One summer night, two men break into an isolated manor house and kill Lady Anne Robinson. Her teenage son, Thomas, convinces the police that Greta Grahame, his father's beautiful personal assistant, sent the killers, but Thomas is known for his overactive imagination, and he has reasons to lie. Thomas's father, Sir Peter Robinson, the British minister of defense, refuses to believe his son. Instead, he marries Greta and is prepared to testify for the defense at her trial. He will be the final witness.