In late 1944, several American prisoners devise a plan to escape from a German prisoner-of-war camp. Unbeknownst to them, among the escapees is Lt. Hans von Kroner, Van Johnson, who is secretly a German spy, quietly reporting their plans to the camp commandant while refining his American English. When the plan is uncovered, he is told he will be removed from the camp and reassigned to a top-secret project that gathers fluent English-speaking members of the Wehrmacht for reasons that remain unclear. The failed escape leaves many dead and unsettled tensions behind the barbed wire.
Von Kroner is then sent to a German castle with others for Operation Greif—the code name the film uses, humorously called "Operation OK Butch"—where they will don American Army uniforms and spearhead the Ardennes Offensive by sabotaging supply lines and grabbing key objectives. He is teamed with three others, including a Waffen SS Officer, Wilitz, who regales his comrades with stories of his terrorizing exploits from his Brown Shirts days. Wilitz, Kerwin Mathews, brings a blend of bravado and menace to the group as they begin to train for and execute their perilous mission.
The initial German assault, aided by the disguise and surprise, meets with early success. However, their forged orders are soon countermanded by a battered American infantry company of the 23rd Infantry Regiment, forcing von Kroner’s group into the role of replacements. Moreover, a shift in weather robs the attackers of the cover that helped their raid, and Allied air power begins to harass the Germans. In the new American unit, von Kroner encounters Sgt. Ludwig, Dick York, and Cpl. Ennis, Larry Storch, who claim they were the only survivors of the escape plan, having been ambushed when the Germans learned of it.
As the infiltration continues, the conspirators obtain plans for an American counterattack against a strategic crossroads. Von Kroner warns the German side by radio about the impending assault but is ordered to stay with the Americans to sabotage the plan from within. The group sabotages a jeep with a bomb, killing the company commander, and then attempts to destroy another vehicle, a weapons carrier, which heightens Ludwig’s suspicion about their true loyalties and motives.
In the ensuing actions, the company fights back at a German position, and Wilitz uses the distraction to shoot their lieutenant in the back, elevating Ludwig to command. Several German prisoners are captured, and the alias of Richardson is brought to the fore as von Kroner volunteers to escort them to Battalion headquarters. He later returns to report that the prisoners did not want to escape; they wanted to surrender to the Americans, and von Kroner decides to escort them in. Wilitz, furious at what he sees as treachery, pushes for harsher action and rails against von Kroner for not simply executing them all.
As news of German infiltrators behind American lines spreads, Ludwig becomes more suspicious when Wilitz seems ignorant of everyday American terms, such as the phrase “hot foot,” highlighting the growing rift and distrust within the unit. The tense balance between deception and loyalty drives the narrative forward, revealing how far individuals will go under the pressure of war, loyalty, and the blurred lines between ally and enemy.
