorway and shuts a grille, locking Rabley in the shaft. To his horror he hears one of the creatures coming up the shaft behind him.

Cowley emerges from the ventilation shafts into a room that leads out onto the back of the beach. She makes her way, still carrying her deadly load, to the water’s edge. She can sense the rage of the creatures in the dome and realises that their anger is targeted at the Doctor.

The Doctor, the robot and O’Keefe ride the sled through the service tunnels, almost to safety. One final creature bars their way. Trying to avoid it, the Doctor crashes the sled. Robot Twelve wrenches a steam pipe from the wall and the vapour literally cooks the creature into a scalded lump, showing that the creatures have little tolerance to heat. Martha and Ku’ra use their disguises to reach the exit from the dome. Ku’ra reads Captain Hassan’s mind to know how to deal with the sentry at the door who blocks their way. Stowing their uniforms under a bush they make their start on tracking down Cowley.

Rabley has fought off the creature that has him trapped by pelting it with anything that came to hand. This drove it down to its eventual death at the hands of the Doctor and the Robot. As Rabley returns down the shaft he overhears the Doctor talking to the robot, mentioning that O’Keefe is still alive. Rabley decides to abandon his plans: his employer’s inexplicable survival means he needs to replace the stolen money and hope to pick up where he left off.

The giant Robot Twelve bursts into the technical rooms of SnowGlobe Six. The Doctor follows, briefs technician Roberts on the situation: the Doctor needs to get O’Keefe to sickbay and Roberts needs to employ other robots with plasma heaters to set up a heat barrier to prevent the pursuing creatures entering the dome. The Doctor leaves him to it and arrives at the sickbay where it is his turn to be told all that has happened in his absence by Marisha. She helps him set up a neurological feedback integrator (a device that she has never heard of before). He uses a jumble of equipment to concoct a device that he connects to O’Keefe’s head, telling the nurse that he hopes to use the telepathic links between the embryo in O’Keefe’s head and the parent creature to help him access the older creature’s mind. In this way he can find out what the creatures are, where they came from and how they came to Earth in the first place.

Ku’ra’s mixed parentage has weakened his telepathic abilities in general but given him a stronger understanding of the human mind. On the beach he drops into a trance to try to locate Cowley. Martha wanders off down the beach where it strikes her that the skulls in Cowley’s case must have decayed into dust by now. As such she will need the wind to disperse it. Just as she is turning to look up to the top of the Burj Al-Arab hotel Ku’ra races up to say he has located Cowley.

Inside the mind of the creature the Doctor is learning of their origins. They come from a cold and hostile world with at least six moons. They call themselves the Gappa and hunted their prey so successfully that only cannibalism was left. By the time that only one Gappa survived a humanoid race of space travellers arrived and took the lone survivor to Earth, but their ship crashed. The Gappa was alone among Homo sapiens and at first it killed and fed, but the humans had already discovered fire which they used to drive it underground where it was buried under a fall of ice.

Roberts has used the plasma heaters to create a wall of heat that is keeping the Gappa at bay until two security guards enter the technical area. They rush to the hole in the wall and see the Gappa howling impotently in the corridor beyond. One of the guards throws a grenade before Roberts can stop him which damages the heaters. With a roar of triumph the Gappa come through the gap and kill the guard. In the confusion Rabley has made it into the workshop from the corridor and grabs an aerosol on his way through to the other side and out of the door into the SnowGlobe.

Martha and Ku’ra run into the lobby of the hotel and take a lift to the penthouse. The door is open, as is the balcony door. They see a spiral stair to the landing pad on the roof. Up there they find Cowley. In a dreamy voice she tells them that the Gappa deserve the Earth more than the humans that have destroyed the planet and that her legacy is to ensure the Gappas’ survival. As Ku’ra makes a desperate attempt to reach her, Cowley steps back, trips and falls over the edge, the case flipping open and wreathing her in dust.

The Doctor awakes from his communication with the Gappa. He tells Marisha that they are a hermaphrodite species from Hydropellica Hydroxi on the other side of the galaxy. He is astonished to find that he has been communicating for hours. He is taken down to see Captain Hassan who tells him what has been happening in the time that he was ‘asleep’. The patients from the ward are being taken away in metal capsules. O’Keefe has had a seizure and is locked in the morgue. The dust that Cowley dispersed has infected thousands of people who are outside SnowGlobe 7 howling to be let in. The Gappa that attacked SnowGlobe 6 were driven off by guards with flamethrowers and are also back in SnowGlobe 7. The King has ordered an airstrike to destroy the dome and kill all the creatures within. The Doctor argues that this is not the way and pleads to be allowed to get back to the TARDIS but Hassan orders him to be held as a prisoner.

Rather than replace the money Rabley has decided to track down O’Keefe. Using the aerosol of red paint to feign injury he has himself taken to the sickbay where the last of the patients are being wheeled away in their metal containers. There is no sign of O’Keefe in the sickbay but Rabley is sure he must have been taken there so he picks the lock on the door to the morgue and finds a drawer with O’Keefe’s name on it. He hears a knocking in the drawer and opens it. A young Gappa bursts out. As the guards race to the incident the Doctor uses the diversion as a chance to escape.

He runs down to the foyer and sees two things he needs: an army ambulance and the deactivated Robot Twelve. Using his sonic screwdriver, he reprograms the robot which bursts into life, smashing everything around it and shouting that it intends to destroy everything. In the chaos and confusion the Doctor gets into the ambulance and drives through the main door as the robot clambers into the back. Robot Twelve is annoyed, saying that it cannot harm humans so the reprogramming of its voice control was a lie. The Doctor merely grins in reply as he drives across to SnowGlobe 7. He has to swerve around many possessed people to get there. He crashes through the perimeter fence and then smashes into the main doors of the dome.

All of this is watched by Martha and Ku’ra. Unfortunately, at that moment the wind changes. Dust blows back into Ku’ra’s face and he immediately snarls and launches himself at Martha.

The Doctor realises that the ambulance is wedged in the doorway, effectively barring the crowds that are trying to get in. Twelve steps from the wreckage by peeling back the roof. They open the airlock doors and step into the blizzard. Outside a Eurofighter is screaming towards the dome. Two Gappa are guarding the TARDIS but as they attack the robot grabs them and crushes them to death. Inside the TARDIS Robot Twelve expresses perplexity that the interior dimensions do not correlate with the exterior. The Doctor quickly enters details into the TARDIS console and then links his mind with the telepathic controls of the TARDIS. He pushes one final button.

Outside, the possessed humans fall to the floor. On the hotel roof Ku’ra collapses, too. Suddenly a brilliant white light erupts from within the dome of SnowGlobe 7, just before the Eurofighter launches its attack. Inside the dome the ice turns to steam, and the Gappa are vapourised.

Martha leaves Ku’ra to sleep in the penthouse. She makes her way through the summer rain that the Doctor has caused to fall on Dubai and arrives at the wreckage of the SnowGlobe. Outside, the first medical teams are discovering that all trace of the Gappa has disappeared from the humans. Inside, the Doctor is standing beside the TARDIS. He tells Martha that the ice that contained the lone Gappa had also held the fusion core of the crashed spaceship. It was this that he blew up to destroy the dome. He is saddened by the fate of the Gappa, he had hoped to save them, but he knows that they were an evolutionary dead end. Martha asks him if he knows who the misguided humanoids were that brought the Gappa to Earth 100,000 years earlier but he says he hasn’t a clue. As she asks if he wants to find out Robot Twelve squeezes out from the TARDIS.