In early 20th-century Vienna, Stefan Brand (Louis Jourdan) is about to leave the city because he faces a duel that he wants no part of. However, before he can do so, he gets an anonymous love letter that changes his life. Though Stefan is moved by what he reads, he doesn't realize that it was written by Lisa Berndle (Joan Fontaine), a young woman he has known but disregarded for most of his life. When he finally figures out who his admirer is, it may be too late to prevent a tragedy.