One summer night, two men break into an isolated manor house and kill Lady Anne Robinson. Her son, Thomas, convinces the police that 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, refuses to believe his son. Instead, he marries his assistant, Greta Grahame, and will be giving evidence for the defence at her trial. He will be the final witness.