As stated, the ultimate parlor whodunit by Agatha Christie; the title of the play has its origin in Hamlet, being the play within that play, with which the Melancholy Dane will catch Claudias' conscience. Wonderfully pariodied in Tom Stoppard's The Real Inspector Hound. (Stoppard also utilized the idea of the play-within-a-play in Rosencrantz and Guildenstern Are Dead.)