In the winter of 1943–44, U.S. Army Brigadier General George Carnaby (Robert Beatty), a chief planner for the Western Front, is captured by the Germans. He is taken for interrogation to Schloß Adler (Hohenwerfen Castle), a mountaintop fortress accessible only by cable car. A team of seven Allied commandos, led by British Major John Smith of the Grenadier Guards (Richard Burton) and U.S. Army Ranger Lieutenant Morris Schaffer (Clint Eastwood), is briefed by Colonel Turner (Patrick Wymark) and Vice Admiral Rolland (Michael Hordern) of MI6. Disguised as German troops, they are to parachute in, enter the castle, and rescue General Carnaby before the Germans can interrogate him. After their Ju 52 transport plane drops them into Germany, Smith secretly meets Mary Ellison (Mary Ure) with whom it’s implied he is in a relationship and Heidi Schmidt (Ingrid Pitt), their presence known only to him; Heidi arranges for Mary to be a secretary at the castle.
Although two of the team are mysteriously killed, Smith continues the operation, keeping Schaffer as a close ally and secretly updating Rolland and Turner by radio. The Germans eventually surround the commandos in a tavern, and they are forced to surrender; Smith and Schaffer (being officers) are separated from the other three operatives, Thomas (William Squire), Berkeley (Peter Barkworth), and Christiansen (Donald Houston). Smith and Schaffer kill their captors, blow up a supply depot, and prepare an escape route for later use. They reach the castle by riding on the roof of a cable car, and climb inside when Mary lowers a rope.
German General Rosemeyer (Ferdy Mayne) and Standartenführer Kramer (Anton Diffring) are interrogating Carnaby when the four new prisoners arrive. Smith and Schaffer intrude, weapons drawn, but Smith forces Schaffer to disarm. He identifies himself as Sturmbannführer Johann Schmidt of the SD of the SS intelligence branch, and a spy too. As proof, he discreetly shows the name of Germany’s top agent in Britain to Kramer, who silently affirms it. To test them, Schmidt proposes that they write down the names of their fellow agents in Britain, to be compared to his own list in his pocket. After the three finish their lists, Smith reveals that he was bluffing and that the lists were the mission’s true objective.
Meanwhile, Mary is visited by Sturmbannführer (Major) von Hapen (Derren Nesbitt), a Gestapo officer attracted to her, but he becomes suspicious of flaws in her cover story. Leaving her, he happens upon the scene of Carnaby’s interrogation just as Smith finishes his explanation. Von Hapen puts everyone under arrest but is distracted when Mary arrives. Schaffer seizes the opportunity to kill von Hapen and the other German officers with his silenced pistol. The group then makes its escape, taking the American General (who is actually a corporal named Jones, a trained actor) and three British Captains to the escape route. Schaffer sets explosives to create diversions around the castle, while Smith leads the group to the radio room where he informs Rolland of their success. The group reunites with Heidi on the ground, boarding a bus they had prepared earlier as an escape vehicle. They battle their way on to an airfield and escape via their Ju 52 transport, where Turner has been waiting.
As Turner debriefs Smith about the mission, Smith reveals that the name Kramer confirmed as Germany’s top agent in Britain was Turner’s own. Rolland had lured Turner and the others into participating so 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 gun at Smith, who reveals that Rolland had the gun’s firing pin removed. Smith permits Turner to jump out of the airplane to his death rather than face trial for treason, and subsequent hanging.