The film involved some of the top moviemaking professionals of the time and is considered a classic.
Major contributors included Hollywood stuntmanYakima Canutt, who, as second-unit director, shot most of the action scenes; British stuntman Alf Joint, who doubled for Burton in such sequences as the fight on top of the cable car; award-winning conductor and composer Ron Goodwin, who wrote the film score, and future Oscar-nominee Arthur Ibbetson, who worked on its cinematography. The film is noted for the phrase Broadsword calling Danny Boy, used by Richard Burton several times throughout.
U.S. Army Brigadier General George Carnaby (Robert Beatty), a chief planner of the second front, is captured by the Germans when his aircraft is shot down en route to Crete. He is taken for interrogation to the Schloss Adler, a fortress high in the Alps of southern Bavaria. A team of commandos, led by Major John Smith (Richard Burton) and U.S. Army Ranger Lieutenant Morris Schaffer (Clint Eastwood), is briefed by ColonelTurner (Patrick Wymark) and Admiral Rolland (Michael Hordern) of MI6. Their mission is to parachute in, infiltrate the castle, and rescue General Carnaby before the Germans can interrogate him. MI6 Agent Mary Elison (Mary Ure) accompanies the mission in secret, her presence known only to Major Smith.
Early in the mission, the two NCOs, MacPherson (Neil McCarthy) and radio operator Harrod (Brook Williams), are mysteriously killed; but Major Smith is unperturbed, keeping Lt. Schaffer as a close ally and secretly updating Rolland and Turner on developments by radio. After seeming to give up and allowing themselves to be captured, Maj. Smith and Schaffer, being officers, are separated from the three remaining members of the group — 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 before hitching a ride on a cable car—the only approach to the castle. Mary, posing as a maid, had been brought into the castle by Heidi (Ingrid Pitt), a deep-cover MI6 agent working as a barmaid in the nearby village; Major von Hapen (Derren Nesbitt), a Gestapo officer whom Heidi has been cultivating, becomes infatuated with her. Mary helps Schaffer and Smith to climb up a rope and in through a window overlooking the castle's cable car station.
Carnaby's interrogation, carried out by General Rosemeyer (Ferdy Mayne) and Colonel Kramer (Anton Diffring), is underway when the three NCOs arrive and reveal themselves to be German double agents. Smith and Schaffer intrude, but Smith then forces Schaffer to disarm and establishes himself as Major Johann Schmidt of the SD, the intelligence branch of the SS. He exposes the true identity of "General Carnaby" — that of Cartwright Jones, an American Corporal. He also claims that Thomas, Berkeley and Christiansen are British impostors. To test them, Smith/Schmidt proposes they write down the names of their fellow agents/conspirators in Britain, to be compared to the personal list in his pocket (having discreetly divulged the name of Germany's top agent in Britain to Kramer, who silently affirms it). After the three finish their lists, Smith reveals his own to Kramer, which is in fact blank. When Kramer realises he has been bluffed, Smith and Schaffer re-secure the room, the former finally revealing the mission's true objective: to uncover the identities of German spies operating in Britain.
Meanwhile, Mary, while preparing the explosives, is visited by von Hapen. He takes her to the castle's cafe and persuades her to recite the tale of her assumed identity. Finding faults in her story, he investigates and happens upon the meeting over Carnaby's interrogation just as Smith finishes his explanation. Von Hapen puts the room under arrest, but is distracted by Smith crossing between himself and Schaffer, enabling Schaffer to draw his silenced side arm and kill him and the other German officers. The group then escapes with Thomas, Berkeley and Christiansen as prisoners. Schaffer sets explosives to create diversions around the compound, while Smith leads the group to the radio room, where he informs Rolland of their success. They then battle their way to the cable car station, sacrificing Thomas as a decoy. Berkeley and Christiansen attempt their own escape in a cable car, but Smith, after a roof-top fight with its passengers, destroys the car with an explosive, hurling himself onto a returning cable car and subsequently riding back down with the others. They abandon the car mid-descent to avoid a party of armed Germans and to reunite with Heidi, boarding the bus they had prepared earlier as their escape vehicle. With enemy soldiers in hot pursuit, they wreak havoc by exploding booby traps on the road to thwart the pursuers and finally escape on a disguised extraction plane, where Col. Turner is waiting for them.
Smith briefs Turner on the mission and confirms a suspicion he and Rolland had shared since before the start: that Turner is the Nazi's top agent in Britain, whose name the late Colonel Kramer had agreed to before. Turner had been lured into participating so MI6 could expose him, with Smith's trusted partner Mary and the American Schaffer, who had no connection to MI6, specially assigned to the team to ensure the mission's success. Turner intends to shoot Smith with a STEN Gun given to him by Rolland, only to be told by Smith that the firing pin had been previously removed. Deciding to save face, Turner is permitted by Smith to commit suicide by jumping out of the plane without a parachute.
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