In the winter of 1943–44, a team of seven Allied Special Operations Executive commandos, led by British Major John Smith of the Royal Engineers, with one man being U.S. Army Ranger Lieutenant Morris Schaffer, are briefed by Colonel Turner and Vice Admiral Rolland of MI6 that U.S. Army Brigadier General George Carnaby, a chief planner for the Western Front, has been captured by the Germans after his plane was shot down and is being held at the Schloß Adler, a mountaintop fortress in the town of Werfen accessible only by cable car. Disguised as Wehrmacht mountain troops, they are to parachute into the German Alps, and infiltrate the castle to rescue Carnaby before the Germans can extract any information from him. After their transport plane drops them off, a woman comes out of hiding in the rear of the plane and parachutes out of sight of the others. On the ground, Smith and the others find one of them, Harrod, is dead, his neck broken. Smith finds a bruise at the back of his head and determines his neck was broken by someone to make it look like an accident. Making their way to an outpost, Smith secretly meets with the woman, agent Mary Ellison, his lover, and arranges to meet her at a woodshed in Werfen the next night.

The next day, the team see the fortress; Smith continues the operation, keeping Schaffer as a close ally and secretly updating Rolland and Turner by radio. German General der Gebirgstruppe Rosemeyer arrives to SS-Standartenführer Kramer. Infiltrating the village. Smith meets up with Heidi Schmidt, the top agent in the area since 1941, posing as a barmaid. Heidi has arranged for Mary, posing as her cousin Maria, to work temporarily at the castle so the commandos can gain access. He reveals to Mary that the crash was arranged and that Carnaby is actually an American corporal named Cartwright Jones, an ex-actor and lookalike of Carnaby trained to impersonate him. Smith finds a dying MacPherson, and begins to reveal all to the suspicious Schaffer. Major von Hapen, a Gestapo officer, attracted to Mary, offers to escort her and Heidi to the castle in the cable car. The Germans, tipped off to the operation, surround the commandos in a gasthaus. Smith and Schaffer decide to take their chances by surrendering. Smith and Schaffer are separated from the rest of the team, Thomas, Berkeley, and Christiansen. Smith and Schaffer kill their captors, blow up a supply depot, and prepare an escape route. They reach the castle by riding on the roof of a cable car and climb inside using a rope lowered by Mary. Kramer and von Hapen have a conflict, due to Kramer's tipping off his staff.

Rosemeyer and Kramer interrogate Carnaby when the three operatives, who claim to be double agents working for the Germans, arrive. When Kramer suggests for his secretary, trained nurse Anne-Marie Kernitser to use truth serum on Carnaby, Smith and Schaffer intrude, weapons drawn, but Smith forces Schaffer to drop his weapon. He identifies himself as SS-Sturmbannführer Johann Schmidt of the SD, the SS intelligence branch and shows Kramer the name of Germany's top agent in Britain. Kramer affirms it and calls a high-ranking officer on Kesselring's staff who confirms that Smith is indeed Schmidt. To ensure the three agents are who they say they are, Schmidt proposes that they write down the names of their fellow agents in Britain, to be compared to the list he has in his pocket. After the three finish their lists, Schmidt kills a guard and reveals that he was bluffing; he is in fact a double agent for the Allies and that obtaining the list of agents was the mission's true objective. Mary is visited by von Hapen, who is suspicious of flaws in her cover story, and stumbles upon Carnaby's interrogation just as Smith finishes his explanation. The suspicious von Hapen orders Kramer to "Sit down, Colonel!" as Smith tries to say he and Schaffer have uncovered a plot to assassinate Hitler and the enraged Rosemeyer exclaims, "This is preposterous!" Smith keeps von Hapen occupied before Mary arrives. Schaffer seizes the distraction to kill von Hapen, Kramer, Rosemeyer and Kernitser with his silenced pistol. The group then make their escape with Jones and the German agents. Schaffer sets a series of explosives to create diversions around the castle while Smith leads the group to the radio room, where he informs Rolland of their success and asks for transport. During the escape, Thomas is sacrificed as a decoy, and Berkeley and Christiansen both attempt to escape before Smith kills them. The team reunites with Heidi on the ground, boarding a Reichspost bus they had prepared earlier. They battle their way onto a nearby airfield and take off in their transport plane.

On the plane, Smith reveals to Turner that Kramer named him as Germany's top agent in Britain. Rolland lured Turner and the others into participating in the fake mission so that MI6 could expose them; Smith's trusted partner Mary and the American Schaffer, who had no connection to MI6, had been assigned to the mission to ensure its success. Turner aims a Sten gun at Smith, who reveals that Rolland had the gun's firing pin removed earlier. Rather than have Turner face execution, and public humiliation for MI6, Smith permits Turner to jump out of the plane to his death, but not before he takes the books from him. Schaffer half-jokingly asks Smith to keep his next mission "an all-British operation", as the two men sit exhausted.