Poirot is asked for assistance by a young lady, Diana Maberly. She was engaged to marry Hugh Chandler for over a year but he has broken it off as he thinks he is going mad. There is a history of insanity in the family, with his grandfather and a great aunt being afflicted, and his father, Admiral Chandler, has insisted that his son leave the navy before his condition gets worse but the reason was hidden under the pretext of having to manage the family country estate – a reason no one believed, including Colonel Frobisher, a great family friend and Hugh's godfather. At Poirot's prompting, Diana admits that there have been some unusual occurrences on nearby farms with the throats of sheep cut and the like but insists it has nothing to do with the situation. The Admiral refuses to let a doctor see his son.
Poirot travels with Diana to the family seat of Lyde Manor where the meets the people involved. Hugh strikes Poirot as a fine young bull of a man. He learns further details of the history of insanity in the family from Colonel Frobisher, including Hugh's grandfather being so bad that he had to be committed to an asylum. Poirot also hears that Hugh's mother died when he was ten years old in a boating accident when she was out with the Admiral and that she was previously engaged to Frobisher before he went off to India with the British Army but when he came home he found that she had married Admiral Chandler, however this incident did nothing to lessen the ties of friendship between the two men. Poirot forces Frobisher to tell him more details of the incident with the sheep and finds out that on the night concerned, the Admiral found his son in bed with blood on his clothes and blood in the washbasin but Hugh remembered nothing of what he had done. Poirot also questions the Admiral who has aged immensely since these incidents started and who feels that breaking the engagement is best for everyone and that there will be no more Chandlers at Lyde Manor after he and his son have died.
I n questioning Hugh, Poirot hears of his dreams which always seem to include elements of hydrophobia . He also suffers from hallucinations and has one while speaking to Poirot of seeing a skeletal figure in the garden. Poirot however is convinced that Hugh is sane and begins his investigations, asking Diana to arrange for him to spend the night in the manor. He searches Hugh's room and also makes a trip to a local chemist, supposedly to pick up a toothbrush... That night, Hugh somehow manages to get out of his locked room and is found outside Diana's room, a bloodied knife in his hands from having killed a cat. Hugh recovers consciousness and tells the others he intends to go out shooting rabbits.
It is clear that his real intention is to commit suicide in the woods with a shotgun and therefore save himself and the others further pain. Poirot stops him and tells them all that Hugh is being set up to carry out the very act of suicide. He is being poisoned with Datura . The alkaloid produces hallucinations and causes great thirst. It is being given to Hugh as part of his shaving cream and thereby continually entering his system with each day's application. Poirot took a sample to the chemist for testing. As to who is responsible…Poirot reveals that Admiral Chandler has inherited the insanity of his family but that Hugh is not his son. Hugh is the natural son of Colonel Frobisher who had an affair with his mother before he went to India. The Admiral recognised the resemblance in his son to his friend and learned the truth from his wife before he murdered her in the boating "accident". He further blocked Hugh from seeing a doctor who would have confirmed his sanity. The Admiral takes the shotgun and repeats his son's excuse that he is going outside rabbit shooting. The last of the Chandlers commits suicie.