1918. World War I has just ended. An amnesiac war veteran, known as John Smith (Ronald Coleman), has just escaped from the Melbridge County Asylum. The only reason he is there in the first place is because of his amnesia and having nowhere else to go.
Once outside, he is quickly befriended by Paula (Greer Garson), a traveling music hall performer who nicknames him Smithy and who wants to nurture his ability to regain his memory. To hide him from the authorities who are after him, she quits her job and takes him out to the countryside. They soon fall in love and despite not knowing his true identity or his past, they get married and start a family where Smithy begins a career as a writer.
Soon, Smithy receives a telegram from the Mercury Newspaper asking him to come to Liverpoool to discuss a permanent position on the paper. Because Paula is still recovering from a difficult birth, he reluctantly travels alone, planning to return the following night. After checking into his hotel, Smithy walks toward the Mercury office but is hit by a car and knocked unconscious. When he comes to, he has no memory of the past three years and recalls only his life as aristocrat Charles Rainier.
Charles returns to his life as a wealthy Rainier and begins his career as head of the Rainier family business. By 1932, Charles has become known as "the industrial prince of England" for vastly increasing his family's fortunes, but is haunted by the missing past that is tied to a latchkey he found in his vest pocket after the accident.
However, Charles must go on with his life and plans to marry longtime family friend, Kitty. But Charles' new secretary, Margaret Hansen, has other plans. She wants him to remember the life he left behind with Paula, because Margaret IS Paula.