- Philo Vance, the dilettante detective
- John F. X. Markham, District Attorney
- Ernest Heath, sergent, Homicide squad
- Mrs. Anthony Llewellyn - prominent social worker
- Richard Kincaid - her brother, and owner of the Casino
- Amelia Llewellyn - her daughter, an art student
- Lynn Llewellyn - her son, a night club lizard and gambler
- Virginia Llewellyn - Lynn Llewellyn's wife, formerly Virginia Vale, stage star
- Smith, the Llewellyn butler
- Morgan Bloodgood - croupier at Kincaid's Casino
- Dr. Allan Kane, friend of the Llewellyns
- Dr. Rogers
Locale: New York City
Synopsis: Philo Vance receives an anonymous letter indicating some harm is to come to members of the Llewellyn family and they should be watched, especially on a certain date. Vance contacts D.A. John F. X. Markham. Vance then visits Kincaid's Casino on the indicated date, to keep an eye on Lynn Llewellyn. While gambling, Lynn collapses and a doctor immediately recognizes it as poison. No sooner is he hospitalized when word comes that his wife, Virginia Llewellyn, has died at home from poisoning.
Soon after that, Amelia Llewellyn also is poisoned, but recovers. The common thread to the poisonings is that each victim drank water just before falling ill.
This mystery is quite enjoyable for a S. S. Van Dine story. Philo Vance is more human and less pedantic than we have seen him in previous books, with none of the long, diversionary sermons on antiquities or other unrelated topics. As this is the eighth book in the series, perhaps the writer had taken some of the reviews into consideration by this point.
The plot takes on an interesting twist as suspicion is placed in one direction, and it appears the solution is at hand - but at the last moment it turns out to be a false trail. The final scene contains high tension and an alarming development as Vance unmasks the killer.