The time is the late '60's, deep in the chill of the Cold War. As observers in both Russia and the USA watch closely, an orbiting spy satellite re-enters the atmosphere, parachuting down to a soft landing in the desolate Arctic wastes. Soon, a hooded figure fights his way through the ice-storm and retrieves the capsule, unaware that, hidden nearby, another figure is watching his every move.
Not long afterwards, the world is shocked to learn that Drift Ice Staton Zebra, a scientific weather station at the North Pole, has met with disaster. The weak distress calls first issued have fallen silent, and the nuclear sub Tigerfish, commanded by James Ferraday (Rock Hudson) is being sent to attempt a rescue. Oddly, the mission also includes a platoon of Marines, as well as a mysterious British agent known only as Mr. Jones (Patrick McGoohan), and a boisterous Russian defector, Boris Vaslov (Ernest Borgnine). Ferraday's orders are to get Mr. Jones to Zebra as swiftly as possible, but without any explanation as to why, a situation he is none too pleased with.
Submerging, the submarine travels under the Polar ice cap, hoping to break through the thin ice near Zebra. As they near their destination, a torpedo is readied in order to blow a hole through the ice. Checking the torpedo tube, a slight trickle of seawater is noted draining from it, but all indicators show clear. The hatch is hard to open, and as the crewman thrusts against the handle, it suddenly slams open, ice-cold seawater flooding the torpedo room.
Fighting for their lives, the men struggle to close the outer hatch while Ferraday orders engines full astern, hoping to prevent the crippled boat from heading for the bottom. Pumping pressurized air into the flooded forward rooms and red-lining the engines does little, and the Tigerfish plummets deeper and deeper into the darkness. Passing far below their dive limit, the outer torpedo door finally begins to close, and once sealed the boat slows its deadly descent. At extreme depth, and withstanding incredible pressures, they manage to halt the dive, and slowly head back up. Badly shaken, they are further stunned at Jones' claim that this was no accident. Someone very capable had sabotaged the tube controls, and tried to murder them all.
Ferraday has had enough of Jones and Vaslov's cloak-and-dagger tactics and demands to know what the hell they are really doing here. "You seem to know an awful lot about the business of operating a submarine", snarls Ferraday.
"I know how to wreck them," replies Jones, shivering under a blanket. The situation begins to turn ugly, but the sonar man breaks in to report they are now under Zebra's position. Disgusted, Ferraday leaves for the control room.
With consummate skill, the crew locate and surface through a relatively thin layer of ice, exposing the submarine's conning tower to the blast of an Arctic ice storm. Quickly taking a fix in Zebra's weak signal, a rescue team sets out across the rugged ice pack, led by Jones, Vaslov and Ferraday. Only a few miles need to be covered, but the way is filled with danger, and several men are nearly lost when an unseen crevasse opens up at their feet. Tied together for safety, the other manage to pull the trapped men out, but back at the Tigerfish, the ice pack is shifting, squeezing the submarine. Unable to contact Zebra or the rescue party, they are forced to submerge once more.
As they near the station, Jones can smell burned rubber in the tearing wind. Finally stumbling into the camp, they see through the storm that the station is in a shambles. Several of the huts are wrecked or burned, and there seem to be no survivors. Cautiously searching the huts they find one is still intact and sheltering a few numbed, barely alive scientists, and these are quickly warmed and revived. Jones and Vaslov both find familiar faces and begin to probe for answers as to what happened, but the frozen men are still too much in shock to provide any information. Leaving the scientists to the rescue team, Jones begins to hunt through the rest of the camp, desperately searching for something. In what's left of the laboratory, he turns to find Ferraday watching him.
"What size is the film?" Ferraday asks.
Surprised, Jones replies that it's a standard film canister, "And all of a sudden, you know a whole damn lot about my business." Ferraday confesses that he has been briefed on the real nature of their mission, despite all the precautions taken by Jones, so he continues to elaborate. The film he's searching for came from a Russian spy satellite, equipped with a special camera stolen from the British, and using a new type of film developed by the Americans, "all very hush-hush," says Jones. Launched by the Russians, it was capable of taking pictures of incredible detail, and it had photos of all missile bases in North America. Unfortunately for the Russians, something went wrong, "and that darling little satellite just kept on taking pictures of Soviet missile bases, even when it wasn't supposed to, making it the most valuable piece of film in the world." Attempting a recovery, the satellite merely shifted to a new polar orbit, meaning that the only choice the Russians had to bring it down was over the North Pole. The British sent an agent up to Zebra, as did the Russians, and the satellite was brought down. "Then disaster, fire, silence," mused Jones, "and here we are. No agents, no clues, and no film." As they think about their next move, a deathly silence settles over the camp: the storm is finally dying down.
Ferraday uses an ice drill from the tool shed, and has his men drill a small hole in the ice a short distance from the camp. Dropping a transponder down the hole, they are quickly located by the Tigerfish which surfaces through the ice a few minutes later. "I want a hundred men out here on the double," orders Ferraday. The entire crew is sent out through the camp and across the ice, looking for the hidden film. Meanwhile Jones has discovered a homing device hidden in the fuel tank of the camp's tractor, and he turns to leave the hut.
A crowbar strikes him across the chest, flinging him back across the room and knocking him out. The attacker cautiously approaches and pulls his hood back; it is Vaslov, the Russian defector, and Jones' supposed friend. Now revealed as a double-agent, he takes the homing device to search for the hidden film but is caught by the Captain of the marine platoon, and they fight to the death. Just as the marine gains the advantage, shots ring out and he falls to the floor, dead. Jones had come to and shot the marine, thinking he was the traitor. Ferraday and the others hear the shots and come running in before Vaslov can regain the homing device, and he quickly re-assumes his old character. As the others take the homer out to search, Ferraday looks over the fight scene with suspicion. Re-checking the tractor fuel tank, he finds another device wrapped in plastic.
The homer quickly finds the film canister, buried in the ice a few yards from the camp. Torches are brought out to melt the ice away, but another problem looms. The clearing storm has allowed the Russians to fly over the camp and drop a squadron of paratroops to take the camp and the film by force. As the Russians approach the camp, Ferraday orders most of the men back into the submarine. The Russian Captain steps quietly up to the edge of the camp, and calmly asks for Ferraday by name, claiming the satellite is Soviet property and stating that they wish only to retrieve what is rightfully theirs. A tense situation ensues, and Ferraday stalls for time while the canister is melted out of the ice. A tricky self-destruct device had been attached to the canister, making things even more difficult. The Russian captain gives Ferraday two minutes to turn over the canister.
Vaslov, still playing the double agent, disarms the small bomb and opens the canister removing the priceless film. Still shaky, Jones walks up behind and observes. Ferraday calmly re-arms the bomb and carries the canister to the Russian captain, tossing it lightly to him. But the captain is no fool, he too disarms the bomb and checks the canister and when he finds it is empty, an order is barked and the Russians open fire.
Smoke bombs are fired into the camp, filling the air with dense fog and hiding everything. Vaslov sees his chance in the confusion, and runs for the Russian troops outside the camp. Another man looms up in his path: Jones. Smiling grimly at each other, they both realize the charade is over, and they begin a fight to the death as machine gun fire whistles all around them. As the smoke clears and everyone is ordered to hold their fire, Vaslov is lying on the ice and Jones is holding the film.
Covered by the Russians, Jones cannot escape. Ferraday takes control of the situation and orders Jones to drop the film. Shooting a venomous look at Ferraday, Jones is forced to toss the film at the Russians in defeat. The film is quickly returned to the canister, still attached to its self-destruct unit, and run up a thin line attached to a weather balloon for collection by a circling Russian plane. As the aircraft approaches, Ferraday turns his hand and reveals the object he'd found hidden in the fuel tank: a remote trigger for the self-destruct unit on the canister. Jones looks at the device and then at Ferraday, understanding dawning on his face.
Just as the Soviet plane swoops down to snare the canister, Ferraday triggers the bomb. In a brilliant flash, the capsule and its film are destroyed, startling everyone. The Russian captain looks across to Ferraday, also beginning to understand. Stepping back across the ice, they contemplate each other for a moment.
"Our mission here is completed" he says, "at least in part."
"Agreed", replies Ferraday. With the precious film now gone, both sides realize it is time to leave. The Americans move to pick up their wounded, and Ferraday tells the Russian that "you sir, may pick up your man", indicating Vaslov, still laying on the ice. They separate, and make ready to depart.
The Tigerfish, carrying the Zebra survivors and wounded marines, submerges once more and begins its long voyage back home.