The ordered calm of Gloucester Abbey is shattered by the murder of one of its monks. The Abbot, ill-equipped to deal with such a heinous crime, is stunned by his conversation with the Sheriff, who is convinced that one of the monks must be guilty. This is the confusion that greets Domesday Commissioners Ralph Delchard and Gervase Brett when they enter Gloucester, having been sent there on the King's business. Ralph and Gervase, doubting the Sheriff's ability to solve the mystery, start their own investigations. It isn't long before they both realise that the killing is just a symptom of a sinister presence that threatens the whole community and must be stopped at any cost.