- Philo Vance, the dilettante detective
- John F. X. Markham, District Attorney
- Ernest Heath, sergent, Homicide squad
- Sanford "Monty" Montague, engaged to Bernice Stamm
- Rudolf Stamm, hard-drinking fish collector, and owner of the Stamm estate
- Matilda Stamm, his mother, who seems to predict the future with 100% accuracy
- Bernice Stamm, his daughter
- Gale Leland, neighbor and friend of the family
- Alex Greeff, stockbroker and guest of the Stamms
- Kirwin Tatum, guest of the Stamms
- Teeny McAdam, guest of the Stamms
- Ellen Bruett, writer of love letter to Sanford Montague
- Ruby Steele, actress
- Trainor, the Stamm butler
- Mrs. Schwarz, nurse-companion to Mrs. Stamm
- Doctor Holliday, Stamm family physician
Locale: Inwood (northern Manhattan), New York City
Synopsis: This is a locked-room mystery, but the room is a pool! Sanford Montague dives into the "Dragon Pool" (a small pond) on his property and never comes up. Where did he go? Is he dead or alive? He is thought to have run off with Ellen Bruett, who wrote a note arranging a meeting that night. When he does not reappear, the pool is drained, and his body is not in it. However, there are strange foot and claw marks on the hard bottom of the pool.
Matilda Stamm, elderly mother of hard-drinking Rudolf Stamm, is convinced there is a dragon that lives in the pool, and protects the Stamm family by killing its enemies. This is supported by the fact there have been two deaths in the pool already. She states the dragon then flies away with its victims to dispose of their bodies elsewhere.
Nearby are several deep glacial pot-holes. The body of Montague is found in one, mutilated by claws, and apparently dropped from a height. Some time later, Alex Greeff goes missing. His body is likewise found in the same place, again mutilated and dropped.
Philo Vance rounds up the key to a family vault located near the pool, and enters it to find that it is connected with the deaths.
This is my favorite Philo Vance novel. The murders are so unique and outlandish, and all evidence points to the legendary dragon being the culprit. Philo Vance seeks out the truth by eliminating all other possibilities. The novel stays on topic for the most part, except for a several page diversion as Vance displays his knowledge of various tropical fish.