As concert pianist Ellen Ewing and architect David McLean are about to be wed, a stranger interrupts the ceremony to declare that Ellen is already married to a man named Lucian Randall. Ellen is stunned by the accusation, but because a telephone inquiry to Fairview, California, where the marriage supposedly took place, supports the man's statement, she is forced to stop the wedding. That same day, Ellen and David drive to coastal Fairview, where Ellen has a vacation home, with Ellen's aunt Clara and her longtime guardian, lawyer Gregory Kent. After confirming that Ellen's signature is indeed on the marriage license, the group questions Roy Palmer, the justice of the peace who supposedly performed the March wedding. Palmer, his wife and a witness all claim to remember Ellen and address her as "Mrs. Randall." That night, in a hotel room that Gregory has reserved for her, Ellen tries to recall her previous wedding day. Before she can share her recollections with David, however, Leah, a hotel maid, comes in wearing Ellen's mother's pin, which she claims Ellen gave to her on her wedding night. Leah then insists that Ellen had been there in March with her husband, "Mr. Randall." After Leah leaves, Ellen describes to David the walk she took the day before the wedding and suggests that if she can clearly remember the walk, she could not have forgotten getting married. At dinner, however, a distraught Ellen confides to David the possibility that she may have amnesia. Although David believes in Ellen's innocence, Ellen insists that they visit Randall in person. While they wait to talk with Randall at a private party, Ellen describes a boat she saw during her beach walk, whose name she remembers as "Noosnom." After Randall tells David that Ellen is indeed his wife, she leaves for a private talk with her "husband." As he waits for Ellen, David hears a gunshot and is horrified to discover that Randall has been shot to death, and Ellen is the prime suspect. Ellen is prosecuted for murder by District Attorney Eric Lowell, who also happens to be her former suitor, and is defended by Gregory. During the trial, Randall is revealed to be a convicted blackmailer, and Eric accuses Ellen of killing him in order to stop his blackmail of her. At first, Ellen denies all of Eric's charges, but as the district attorney's words become more pointed, she becomes hysterical and admits she is unsure about what happened to Randall. The convicted Ellen is then sent to an insane asylum, where she falls into a semi-catatonic state. Although Ellen's doctor speaks discouragingly to David about her condition, David decides to fulfill Ellen's request to have her wedding dress brought to her. While he is retrieving the dress from Ellen's Fairview house, David finds a seashell she had described to him earlier. On a hunch, David goes to the beach and spots a boat whose name, when viewed upside down in the water, spells "Noosnom." Inspired, David questions the man who issued Ellen's marriage license and learns that the woman who signed Ellen's name had a scar on her hand. David then returns to Roy Palmer's house, but discovers that no one by that name has ever lived there. Sensing a pattern, David finds Leah at the hotel, and for a fee, she offers to tell him the whole story after work. Just as David is about to meet with Leah, however, she is strangled by a wire-wielding man. After David finds Leah's body and notices the scar on her hand, he is attacked by the same man while driving his car. David saves himself, but loses his would-be killer when he jumps out of the car after it crashes into a ditch. Later, David and Gregory visit Ellen at the asylum to tell her she has been cleared of all charges. Ellen is at first overjoyed, but then cries out in fear and runs from the room. That night, at her aunt Clara's house, Ellen appears with a gun and accuses Gregory, whose distinct mannerisms she recalled seeing just before Randall was killed and then later recognized at the asylum, of framing her. After admitting his guilt, Gregory reveals that Ellen's father, a judge, had wrongfully sent him to an insane asylum. To exact revenge, he says, he plotted against Ellen, killing Randall himself and hiring another man to murder Leah and David. Gregory then tries to force Ellen to attack him, so that she will shoot him and be forever incarcerated as a mad woman. Before Gregory can complete his deed, however, David arrives and, while fighting with the lawyer, causes a large mirror to fall on top of him. With Gregory's death, Ellen is finally free.