The Cornish Mystery

Hercule Poirot receives a visit from Mrs Pengelley, a middle-aged woman who is afraid that she is being poisoned by her husband, a dentist. She has no proof, only that she suffers stomach pains (diagnosed as gastritis by her doctor) after eating whenever both of them are at home, but not when he is away at weekends. In addition, a supposedly unused bottle of weed killer is half empty.

Poirot and Hastings travel to Cornwall the next day and are shocked to find that Mrs Pengelley died half an hour before. Poirot interviews Mrs Pengelley's niece Freda Stanton, as well as Jacob Radnor, a young man who had been keeping company with Freda. The two are engaged, but Mrs Pengelley developed an infatuation with Radnor, resulting in a row between her and Freda that prompted Freda to move out.

Some time later, Mrs Pengelley's body is exhumed and found to contain high levels of arsenic, and her husband is arrested and charged with her murder. After attending the committal hearing, Poirot meets with Radnor and presents a murder confession for him to sign. Poirot conjectures that Radnor had intended to take control of the family's money by gaining the confidence of both women, marrying Freda, killing Mrs Pengelley, and framing her husband for the crime. He bluffs Radnor into believing that the police have him under surveillance, and promises to wait one day before turning over the confession and letting them track him down. Radnor signs the document and hurries away. Poirot then admits to Hastings that he had no proof of the man's guilt and could only establish it by deception.