Following the murder of a young actress from a touring theatre company, suspicion points to Diana Baring, another actress from the same company, who is found in a trancelike state seated beside the body. At the subsequent trial, the jury are unanimous in their belief of her guilt, with the exception of the eminent actor Sir John Fenier, who harbours doubts. Under pressure, however, Sir John reluctantly agrees to a guilty verdict.
Growing concerned at the verdict, and feeling some responsibility since he had earlier rejected Diana for a part and recommended she tour to gain experience, Sir John resolves to conduct his own investigation into the case. Enlisting the help of the theatre company's floor manager, Ted Markham, and his wife Doucie, Sir John visits the town where the murder took place, in the hope of finding fresh evidence.
After the revelation that a mysterious policeman was seen on the night of the murder, suspicion falls on one of two men in the company with access to a policeman's uniform, and a further clue emerges in the form of a bloodstained cigarette case. Sir John visits Diana in prison to seek clarification, but when she is unwilling to cooperate he becomes convinced that she is shielding someone. Under pressure she reveals that the dead girl had been about to tell her the terrible secret of another member of the company, which she already knew. Although she refuses to reveal the identity of the man in question, she inadvertently identifies the cigarette case as belonging to the actor Handel Fane.
Now convinced that Fane is guilty, Sir John and Markham devise a plan to expose him, inspired by the play scene in Hamlet: Sir John will invite Fane to audition for a role in a new play, which will be based on the incidents of the Baring case. At the 'audition' however, Fane, though visibly shaken, maintains his guard. Later, Sir John and Markham visit Fane at the circus, where he is performing on the trapeze. In the middle of his act Fane, overcome by guilt and shame, hangs himself.
With Diana released from prison, she and Sir John appear together in his new play.
