The Doctor takes Leela to northeastern America, in the belief that she will find much in common with the native tribes; however, the power pylons crossing the landscape indicate that he has miscalculated and arrived centuries too late. It is nearly Thanksgiving in New Hampshire, but the weather is bad and the Doctor and Leela emerge into a terrible blizzard. This is Leela’s first experience with snow, and she doesn’t care for it, but when the Doctor tries to retrace his steps back to the TARDIS he finds that he’s lost his bearings. A pack of coyotes rushes past, apparently spooked by something, and the Doctor and Leela hear gunfire from the opposite direction. They investigate to discover that an elite military squadron, White Shadow, has just moved in on a cottage occupied by UFO cultists -- but the gunfire wasn’t aimed at them, and when they break in, they find that the cultists have all apparently vanished.

The Doctor introduces himself as UNIT’s scientific advisor, and Captain Morgan Shaw decides to accept his help. Leela quickly befriends the psychic Lieutenant Kristal Owl Eye Wildcat of the Pasamaquoddy, while the Doctor questions Lieutenant Joanna Hmieleski and learns that the team is searching for the remains of a plane which went down over this area, carrying something reverse-engineered from alien technology. Kristal sensed some fragments in this cottage, and the team assumed that the cultists had recovered the device -- but not only have the cultists all disappeared, the device itself isn’t here. Shaw deduces that some cultists may still be out on the mountain, searching for more fragments. He thus divides his forces; some will stay at the cottage with Hmieleski to investigate the cult’s plans, while Kristal leads a team out to the mountain to look for the “Stormcore”. Leela accompanies Kristal, while the Doctor and Shaw go to the local town, Melvin Village, to set up a base for White Shadow’s operations.

Captain Shaw was actually born in Melvin Village, and his brother, Makenzie, is now the town’s police chief. Makenzie is in love with Martha Mailloux, a Southerner who recently moved to Melvin Village with her sullen ten-year-old daughter Amber. Martha frequently moves around the country in an attempt to make a clean break from her alcoholic ex-husband Curt Redeker, but she can’t deny his right to visit Amber, especially on Thanksgiving; however, Curt is already two days late, and Amber, upset by this betrayal, storms out of the house which Martha now shares with Makenzie. While wandering in the woods, she finds an abandoned parachute, hides it and goes to town to tell someone of her discovery. However, she’s been observed by two CIA agents, Melody Quartararo and Parker Theroux, who recover the parachute and contact Martha, asking to speak to Amber to find out if she knows what happened to the missing pilot and the Stormcore. Martha accompanies them into town, upset with Amber for getting herself into trouble again.

The bad weather has effectively cut off the village, but despite the risk some tourists and cottage owners decide to make a run for the highway. Makenzie and his deputy, Laurie Aldrich, head out to check on them and find a convoy of abandoned cars on the road outside town. One of the cars is particularly disturbing to Makenzie, as it bears Oklahoma licence plates and is facing towards town rather than away, having apparently swerved off the road to avoid the other abandoned cars; furthermore, the inside smells of bourbon and contains an abandoned present addressed to Amber. It seems that Curt Redeker made it to town after all -- but he’s vanished into the storm, and when Makenzie next turns around, so has Laurie. Makenzie is unable to find her, and must return to town for reinforcements.

There are still two cultists out in the wilderness; Emilie Jacks and Mitch Lagoy, who have found the Stormcore and are returning to the cottage with it. However, on the way back they encounter their leader, Crayford Boyle, who’s running from the cottage in terror; the cult finally succeeded in summoning the aliens he’d worshipped, but they weren’t friendly after all. Emilie realises that Crayford has snapped from fear, and sees this as a betrayal of all he’d convinced her to believe in. She thus abandons the Stormcore to Boyle, hoping that he will act as a decoy, while she and the reluctant Lagoy storm the cottage, intending to destroy both the hostile aliens and the interfering Army soldiers as well. They attack the cottage and take Hmieleski hostage, shooting Private Paul Falvi when he attempts to intervene. Lagoy is also injured in the exchange of gunfire, but Hmieleski orders the others to stand down rather than risk further injuries. Jacks drags her off into the storm, the wounded Lagoy following.

Kristal’s psychic sensitivity leads her towards Boyle, but unfortunately, something else finds him first -- something which seizes control of the Stormcore and sends a wall of snow rushing towards Kristal’s team. Sensing the danger posed by the blizzard, Kristal orders her team to dig themselves into the snow; they thus weather out the storm, although the driving wind and snow destroys their snowmobiles. Kristal reports in to Morgan that there is a new spirit on the mountain, something too large to be fully understood. Unsure how to interpret this report, Morgan orders her and Leela to pursue the escaping cultists and rescue Hmieleski. Meanwhile, the rest of Hmieleski’s team makes a makeshift sledge/stretcher and tries to take the injured Falvi to town for treatment -- but the soldiers all vanish into the storm, one by one...

The Doctor has learned more about the Stormcore, and doesn’t like what he’s heard. According to Morgan, Kristal had established psychic communion with the device, which was apparently an alien navigational system, but she lost contact when the plane went down. The original plan was to use the Stormcore to control the weather, but the Doctor suspects that the military’s motives weren’t as benign as Morgan claims -- and he fears that the Stormcore may now be under the control of something even better at controlling the weather than the human race.

When the convoy arrives in town, Morgan and Makenzie fall into one of their familiar, bitter arguments. Morgan got out of this small town as quickly as he could, while Morgan remained to serve the people he’d grown up with; however, the people of Melvin seem to regard Morgan as the more successful one, which frustrates Makenzie. Amber arrives in town and see them arguing, and although she can’t put it into words, she realises that the brothers are angry because of their love for each other; upset, she claims to have seen the parachute which Morgan is looking for, but only the Doctor believes her. After speaking to Amber, he visits the hotel, where White Shadow’s engineer, Irving Pydych, is trying to reassemble the fragments of the crashed plane to get some idea of what happened to it. Several parts of the plane are still missing, however, and the evidence suggests that they disintegrated in mid-air. Pydych explains that Kristal lost communion with the Stormcore when the plane went down, which suggests that their mental connection was being jammed somehow. The Doctor realises that this may be connected to his own inability to locate the TARDIS.

Martha and the CIA agents arrive in town, and while Martha and Melody search for Amber, Parker visits the general store to buy some candy for the girl. He walks into the middle of a hostage situation; Curt Redeker didn’t vanish into the storm after all. Drunk, he managed to stagger through the biting snowstorm into town, only to realise that he’d forgotten Amber’s present back in his car. He thus tried to steal toys from the store, and when the owner, Hal Byers, confronted him, Curt pulled out a gun. Melody eventually comes looking for her partner, and Parker manages to distract Curt long enough for Melody to get the drop on him. Curt tries to shoot her anyway, and Melody is forced to shoot and kill him first.

While Makenzie investigates the sound of gunfire, the Doctor breaks into the CIA agents’ 4x4, where he finds a graviton distortion sensor and evidence that these agents have a particular interest in cases with an extra-terrestrial element to them. Meanwhile, Makenzie recognises Curt and is forced to break the news to Martha. When Amber learns that her father is dead, she runs away before Makenzie can warn her that this storm is particularly dangerous. Martha, upset by this disruption to her life, misinterprets Makenzie’s fear as exasperation, and blames him for not taking responsibility for Amber.

Jacks eventually abandons the dying Lagoy to the snow, and forces Hmieleski to continue on. They find their way to a cabin, and Jacks breaks in to steal the keys to the truck. There are two civilians in the cabin -- Charlene Lowell and Gary Wentz, who are having an affair -- and although Hmieleski tries to warn them not to interfere, Gary attacks Jacks and is shot dead. Jacks knocks out the sobbing Charlene and forces Hmieleski out to the truck, where she also knocks her hostage unconscious, angered by the lieutenant’s self-righteous attitude. As they drive off, Charlene recovers and uses Gary’s CB radio to call into town for help.

The Doctor notes that the ice crystals on Curt’s body aren’t melting as quickly as they should. Melody conducts an autopsy in the hotel, and finds that Curt’s nervous system was slowly being replaced by ice; it appears that he was infected by something in the storm, perhaps the same force which is responsible for the other disappearances. The Doctor warns Morgan and Makenzie of the danger, and joins the search for Amber, whom they eventually hear screaming for help from the Walsh home. Amber entered the house to get away from a pack of hungry coyotes, but the Walshes are missing, the tropical fish tank has frozen and burst open, and a coyote has entered the house through the dog-flap. It abandons its attempt to lick the frozen fish out of the carpet and pursues Amber to the bathroom -- but as the soldiers approach, the coyote goes into convulsions and dissolves into a storm of thrashing ice tendrils. Some of the tendrils latch into the soldiers Bertelli and Spence, and the ice eats their flesh away, transforming them into more ice. The Doctor sends the others to safety while he rescues Amber, driving the living ice back with a flare. As soon as the Doctor and Amber are safe, Lt. Dermot Beard fires a white phosphor grenade into the house, destroying the ice creature. But there are bound to be more out there.

The Doctor takes Amber back to the hotel, assuming that she’s in shock. He then compares notes with Melody and Parker, and theorises that the alcohol in Curt’s system slowed the progress of the ice; but was it due to the alcohol’s chemical or neurological effects? The Doctor decides to warn the others on the mountain of the real danger, and Morgan directs him to the Wentz cabin. Kristal and Leela have followed Jacks’ trail to the cabin; Lagoy has vanished, but Kristal has sensed his passing and realises that he was taken apart by something which itself dispersed on the winds. Kristal takes the second car from the cabin and she and Leela continue the pursuit. Some way ahead, Jacks, believing that she’s shaken off her pursuers, decides that she no longer needs a hostage and tries to throw Hmieleski out of the moving truck. Hmieleski is suffering from concussion, but realises what’s happening and fights back. In the confusion, the truck goes off the road and crashes -- and is then attacked by the living ice.

Morgan tells the surviving townspeople that the coyotes have brought an infection into town, but as they prepare to evacuate, he realises that the storm is getting worse and snowdrifts are encircling the town. All he’s doing is keeping people busy and staving off the inevitable. Meanwhile, Martha confronts Makenzie, demanding that he fulfill his responsibility to her and Amber by getting them out of town -- but he also has responsibilities to the town which he can’t ignore. As he argues with Martha, he sees that Amber’s hand is frostbitten, possibly infected, but he can’t bring himself to tell Martha, and as he sets off to find the Doctor, Martha decides to get Amber out of town herself. Her daughter protests at being uprooted, bundled up and dragged away from her home yet again, but Martha steals the CIA agents’ car and drives out of town -- only to be forced off the road by a snowdrift. The car ends up on the lake, where the frozen surface gives way beneath it. Martha and Amber escape from the car before it sinks, but the living ice begins to lash out at them from the shoreline...

The Doctor, Kristal and Leela all catch up with Jacks and Hmieleski. Jacks is being eaten away by the ice, but the Doctor plans to rescue Hmieleski by using the spare gas cans from Kristal’s borrowed car to start a fire and drive back the attakcing ice. Despite the danger, Kristal tries to communicate psychically with the ice, but the creature is too alien and the attempt destroys her mind. The Doctor manages to pull Hmieleski out of the car, but the ice is already tangled in her hair; however, her concussion has slowed her neurological processes, and thus the progress of the ice is slowed as well. The Doctor hypnotises her, placing her in a trance, and does the same to Leela when she too is infected. He must leave Kristal’s body where it is, knowing that the ice will eventually disintegrate it.

Makenzie learns that Martha and Amber have gone, and follows them out to the road, where he sees their car sinking into the lake. The living ice retreats, wary of the open water, but Martha tries to force it back further and falls into the lake. Makenzie arrives just in time to pull her out, and Amber, who’d been blaming herself for foolishly wishing her mother dead, sees it all. The Doctor and the rest of the White Shadow soldiers from the mountain arrive, having taken a detour over the lake to avoid the hostile snowdrifts. While the others take the entranced Leela and Hmieleski back to the hotel, the Doctor questions Amber, whose infection turns out to be benign. Amber can sense what the ice creature is feeling; it isn’t hostile, it’s just trying to reach out to other people, unaware that any contact with other life forms is fatal to them. Makenzie privately explains to the Doctor that Amber has spent her entire life being dragged from one place to another by her mother, and seeks some sort of control over her own life.

The Doctor returns to town, where Parker reveals that he and Melody know the Doctor to be the legendary “Doctor John Smith” of UNIT, and know that he’s capable of cellular regeneration; thus, perhaps a transfusion of his blood can save Leela and Hmieleski. The Doctor happily volunteers to donate some of his blood, and then reveals that he has also guessed the agents’ secret; they are in fact the pilots of the alien ship which was shot down all those years ago, the ship which used the Stormcore as its navigational system. In order to get home, they got jobs in the government and began to investigate cases with extra-terrestrial elements, in the hope of one day getting close to their lost navigation system. It took the military several decades to find a use for the Stormcore, however, and now they’re facing with another, greater threat. Despite Parker’s objections, Melody agrees to help the Doctor destroy the threat of the ice creature, although this may mean losing the Stormcore forever.

A chance remark gives the Doctor an idea, and with Melody’s help, he proves that electrolysis can separate the alien crystals from the ice to which they have bonded. Makenzie and Morgan arrange to evacuate the townspeople across the frozen lake, and as Melody and Parker’s 4x4 is still stuck in the ice, the Doctor orders Morgan to wire it up to the town’s power pylons and blow a hole through the ice; he will take care of the rest. Before setting off to confront the living storm, he downs several scotches, in the hope that the alcohol will protect him from infection as it did for Curt Redeker. Parker sets off in pursuit, unwilling to risk the loss of the Stormcore, but Leela has now recovered and follows him, equally determined to prevent him from interfering with the Doctor’s plan, whatever it may be.

Living ice creatures rise from the snowdrifts encircling the town, and the surviving soldiers fight them off, many losing their lives, while giving the townspeople time to evacuate. Amber flees from the town, believing that she’s the only one who really understands the creature, and when the Doctor arrives at the core of the storm she’s there, trying to communicate with it. As the Doctor had suspected, the ice has the Stormcore suspended above the TARDIS, and it’s rejecting Amber’s attempts to reach out to it. He explains to her that the creature is raw, crystallised emotion, drawn to this world through a dimensional rift created by the Stormcore; it seeks a governing intelligence which will control the raging emotional impulses which it doesn’t understand. All it really wants is stability and control of its own life, and when the Doctor puts it like that, Amber finally understands it as well.

Leela fights off Parker, preventing him from interfering as the Doctor plants bombs around the TARDIS and carries Amber to safety. The TARDIS deflects the force of the explosion upwards, knocking the Stormcore out of the creature’s grasp and sending it spinning down the mountain towards the lake. Morgan’s team has done their work; the survivors are safe on the other side of the lake, the agents’ car has been wired to the power lines, and the surface of the lake has been blown open. When the avalanche hits the water, the snow melts and the alien crystals condense around the electrified 4x4. The creature thus falls dormant, and Morgan can arrange for the alien crystal to be taken to a place of safety.

Parker and Melody will have to look for another way off the planet, but that’s not so bad; at home they’re nobody special, but on Earth, they’re unique. Hmieleski blames herself for the deaths she was unable to prevent, and Morgan gives her time off to recover, hoping that she’ll return to his team afterwards. As the Doctor and Leela dig the TARDIS out of the snow, Parker takes Amber back to the recovering Martha, and to Makenzie, whom Amber now accepts as a member of her family.