At a top secret research base in Greenland during World War II, the scientists tell ribald stories to pass a long, cold night, while outside in the stables, Yvan Oluffson visits his beloved horses and expresses his regret over having to spend the war in such a frigid, out-of-the-way location. Suddenly there is a sound like a beating heart, the horses are spooked, and Oluffson begins to scream.
The Professor and Ace have arrived near the base. Ace is not pleased, as the Professor was supposed to be taking her to the south of France so she could recover from a cold. The Professor notices strange discoloration in the water, and the two set off to investigate.
While out looking for the vanished Oluffson, scientist Ray Connor discovers the Professor and Ace and takes them to the officer in charge of the base, Major Hendrix. Connor is sure they’re spies, but the Professor distracts Hendrix by pointing out that the fish have been mutated by something in the water. Gudrun, another of the scientists, reveals that a similarly changed whale beached itself recently; they had thought it was rotten, but closer examination revealed that it was still alive. She also lets slip about the disappearance of Oluffson, who was the base’s wireless operator until his disappearance while on patrol the night before. It is agreed that the Professor, Hendrix, and Connor will continue the search for Oluffson while Ace and Gudrun remain at the base.
As they relax over a meal, Gudrun explains to Ace how she fled Germany in 1937, appalled at the mass madness that seemed to be spreading in her homeland. England granted her asylum because of her scientific skills, and she was assigned to the base in Greenland when the war came. Ace tries to find out what the base’s purpose is, but Gudrun puts her off.
From the beach where Oluffson was last seen, the Professor and the others follow Oluffson’s usual patrol route to the stables. Connor heads back to the base as the Professor shows Major Hendrix the mutated fish and learns that the scientists are conducting “radar experiments.”
At the base, Connor asks Gudrun about the radar and learns that it is unchanged, with the same mysterious low-level interference. He asks Gudrun to get him something to eat, ignoring Ace’s insulted egalitarian protests. Connor is worried that without functioning radar, the enemy could capture them without any warning. Ace notices a pattern in the interference similar to the sound of the tides going in and out.
The Professor and Hendrix investigate the stables and find signs of a struggle, as well as the same strange material they saw on the beach. The Professor collects some samples to examine using the base’s facilities, and the two are about to return when Oluffson unexpectedly reappears. He explains that he must have hit his head when the storm blew the stable door open, and has been wandering around for hours. Refusing medical examination from the Professor, he insists that he’s fine.
Concerned about the possibility that the interference represents an imminent German attack, Hendrix has Ace and the Professor locked in the stockroom while they use the scientific equipment. The Professor knows that Oluffson’s story can’t be true; he should be dead from exposure. Discovering something alarming in the samples, the Professor sends Ace to get Gudrun’s help, only to discover their imprisonment. Ace examines the fish cells herself, and the Professor explains that they were taken over by some organism, the same way cold cells take over part of the human body.
As Connor complains to Oluffson and Gudrun about Hendrix’s refusal to treat the Professor and Ace as the security risk they are, Oluffson becomes increasingly distracted and abruptly flees, attacking the stockroom in an almost animal fashion. He’s looking for something, and the Professor explains the changes in his skin; he’s been taken over by the same thing that affected the fish. Oluffson ultimately breaks the outer wall and flees the base. Connor and Gudrun set to patching up the wall, and Hendrix wants to question the Professor. Once Connor finishes the wall, he claims that he’s about to check the rest of the base for damage, but instead he activates a transmitter, muttering “Where are you?” to some unknown accomplice…
Hendrix explains to the Professor that their research relates to a British effort to develop a sound-based camouflage; the effect they’re trying to obtain is similar to the one that is currently baffling their own radar. Early tests were encouraging, but they caused some sort of disruption that led to an enormous rockfall near the beach. Two men were killed. The experiments were ended, and the base staff is due to be relieved soon, as Whitehall believes the Germans may be aware of the research. Since the rockfall, the atmosphere of the entire area has changed; it’s almost as though someone else is there, someone unseen.
Gudrun discovers Connor’s secret transmitter, and although he begs her to keep it secret from Hendrix, she reveals it at once. Connor explains that he is an undercover security operative from the War Office. The others aren’t quite sure, but they have a larger problem: an entity was released during the sonic experiments, and now it has possessed Oluffson and is desperately searching for something, but no one can figure out precisely what.
The Professor asks Hendrix to take him to the site of the sonic experiments. Hendrix orders Gudrun to remind behind with Connor, but she protests that she doesn’t want to be left alone with a potential spy. Ace volunteers to stay behind with her, and over the Professor’s objections she does. If Oluffson comes back, they have a gun and can deal with him.
At the test site, Connor explains what happened during the last experiment. Hamer and Saxby, the two who died, fired the gun; Connor heard a loud noise like the frightened chatter of many people, and the rock the gun was aimed at exploded. It was the best result they’d gotten yet, but Hamer and Saxby were dead, apparently killed in the explosion. However, their skin displayed the same discoloration as Oluffson’s. Examining the remnants of the destroyed rock, the Professor realizes that it was hollow, and the test released something that had been alive within it.
He, Hendrix, and Connor visit Hamer and Saxby’s graves near the stables, and discover the abandoned body of Oluffson. The creature is gone. The Professor reveals that Hamer and Saxby were not killed in the explosion but merely overwhelmed by the creature. And now their bodies have disappeared.
Ace wonders if the radar blip is a nearby ship; perhaps Connor was contacting his Nazi superiors. Suddenly there is a strange knocking at the door; one of the horses seems to have escaped the stables. Gudrun leaves to lead it back down to the stables, but the horse has been possessed by the creature, and soon it moves to Gudrun.
The men return to the base and discover Gudrun’s absence. They’re worried that they didn’t pass her on the path, but she arrives moments later, apparently healthy. She goes off to prepare a meal as the Professor explains that the creature can’t sustain host bodies for long; they decay, and it must move on. Since Oluffson has been disposed of, the creature must have inhabited someone else. They consider the possibility that the creature was looking for the sound gun, but are interrupted by a break-in; a German submarine crew has just captured the base.
Lieutenant Kell, the leader of the Nazi force, demands that Hendrix surrender and give him the sonic weapon. The Professor tries to explain about the creature and its infection, but Kell laughs off his claims as a pathetic ruse. He also reveals that it is Gudrun, not Connor, who is the Nazi spy in their midst. Kell orders that Hendrix assemble the gun, and the others are locked up.
Trapped in the stockroom, the Professor tells Ace and Connor that the creature is now in Gudrun. When it possessed Oluffson, it tore apart the stockroom looking for the gun, then retreated to the stables, where the dim shreds of Oluffson’s memory told it that it could find other life-forms to inhabit. It then moved to one of the horses, and went off to get Gudrun’s attention. Now that it is trapped in Gudrun’s rapidly-decaying body, it is sticking with Gudrun’s Nazi guise because the Germans are its best chance of getting to the sonic gun. They still can’t figure out what the creature wants with the weapon, but if they can delay things long enough, Gudrun’s body will decay, and the German crew will realize that something is really going on.
As Kell makes conversation with what he believes to be a fellow Nazi, the creature inside Gudrun becomes increasingly strained and distracted. Desperate to get at the weapon, it explains about the deaths during the last test, and suggests that they ought to make one final experiment before turning the gun over to their superiors. Kell agrees. The creature selects a particular rock for the test site, explaining that it is an igneous rock, the hardest on the island and the best subject to prove the gun’s durability. Kell’s second-in-command, Hauser, reports that the gun is assembled, and they set off for the test site.
Back in the stockroom, Ace remembers that one of the outer walls is still weak from where Oluffson smashed through it, and she and Connor start dismantling it as quietly as possible. As they work, the Professor explains that they must delay the removal of the gun to the U-boat; if the creature escapes, there’s no telling how far its infection could spread.
At the site, Gudrun is looking sicker than ever, but adamantly refuses to leave; the creature must be there for the test. The gun slowly powers up, but as it nears 100% the Professor, Ace, and Connor arrive. Kell orders Hauser to shoot them, but the creature’s presence has now become obvious even to the Germans. The Professor tells Hendrix he must stop, but it’s too late; the gun is about to fire.
The weapon hits its target, and the creature is overjoyed, crying “Free!” A number of large insectoid aliens are visible within the area revealed by the explosion. The Professor deduces that these creatures must have traveled with their bodies stored separately from their minds. The earlier test had freed their hive mind, and that group consciousness had set about freeing its physical forms. And now it has succeeded.
Kell orders his men to fire on the aliens, but their weapons have little effect. The Professor moves in closer, determined to stop the conflict before it spreads. One of the creatures takes possession of Kell, and explains that it cannot depart; it must remain, and needs human energy to fuel its bodies. The Professor offers to repair their vessel, but the aliens are only interested in feeding on him. Nearby, Hendrix is badly injured, but he has set the sound gun to overload; the resulting explosion will destroy the aliens, but he has to remain behind to ensure that the failsafe doesn’t kick in. The Professor and Ace flee to a safe distance, and the weapon detonates, killing Hendrix, the Germans, and the aliens. With the crisis averted, the Professor and Ace depart, leaving Connor to await the British relief team and figure out how he’ll explain the mysterious events that have occurred on the island.