Elizabeth MacDonald (Claudette Colbert) is a newly married corporate librarian in 1918 Baltimore working for a chemical company owned by the Hamilton family and managed by Larry Hamilton (George Brent). Just as she is celebrating the armistice and anticipating the return of her husband John (Orson Welles), she learns he was killed in action, just days before the cease fire. Pregnant with their child and alone in the world, she is taken in by Larry Hamilton, who has loved her from afar and is driven by sympathy for her plight. She has her baby, a boy named Drew, and she and Larry marry, raising the child as his own and never telling the boy of his real father. Meanwhile, in an Austrian hospital, a horribly wounded and disfigured American officer (Welles) without any identification insists to the doctor treating him (John Wengraf) that he be allowed to die. The doctor saves his life, but the shock of his injuries and the strain of his recovery causes him to lose his memory, and he ends up adopting a new identity. Cut to 1939, and the outbreak of the Second World War in Europe. Drew (Richard Long) is about to graduate from college and wants to join his fraternity brothers, who are planning on going to Canada, signing up with the Royal Canadian Air Force, and heading to England to fly against the Germans. Drew is not yet 21, however, and needs the permission of his parents, but Elizabeth is appalled by the notion of losing Drew to war the same way that she lost John.
Into their family comes a visitor, Erich Kessler (Welles), a crippled, ailing Austrian refugee and chemical expert hired by Hamilton's company, who arrives in Baltimore with his young daughter Margaret (Natalie Wood). Kessler starts to recognize places in the city, including the home where Elizabeth lived, and when they meet, despite her discomfort at having an Austrian army veteran in the house, she does her best to welcome him. Elizabeth also starts to notice little aspects of Kessler that remind her vaguely of John. But much as she is haunted by these strange similarities, she is appalled when Kessler seems to encourage Drew to pursue his goal of fighting the Nazis. Even Kessler's presence in their home, despite his genial and deferential manner, is a vexation to Elizabeth, bringing the horror of the war and what the Nazis represent into their midst and making Drew even more fervent in his desire to join up and fight. When Margaret displays terrible fears and nightmares, it comes out that she isn't really Kessler's child at all, but the daughter of the doctor who saved his life (he and his wife had been executed by the Nazis).
Larry, meanwhile, must watch from the sidelines, not aware of Kessler's real identity and unable to resolve the conflict between his admiration for Drew's intentions and his love for his wife. When Drew decides to ignore his parents' wishes and go to Canada and enlist without their permission, Kessler follows and stops him (despite his own weakened condition), and brings the young man home. A confrontation ensues upon their return, and Kessler explains to her that, whomever she thinks he might have been, the past has passed. Elizabeth finds the strength and courage to face the future, and the coming of the new war and what it may bring.