World War Two. Professor William Potts, a teacher of modern languages, exactly resembles Muller, a German spy, and is mistakenly arrested by the authorities. After the real Muller is captured and Military Intelligence sees them side by side, Potts is persuaded to substitute for the spy. He parachutes into Germany to take up Muller's task of instructing young spies in British behaviour at a university, with instructions to investigate secret war work being conducted in a research laboratory there.
As Muller, Potts instructs his class in British customs, including the two-finger V- for-Victory salutation which they practise on a portrait of Adolf Hitler. He invites Professor Hoffman, the head of the research laboratory, to join him in a tavern. Before they pass out from excessive drinking, Hoffman ridicules Hitler's moustache while Potts lets out that he is a British spy. Fortunately, Hoffman is unable to remember Potts' slip of the tongue.
Potts cleverly tricks one of the students into revealing details of security at the lab and breaks in. He escapes with what appears to be a sample of a new bomb and is given an alibi by three other students - Max, Hans and Curt - who are actually Austrians seeking to join the British side. When the alibi fails, he successfully maintains that his visit to the lab was a practical test of its security, which has been proved deficient.
Hearing that General Von Goltz is taking a train to show a finished sample of Hoffman's new bomb to Hitler, Potts joins his group and discusses the plans to invade Britain. With the help of the three Austrians, Potts replaces the General on the plane which is to carry them the rest of the way to Berlin. As Goltz, Potts instructs the pilot to fly to London instead, saying the British are preparing to surrender. The pilot realises he is being tricked and is knocked out in a struggle. Potts is forced to take over the controls and accidentally ejects the pilot.
After much flying upside down, being shot at by British anti-aircraft batteries, inflating the only parachute inside the aircraft, climbing, diving, and puncturing some barrage balloons over London, Potts just misses Big Ben and successfully crash lands the plane into a tree, just outside the offices of Military Intelligence in Whitehall. He throws the sample of Hoffman's new bomb which he has brought with him to the astonished intelligence officers, before the tree collapses and he and the three young Austrians fall into a passing salvage truck.
