Simon and Eleanor Ashby are at church, along with their Aunt Harriet, for a service commemorating the death of their parents in a plane crash. Also mentioned is their brother Tony, who disappeared soon after and was believed to have committed suicide. Eleanor is a frail young woman who never recovered from Tony's death. At Simon's insistence, she has a nurse companion to attend her. Unknown to the family, Francoise is Simon's mistress.
Eleanor suddenly spots a young man outside and runs to find him. She insists the man is Tony but no one believes her. Later that day, she spots him in the garden. Again he escapes before she can confront him. Convinced that she is going insane, she goes to the cliffs and prepares to jump into the ocean. She is rescued by the young man, who carries her to the house. The butler recognizes the man as Tony Ashby.
The young man turns up at the office of John Kossett, the family solicitor. He claims to be Tony Ashby, revealing that he did not commit suicide but ran away. He has traveled all over the world but now wishes to return to the family and claim his inheritance. Unfortunately, Simon is due to inherit everything in a few weeks. Tony's reappearance will ruin everything. Simon becomes even more drunk and obnoxious.
While Eleanor is delighted to have her brother back, Aunt Harriet and Simon are hostile. They do not believe the man is Tony. He is summoned to the house and put through the ringer, but answers every question correctly. Later, Tony meets the solicitor's son Keith for a drink. Keith hired him to pose as Tony to prevent an audit of the estate, which would reveal Keith's embezzlement. The impostor's real name is not revealed but he bears a startling resemblance to Tony. Keith gave him a compete family history, adding events that only the real Tony would know.
Tony awakes one night to the faint sounds of a choirboy singing, to an organ accompaniment. He follows the music to the ruined chapel attached to the house. Eleanor joins him, explaining that she has heard the singing too but though she was imagining things. They manage to look inside. Someone wearing a choir robe and a garish mask is standing next to a phonograph, which is playing a record of a child singing. Simon is playing the same song on the organ. Tony and Eleanor don't know what to make of this. Suddenly the door opens and Tony is attacked by the robed figure. Despite a slashed arm he manages to overpower the figure. It is Aunt Harriet.
Upstairs, Aunt Harriet explains that Simon has been tormented ever since his brother's presumed suicide. Having her impersonate Tony, while playing a song that Tony sang as a child, is the only way he can get any peace. Eleanor and Tony are shocked. Aunt Harriet turns on Tony, asking why he bothered to come back.
Simon continues to drink, running up huge liquor bills. One night Tony is forced to bring him home from the local pub, where he has passed. out. It is obvious that Simon hates him and even accuses him of being an impostor. Eleanor confesses that she is now afraid of Simon.
While Eleanor and Tony are on a picnic, the brakes of her car fail and she is nearly killed. Once again Tony saves her life. They are forced to take a taxi home. When Simon sees them arrive from Francoise's window, he is furious. It was he who cut the brake line, hoping they would both die. Francoise is aware of this and threatens to tell the police unless Simon marries her. That night she decides to leave, but Simon catches her in the grounds. He forces her head into a nearby pond and she drowns.
Eleanor begins to realize she is in love with Tony. After they kiss, she runs to her room and locks herself in, intending to cut her wrists. If she is in love with her own brother, she must be insane. Tony breaks down the door and stops her, revealing that he isn't her brother. He asks her if she will leave with him and she agrees. But Simon overpowers Tony and knocks him out. When he comes to, he is tied to a chair in the ruined chapel.
Simon is the one who is insane. After getting Tony to write a fake suicide note sometime earlier, he put the note and Tony's clothing on the cliffs. In the impostor's attempts to free himself, he knocks over a lantern and fire breaks out. Simon screams and begins tearing off the paneling next to the organ console. A skeleton wearing a choir robe is revealed. It is the real Tony Ashby, who was killed by Simon. Eleanor arrives and frees the impostor. They are unable to rescue Simon, who goes to his death clutching his brother's skeleton.