A murderer is on the loose at Cambridge, where the family of the late Professor John Faraday is being murdered. His niece's husband Andrew is found in the river with a bullet in his head, then his daughter Julia is poisoned and his son William is attacked. Campion is called in by a friend, and Caroline Faraday, the Professor's widow, retains hin to protect her eccentric children William and Kitty and to find the murderer. Campion finds himself working on the case with Inspector Stanislaus Oates.

Campion receives a letter from an old Cambridge friend, asking him to help find a missing uncle of his fiancée. Campion is convinced that the murderer is part of the Faraday household.