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,[41] 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.
The 1933 K.B.S. Productions Sherlock Holmes film A Study in Scarlet follows a strikingly similar plot;[42] the victims are tormented by slips of paper inspired by the same poem as Christie's, one says "Six Little Black Boys | Playing With a Hive | A Bumble-Bee Stung One | And Then There Were Five." And it anticipates the largest plot twist in Christie's book, the killer turns out to be one of the "victims" who had already been identified as dead. The film retained no plot points from Arthur Conan Doyle's original story of the same name. The author of the movie's screenplay, Robert Florey, "doubted that [Christie] had seen A Study in Scarlet, but he regarded it as a compliment if it had helped inspire her".[43]
