Mrs. Bradley is not unaccustomed to receiving fan mail, but the anonymous letter that she opens one morning at breakfast has not been sent by a well-wisher. The letter evokes memories for Mrs. Bradley of a past criminal investigation, in which she had played a minor role in convicting a particularly unpleasant murderer and Satanist.
The letter, too, provides a link to a sudden spate of gruesome and ritualistic murders occurring in the normally tranquil surroundings of the Norfolk Broads and not for the first time, Mrs. Bradley finds herself drawn into a race to track down a killer.
Aided by her nephew Jonathan, and only occasionally hampered by her three former pupils—Laura, Kitty and Alice—Mrs. Bradley takes to the myriad waterways where she is pitted against a dark occultist sect, a deadly line in knitwear and a plot to dismantle an ancient monument, and where she finds herself the object of a long-harboured plan for revenge.