It is the winter of 1944. A lone transport plane, in German Luftwaffe insignia, picks its way through the peaks of the Bavarian Alps by night. Inside, seven paratroopers, in German Fallschirmjaeger uniform, wait in grim silence for the jump signal. Among them is their leader, Major Smith (Richard Burton) and his junior officer, Lieutenant Schaeffer (Clint Eastwood). The others: Christiansen (Donald Houston), Barkely (Peter Barkworth), Thomas (William Squire), Mac Pherson (Neil Mc Carthy) and Harrod (Brook Williams) are enlisted men. In the silent tension, Major Smith ponders his mission briefing.
Flashing back to headquarters in London, the Fallschirjaeger paratroops turn out to be a team of highly trained British MI6 agents (except for Shaeffer, who is on loan from the American OSS). Briefed by intelligence chief, Admiral Rolland (Michael Hordern), and his executive officer, Colonel Turner (Patrick Wymark), the team's mission is to rescue a captured American, General Carnaby (Robert Beatty), an Allied planner for Operation Overlord (D-Day). The General's plane has been shot down, and he is being held prisoner at the Schloss Adler (Castle of Eagles), an impregnable mountain-top fortress that also serves as headquarters for the German Alpine Korps. The prisoner must be rescued before he can divulge any information about Operation Overlord.
Back in the present, the plane reaches its drop zone and the seven commandos parachute into a snow-covered mountain wilderness. Unseen by any of the men, a mysterious woman, also in Fallschirmjaeger gear, emerges from the planes cargo compartment and jumps after them. Smith's team gathers on their landing zone, only to find that their radioman, Harrod, is missing. After a quick search, he is found dead, his neck apparently broken in the drop. However, Major Smith is suspicious. Though the radioman's neck is bruised, there are no logs or large rocks buried beneath the snow to account for such an injury. He keeps silent, however, about his misgivings, and the group carries on to shelter in a high Alpine farmhouse that remains unoccupied in winter.
Once there, Smith claims he's forgotten the code books carried by the dead radioman and goes back out into the night to get them, but instead detours around to the barn where he meets the mystery woman from the plane, Mary (Mary Ure). They obviously know each other very well, and after an hour of her company, Smith returns to the farmhouse. A skeptical Lt. Schaffer sits there patiently waiting, cleaning his gun, He obviously doesn't believe a word of Smith's story, but says nothing.
The next day, the team climbs over the next ridge in the mountains, and see the Schloss-Adler castle on a peak across the valley. Waiting until dusk, they descend and sneak into the village at the base of the mountain. Changing into Wermacht uniforms, they bluff their way into the village and enter an inn, listening for information on Gen. Carnaby. Smith has a coded exchange with the barmaid (who turns out to be a British agent as well), and shortly thereafter meets both her and Mary in the woodshed behind the inn. He explains that he's in a hurry to get into the castle above because the Germans don't really have an American General, but instead have an impersonator named Cartright Jones who knows nothing about the second front. Astonished, Mary can't believe they've gone to all this trouble to rescue an imposter, but Smith has no time for further explanations, and produces forged papers that will get Mary into the castle that night with a new job as a servant.
Exiting the woodshed, Smith finds another member of his team murdered, laying in the snow behind a scout car. He rejoins Schaffer in the inn, who has had enough of Smith's deceptions, and demands to be told the real story behind this mission, or he's out. As Smith starts to explain, the SS bursts in, capturing Smith, Schaffer and the other three agents. Smith and Schaffer are driven away in a staff car, while the other three men are taken away for questioning. Mary meanwhile rides the cable car up to the castle and begins to make her preparations.
A mile from the village, Schaffer and Smith manage to start a fight in the car and to kill the German soldiers. They hike back to the village, pick up their hidden gear, blow up a warehouse as a diversion, steal a motorcycle and drive back out of the village along the road to the airport. They install small bombs on the telephone poles along the road, and rig trip wires to snow-poles along the road edge. Returning once more to the village, they find their three fellow agents being led into the cable car for a trip up to the castle, and make their way quickly up onto the roof of the terminal. As the car pulls out, Smith and Schaffer climb onto its roof, riding unseen up to the castle.
After a harrowing ride on top of the cable car, they leap to the ledge above the terminal and claw their way up. Far above, Mary lowers a rope from a castle window, and they climb up the cliff face. Laying their final plans, Smith and Schaffer quietly make their way to the castle's great hall, where the Germans are talking to "Carnaby" over brandy and cigars. Since he politely but firmly refuses to tell them anything, they must regrettably resort to Scopolamine to get the information they want. The three captured members of Smith's rescue team are also brought in, and it is revealed that they are really double agents, posing as British but working for the Germans. As they are about to inject the fake General with truth serum, Smith quietly walks up out of the shadows surprising everyone, and declares that the double agents are still lying. Smith himself is tracking them, and he is really working for SS Military Intelligence. Pointing his gun at an astonished Schaffer, Smith tells him to sit down and shut up.
Smith goes on to prove his true loyalties to the German High Command, and manages to convince the three traitors that they have been trapped. Desperate to prove they are really on the same side, they write down all the info they have on Nazi agents working in England. Smith gathers this up and asks German Col. Kramer (Anton Diffring) to review it, then quietly signals Schaffer to get ready. "Compare their notes with my original," he tells Kramer, handing him another notebook. Leafing through it, Kramer finds all the pages are blank, and is stunned by the realization that Smith had duped them all. He really is a British agent, and has tricked the Nazis into revealing their entire spy network in England.
In the ensuing confusion, Smith and Schaffer (who tells Smith that he's "about as confused as I ever hope to be") battle their way back out of the castle and onto the cable car. Escaping back down to the village, they highjack a large bus and speed out of town with the Germans in hot pursuit. Tripping the bombs they'd laid ealier as they drive by, the telephone poles are blasted in half and fall to block the road. The Germans are slowed but not stopped, and use dynamite of their own to blast through the obstacles. The team manages to beat them to the airport, where the same plane that had flown them in now swoops down and lands, allowing them to clamber in and escape through a hail of gunfire.
Taking a long sigh of relief, they sit back as they're met in the plane by the man who'd sent them on this mission, Col. Turner (Patrick Wymark), second in command of British Intelligence. As he reviews the list of agents in disbelief, as Smith calmly tells Turner that there's one name that isn't on the list: that of the top German agent in England, confirmed by Colonel Kramer back at the castle. He shows the note to Turner, who shoots an alarmed look back at Smith.
"It's your own name Colonel," he says. "Don't look so surprised."
Pointing a gun at Smith, Turner calmly tries to talk his way out, but cannot. When in desperation he pulls the trigger to shoot Smith, the gun doesn't fire; Smith had removed to firing pin before the mission. With no cards left to play and facing certain hanging, Turner, sweat beading on his forehead, calmly asks if there is an alternative.
"Do you want it?" asks Smith.
Turner nods. He slowly gets up, walks to the door in the rear of the plane and opens it. Taking a last bitter look at Smith, he jumps to his death.
"Is that it Major?" asks Schaffer tiredly as he replaces the door.
"Yes, that's it," replies Smith. He leans back in his seat and closes his eyes. The plane lumbers on through the night, returning to England.