In 1940, Captain Karel Hasek of the Czechoslovak Army remains incarcerated in the Dachau concentration camp. He manages to escape and assumes the identity of Captain Geoffrey Mitchell, a recently-deceased British officer. When recaptured by the German authorities, they assume that he is the real Mitchell. Karel is relocated to prison camp in western Germany, along with thousands of British prisoners of war.Karel is fluent in the English language, but his unfamiliarity with the British culture makes his fellow prisoners suspect that he is an impostor. He is mistaken for a German spy, and is forced to explain his story to Major Dalrymple, the senior British officer. He agrees to help him maintain his cover.The British prisoners are allowed to maintain correspondence with their loved ones in the UK. To continue the ruse, Karel starts corresponding with Celia Mitchell, Geoffrey's estranged wife. The real Mitchell had abandoned his wife and children. Karel's letters convince Celia that her husband wants to reconcile with her, and she starts falling in love with him again.In 1944, the prison camp is visited by Herr Forster, a former commandant of Dachau. Karel fears that Forster will recognize him, but Forster seems to have forgotten about him. Forster eventually realizes that Karel is not the real Mitchell, and sends Karel's photograph to Berlin for identification.Shortly after the Germans decide to repatriate a number of the British prisoners. Karel's name is not in the list. Private Mathews reveals that he was a burglar in his civilian life. With the help of others, Mathews breaks into the commandant's office and adds the name of "Mitchell" to the list. Karel is send to Great Britain, with the German still not realizing that he is not British.Karel visits Celia, explaining that her real husband is dead and that she has been corresponding with him for four years. He claims to have fallen in love with Celia through their correspondence. She is shocked, and he leaves her alone. Afterwards she re-reads his letter, and realizes that she genuinely loves Karel.In 1945, Germany surrenders and the European theater of World War II ends. Karel phones Celia, and she is eager to speak to him. The film ends in a hopeful note concerning their relationship.