In Dijon, Inspector Hanaud investigates Betty Harlowe after her uncle, Boris Waberski, accuses her of poisoning her wealthy aunt for an inheritance. While an initial autopsy finds nothing, Hanaud discovers the use of Strophanthus Hispidus—a rare, undetectable poison applied via an antique arrow. Amidst a flurry of "poison pen" letters and deception, Hanaud proves the murder occurred despite the lack of conventional medical evidence.