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.