The story began 13 years ago, on a clifftop in Eastbourne. A couple tragically died - but the question has always remained - did the husband, General Ravenscroft, shoot the wife, or did the wife, Molly, kill the husband? Present day, and Poirot has been called in by his old friend Dr Willoughby, whose elderly father, an eminent Professor, has been found drowned in his own hydrotherapy treatment bath - a rather brutal contraption he used decades ago to treat psychiatric patients. Poirot is intrigued by the case, and keen to help his friend, although the doctor himself does seem to be the prime suspect in the eyes of the investigating officer... Meanwhile, Poirot's old crime-writing friend, Ariadne Oliver, is dealing with a case of her own. At a dreaded literary lunch she is approached by one Mrs Burton-Cox, who quite forcefully insists that Ariadne discover the truth about the Ravenscrofts - as her son, Desmond, is about to marry their daughter, Celia - and what if young Celia has inherited the "madness gene"? A haphazard Ariadne sets off on a mission to track down the truth via half-remembered stories from 13 years ago, whilst the methodical Poirot pursues his modern-day murder with quiet logic. Until gradually he pieces together the puzzle, and realises that only when examined together will these two cases reveal the truths which everyone has been seeking - who was murderer 13 years ago, and who is killing again..?