In the Second World War, Sally Maitland (Anna Neagle) appears to signal Nazi planes to bomb England after murdering an innocent citizen in his home. The next morning, Sally boards a ship bound for Canada. Two of her fellow passengers, Jim Garrick (Richard Greene) and Polish officer Jan Orlock (Albert Lieven), seek her acquaintance, despite her long-time and well-known admiration for Nazi Germany. It soon becomes common knowledge that Jim is a British intelligence officer. Sally rebuffs his advances, but welcomes Jan's attention. Sally is, in fact, a deep cover British agent on a secret mission shadowing her quarry: Jan. Unbeknownst to Sally, Jim has been assigned to help and protect her.