An old friend's promise of access to some rare texts of Aristotle's has taken Master William Falconer far from Oxford to the remote Furness Abbey. While crossing the dangerous shifting sands of Lancaster Bay, he witnesses the recovery of a body from the sandbanks that mark the changing course of the River Kent. Back at the abbey, the body is identified as that of a monk who disappeared twenty years earlier. His death may not have been an accident. The dead man was to have been the new abbot, and the present incumbent, who took his place exactly two decades ago, is the prime suspect. But Falconer's friend is convinced of the abbot's innocence. Will Master William look into the matter?