Mystery writers are supposed to play by a set of rules, ie give the reader all the clues, no rare, undetectable poisons, the murder can't be the butler, detective, victim or someone we've never met etc. Agatha Christie always played fair in that all the clues were present for the reader, and she didn't rely on obscure poisons unknown-to-science but she was somewhat more creative in her interpretations of some of the others. PERIL AT END HOUSE is an example of Christie's creativeness with one of the rules of mystery writing (but you'll have to read the book to find out which rule).
Poirot and Hastings are spending some time at a seaside resort. Poirot is still insisting that he has retired but concedes that "...if a bullet should strike the wall by my head, I would ...investigate the matter!" Needless to say one does and Poirot is soon investigating the numerous attempts on the life of a young woman. Poirot sorts his way through a murder, drug trafficking, false identities, secret engagements and attempted frauds to reach the truth.
The ending is clever and we are treated to Poirot being forced to ask someone else for the answer to a minor secondary puzzle.