In Norway, Nazi official Oberst von Ritter executes a British spy, and takes Norwegian general Heden prisoner in order to crush the Free Norwegian movement. News of Heden's capture reaches Britain, where it is decided to send a troop of three commandos, all of whom speak Norwegian, to free Heden. Captain Robert Owen heads the team and is joined by Cockney communications officer Harry, and a young Norwegian lieutenant named Eric Falken. The three commandos are flown by night to Norway, and parachute into the country. They are spotted immediately by German soldiers, but after Bob shoots one, they escape to town, where they pose as mountaineering tourists. As Bob meets their secret contact, Sandling, Eric encounters his former fiancée Inga, whom he left because she wanted him to join the Nazis. Unknown to Eric, Inga is a collaborator, and she informs von Ritter of their location. Von Ritter sends three men to capture the spies, but they escape with the help of Sandling, who sacrifices his life to kill the Nazis with a grenade. Now dressed in Nazi uniforms, the commandos go to the prison camp where Heden is being held, and he is released into Bob's custody, while Harry captures the enemy radio and broadcasts a code to British Intelligence, signaling them to begin their bombardment of the nearby naval base. The commandos are forced to shoot their way out of the prison camp, and destroy the camp with grenades. Heden is wounded during the escape, and after abandoning their car, they hike through the mountains. As Heden's injury is serious, Eric returns to town to find a trustworthy doctor, but he makes the mistake of confiding in Inga, who has him arrested. When the torture becomes unbearable, Eric reluctantly tells the Nazis where his compatriots are located. Only Bob is captured, as he has sent Harry and Heden ahead, and von Ritter uses a lie detector and threatens to shoot Eric if Bob does not answer his questions truthfully. To mislead von Ritter, Bob secretly burns himself with a lighted cigarette every time he has to answer a question which would register as the truth on the machine. That night, von Ritter's right-hand man, Dalberg, a Nazi collaborator, frees Bob and Eric in order to help his country. Although Dalberg is shot by von Ritter, Eric and Bob escape and rendezvous with Harry and Heden. The four men finally reach the coast as British troops begin their aerial bombardment of the naval and military bases, but because they have missed their arranged meeting time, they have to signal the troops by lighting a fire. While Heden and Eric are alone on the beach, they are spotted by von Ritter, who shoots Eric as he throws himself in front of Heden. Harry avenges Eric's death by shooting von Ritter, who survives and is captured by British forces as they rescue the commandos.