WWII is entering its last phase: Germany is in ruins, but does not yield. The US army lacks crucial knowledge about the German units operating on the opposite side of the Rhine, and decides to send two German prisoners to gather information. The scheme is risky: the Gestapo retains a terribly efficient network to identify and capture spies and deserters. Moreover, it is not clear that "Tiger", who does not mind any dirty work as long as the price is right, and war-weary "Happy", who might be easily betrayed by his feelings, are dependable agents. After Tiger and another American agent are successfully infiltrated, Happy is parachuted in Bavaria. His duty: find out the whereabouts of a powerful German armored unit moving towards the western front.

On 8 Dec 1944, Lt. Dick Rennick of the U.S. Army is being driven to his new assignment with an intelligence division in Armentières, France, near the German front, when his driver gets lost. As the Americans consult a map, two German soldiers, also lost, surrender to them. The Germans, Corp. Karl Maurer and Sgt. Paul Richter, thank Rennick for his kindness when he leaves them at a P.O.W. camp before joining his unit. At the convent where the intelligence division operates, Rennick then meets Col. Devlin, who explains that their mission is to train German P.O.W.s to spy on German military operations. Rennick is dubious, but Devlin, who insists that only volunteers will be used, believes that the plan will help the U.S. to end the war. At the P.O.W. camp Rennick had visited earlier, the officers interview candidates, including Sgt. Rudolf Barth, a cynic who has more faith in a paycheck than in politics. That night, Richter is murdered by prisoners angered by his negative statements about Germany. Disgusted with the Nazis and the futility of the war, Karl becomes one of Devlin's volunteers, even though it will mean becoming a traitor to fellow Germans. Karl explains to the Americans his belief that fighting against his people will be fighting for them. Despite Rennick's unease over Karl's sensitive nature, he is accepted and given the code name "Happy." Barth, who is called "Tiger," also undergoes the rigorous training, but the Americans grow suspicious when, during his first mission, his partner disappears. Devlin receives a report that confirms Tiger's explanation of the incident, however, and he is chosen to accompany Rennick on a mission to Mannheim, where they are to establish a radio liaison to an important German general who wants to surrender. Happy is chosen to discover the location of the 11th Panzer Corps, which could reinforce the general's troops and make the surrender difficult. As the trio prepares to parachute behind enemy lines, Happy overhears Tiger and Rennick discuss the location of the safe house in Mannheim, and they then split up. Stating that he was on medical leave and is trying to rejoin his unit, Happy passes through a check point and takes a bus to Munich, then boards a train bound for Nuremberg. Also on board is a friendly soldier, Heinz Scholtz, who is curious about Happy's supply of banknotes. Upon reaching Nuremberg, Happy makes inquiries and learns that he has just missed the Panzer Corps. As Happy encounters difficulties at another check point, Scholtz offers him a ride and whisks him away from his questioners. That night, Scholtz obtains rooms for them at an inn, where a nervous Happy, who has just heard a radio report about a parachutist spy, dances with Hilde to avoid Scholtz. Hilde, a dispirited "hostess" who follows the military from camp to camp, tries to spark a relationship with Happy, and bitterly tells him about her sad life when he rejects her. The next morning, the regretful Hilde warns Happy that Scholtz, who is an S.S. officer, is watching him. Happy sits with Hilde on a truck bound for the next town, but the truck is stopped to "recruit" the men for a nearby regiment. With time running out before he is to return to the Americans, Happy is ordered to serve as the medic for ailing Oberst von Ecker. That night, Happy saves von Ecker's life through quick medical attention, and in the morning, learns the location of the Panzer Corps. Desiring to reward the young soldier, von Ecker accedes to his wish to go to the front and releases him. On the way to Heidelberg, Happy and the other men aboard the transport are forced to disembark, and as they are walking, an American air strike begins. During the confusion, Corp. Ernst, an S.S. man alerted to Happy's presence by Scholtz, tries to kill Happy, but Happy shoots him first. Searching Ernst's belongings, Happy realizes that his name is on the dreaded security list, and that he is in danger. Knowing that he cannot reach his rendezvous point in time, Happy travels to Mannheim to give his valuable information to Rennick. Happy reaches the safe house, but Rennick informs him that their radio is damaged, so they cannot relay his information. The men are then contacted by a German soldier, who states that Gen. Jaeger, the officer attempting the surrender, has been placed under S.S. guard and cannot carry through with the plan. Realizing that the entire scheme was a trap, Rennick tells Happy and Tiger that they must return to Armentières, which entails swimming across the Rhine. They go to the apartment of Tiger's sister-in-law, which has a view of the river, but their presence is revealed by Tiger's nephew, a Hitler youth. The men are chased by soldiers, but when the boy comes face to face with the hiding Rennick, he cannot betray him again and breaks down in tears. Believing that the boy was pulling a prank, the soldiers leave, and the men reach the river bank. There, Tiger attempts to flee, and Rennick is forced to shoot him. Rennick and Happy are shot at by pursuing soldiers as they dive into the water, but they make it to an island midway through the river. Happy, who has been disabled by a cramp, knows that he cannot continue, and distracts the soldiers on the island so that Rennick can escape. Happy is captured, while Rennick reaches headquarters and relays Happy's information. Rennick's compatriots tell him to forget about Happy, who will be shot as a traitor, but as he leaves, Rennick ponders Happy's sacrifice, and promises that he will live on in his memory.