While it certainly seems that Christie's infinitely superior work was influenced by this 9 years earlier work of fiction, it may or may not be so. Almost everyone is inspired by something seen or heard which later germinates. Christie may or may not have seen the film in question or read the book, who can know. Did the two American authors and Conan Doyle (who wrote the similar plotted "A Study in Scarlet") also plagiarize each other? As it only ran for a dismal 72 performances on Broadway she surely did not see the play.
I also disagree with some of the previous comment that begins with "Though it runs just over an hour, nearly every element of the film's plot was replicated in Agatha Christie's 'Ten Little Indians'":
1) "a coward who offers to collude with the murderer in return for his life being spared" - Totally inaccurate description of the relationship between Dr. Armstrong and Justice Wargrave in ATTWN.
2) "a male character managing the tension by drinking to excess" - Not in ATTWN.
3) "the two would-be lovers unraveling the solution to the mystery before they can be killed" - Not in ATTWN novel - not in original ending. Bleak ending changed later for stage, film, and TV productions.
4) The 1930 work relies on elaborate electronic devices more appropriate to the late 20th-early 21st century which are used to constrain and inject poison while Christie's has nothing of that sort.
5) The characters, some of whom know each other intimately, targeted for death in "The Invisible Host" are guilty of such serious but not capital crimes as conspiracy, corruption, and bigamy, and the killer is seeking revenge on those who directly impacted his life, whereas in Christie's ATTWN, each and every guest to the island is a stranger to each other (except the married couple of servants) and each has evaded justice despite being responsible for intentionally causing the death of at least one other human being (except, ironically, the actual killer, who took no innocent life nor was the ATTWN killer directly affected by the actions of his guests/victims).
6) In 'The 9th Guest', at least one completely innocent person is killed (later revealed as the electrician who wired the apartment so a high-voltage charge ran through the metal gate that was the only way to leave), and the fate of the two apparently innocent butlers is not known to those of us who haven't seen the film.
7) Unclear and illogical to invite the young man and woman who become the heroes and lovers, as they had done nothing to merit punishment.
Serious differences. Also, there is no record online of any accusations, much less lawsuits, filed by either Owen Davis and/or Gwen Bristow & Bruce Manning, for plagiarism or any similar such offense against Christie and her publisher, which, based on the above, would have been relatively easy to prosecute, with a good chance of success, based upon an apparent preponderance of evidence. See more