The House of the Arrow (1940)

If the murderer of wealthy widow Mme.Harlowe (Louise Hampton) had not relied on information obtained from a book in her library, the old lady might have lived for years, as chronic invalids often do. The killer relied on the false information that surare, the deadly native poison, leaves no traces after death. And so, Inspector Hanaud, the eminent Paris detective, arrived in Dijon to solve the mystery of the Maison Grenelle. He finds Betty Harlowe (Diana Churchill), the victim's niece, suspected by the police, and Ann Upcott (Belle Chrystall), the dead woman's companion, in a state of semi-hysteria. And there is also an anonymous letter writer known as The Scourge. For his clues, Hanaud has a scrap of charred paper, an anonymous letter and a pen-holder that is not a pen-holder at all. From these, he builds a case against the guilty one, until he is certain enough to risk the life of Betty to make the killer confess.