Neptune King and the Lunatic Asylum is the 22nd book in the series. Neptune and Shark are retained by Mrs Cartwright, the distraught mother grieving the loss of her son who supposedly killed himself whilst an inmate in Willow Downs Asylum. She believes he was murdered, despite his history of self-harm and attempted suicide. Doubtful at first they undertake a preliminary investigation not expecting to find anything much. To their surprise they learn that there have been eighteen suspicious suicides in the past year. That is enough to convince them that something serious is going on. Joining forces with a young enthusiastic policeman who also has doubts about the asylum, they go undercover, Neptune as a patient, Shark as an orderly, and from then on begins their terrifying ordeal. Will they survive the asylum with their health and sanity intact? Their only ally is a deranged patient who thinks he is a spy deep undercover. This is one of their toughest cases yet.