Zoe Stock,a brilliant pianist,is attending a music school run by the Fielding family when she believes she sees a woman drown herself in the river,having left her baby on the bank. When the police arrive there is nothing to support her claim but Barnaby recalls that a mystery woman died in identical circumstances eighteen years earlier. Her body was found by Catholic priest Father Gregory,outside whose church an attempt is made to kill Zoe. A lucrative prize awaits the young musician adjudged the best by the Fieldings and it is assumed that another student is responsible.But Zoe seems to know more about the dead woman,'Mad Molly',than she should and then another student is murdered,as is Father Gregory, before he can give important news to Zoe's parents. Zoe is,in fact,Mad Molly's daughter, and the centre of a web of incestuous relationships that threaten the master class.