The Cornish Mystery

S2, E5: Much to Poirot's chagrin, he arrives too late in Cornwall to prevent Mrs Pengelley's death. She had asked for his help. She suspected her husband, a dentist, was poisoning her so that he could marry his young assistant. After conducting preliminary interviews, Poirot is sure that Mrs Pengelley was not murdered by her husband and sets about finding the real killer.