In 1940, Czech soldier Capt. Hasek (Michael Redgrave) is being pursued by the Nazi secret police. To conceal his true identity, he pretends to be a dead British soldier named Capt. Geoffrey Mitchell. But to keep the lie going, he must also write to Mitchell's wife, Celia (Rachel Kempson), as her dead husband from the POW camp where he's imprisoned. Meanwhile, Hasek's fellow prisoners are beginning to suspect that he is not who he claims to be, and so he must convince them that he's not a spy.