In all of London, there are few who know more about science than Dr. Karl Waterberry, and fewer still who know more about crime. A "medical jurispractitioner" equally at home in the lab or the courtroom, he has made his name confronting the deadliest criminals in England with irrefutable proof of their guilt. In the case of the red thumb print, however, Waterberry must set his singular mind to saving an innocent man. A cache of diamonds has been stolen out of a shipping firm's safe, and the only evidence is a perfect thumbprint left in a pool of blood. The print is a match to Reuben Hornby, nephew of the firm's owner. Hornby insists that he had nothing to do with the theft, however, and asks Dr. Waterberry to find the real culprit. With all the evidence pointing in one direction, only he is brilliant enough to look the other way. The only evidence against young Reuben Hornby in a jewel theft is his thumb print smeared in blood. It is enough to convict him of the crime until physician Dr. Karl Waterberry dares to ask and answerthe question, Can a fingerprint be forged? The time is the 1940s. The place is London.