the 1930 novel "the invisible host" by gwen bristow and bruce manning has a plot that strongly matches that of christie's later novel, including a recorded voice announcing to the guests that their sins will be visited upon them by death. "the invisible host" was adapted as the 1930 broadway play "the ninth guest" by owen davis, which itself was adapted as the 1934 film "the ninth guest". there is no evidence christie saw either the play (which had a brief run on broadway) or the film.