Christie allows some culprits to escape earthly justice for a variety of reasons, such as the passage of time (retrospective cases), in which the most important characters have already died, or by active prescription. Such cases include The Witness for the Prosecution, Murder on the Orient Express, The Man in the Brown Suit, Elephants Can Remember, and The Unexpected Guest. There are instances in which a killer is not brought to justice in the legal sense but does die as a direct result of his plot, sometimes by his own hand at the direction or with the collusion of the detective (usually Hercule Poirot). This occurs in The Murder of Roger Ackroyd, Death on the Nile, Dumb Witness, Crooked House, The Hollow, The Mirror Crack'd from Side to Side, Cat Among the Pigeons, Peril at End House, Nemesis, Appointment with Death, The Secret Adversary, and Curtain. In the last of these (Curtain), no fewer than three culprits die during the course of the story.