In the waning days of World War II, American soldiers Sgt. Leonard Richardson, Sgt. Ludwig and Ennis are incarcerated at a German POW camp. In reality, Richardson is a German soldier, Sgt. Hans von Kroner, who had lived in the United States prior to the war and has been assigned to pose as an American POW to spy on the inmates. When Kroner reports to the camp commandant that Ludwig, an American who is of German ancestry and therefore fluent in the language, is digging an escape tunnel beneath the camp, the commandant assures him that no harm will come to Ludwig and the others. Soon after, Kroner and all other English-speaking German personnel are transferred to a secret operation known as "OK Butch." Colonel von Ruppel, the head of the operation, explains that their mission is to pose as American soldiers and infiltrate and sabotage Allied lines. After coaching the Germans in American pronunciation and comportment, Kroner reassumes the identity of Leonard and is teamed with three German soldiers: Wilitz, Kirsch and Hoffner. As they impatiently await their assignment, Wilitz, a sadistic "brownshirt," taunts Kroner about being the son of a famous German general. One day, as Germany's final Blitzkrieg begins, Kroner and his men are ordered to prevent the bridge at Naimure from being destroyed by the Americans. Posing as lost American GIs, the four infiltrate American lines in Belgium and proceed to wreak havoc by moving signs warning of land mines, resulting in the loss of many American lives. After systematically destroying fuel lines and ammunition dumps, the four, cold and hungry, come upon a small cottage. Still posing as Americans, they ask for shelter, and Monique, the young woman who lives there with her brother, invites them in. After Monique tells them that her brother and father are off fighting the Germans, she confides that she believes the German Luftwaffe has been disabled because she has seen no airplanes for weeks. Later, alone with Monique, Wilitz demands a kiss and when she resists, he assaults her. Kroner comes to her defense, and chagrined and disgusted by his subordinate's behavior, he profusely apologizes to Monique. While they are driving their jeep along the road one day, Kroner's company is directed to a village where an American captain attaches them to the 23rd Infantry unit, then confiscates their jeep. Upon reporting to their new unit, Kroner is stunned to be greeted by Ludwig and Ennis, who address him as Richardson. After Kroner lies that he fled the POW camp hidden in a laundry truck, Ludwig tells him that he and Ennis escaped through the tunnel, but all the others were gunned down by the waiting German commandant. Kroner then bitterly recalls the commandant's assurances that none of the tunnelers would be harmed. Although Kroner senses that the Germans are losing, the others refuse to accept the notion of defeat and Kroner accuses Wilitz of being drunk on the brutality of war. To prevent the Americans from using their confiscated jeep, the Germans place a bomb in it, killing the American captain who was behind the wheel. As they proceed on their mission to secure an area known as "Crossroads," the Americans discover a troop of Germans holed up in a shack. The Americans start to pepper the enemy with machine gun fire, and in the ensuing shootout, the American lieutenant is killed and Ludwig assumes command. Once the Germans surrender, the Americans discover that they have been fighting a ragtag band of boys and old men. Kroner offers to escort them back to the battalion, intending to free them along the way. When he returns, however, Kroner informs Wilitz and the others that the Germans refused his offer of escape, preferring to be prisoners of war rather than soldiers. Ludwig, who was wary of Richardson when they were POWs together, becomes suspicious of Wilitz when Wilitz fails to recognize American slang for "a hot foot." Later, when one of their trucks is found sabotaged after a soldier swore that he saw one of Kroner's men near it, Ludwig recalls his earlier suspicions about Kroner and sets a trap for him. After sending Kroner and his men on a reconnaissance mission to the top of a hill, Ludwig, yelling in perfect German, tricks them into believing that the Germans are attacking. When Kroner responds back in fluent German, he exposes his true identity. While trying to escape the Americans, Wilitz, Kirsch and Hoffner are killed and Kroner is wounded. As Ludwig treats Kroner's injuries, Kroner has difficulty comprehending that enemies can behave humanely. Soon after, the Germans attack, and after the American machine gunner is shot, Kroner grabs his weapon. The German officer in charge orders the Americans to surrender, but after they peaceably drop their guns, he shoots them down. Aghast, Kroner trains his machine gun at the Germans and mows them down. When Kroner is shot in the answering barrage, Ludwig runs to his side to comfort him. As he dies, Kroner shouts to the surviving Germans that Hitler is insane and that they are doing the work of the devil. Ludwig then notifies headquarters that their mission has been accomplished and that Crossroads has been secured.