It's been ages since I read Peril at End House, but apparently, this filmed adaptation has been changed only slightly from the actual story.
While staying at a Cornish resort, Poirot and Hastings meet a beautiful young woman, Magdala "Nick" Buckley (Polly Walker). She tells them that she's had a series of strange accidents lately. Poirot has reason to believe that someone is trying to kill her. When she leaves, she forgets her hat, and Poirot notices a bullet hole in it -- while sitting with them, she had been annoyed by a bee. Poirot points out to Hastings that it was a bullet, and actually finds it.
Concerned, Poirot becomes concerned with protecting Nick. One night, when she returns to her home to get something, her cousin, who was wearing Nick's coat, is killed. Nick tells Poirot that she doesn't care if she lives or dies, which Poirot finds a strange statement. She is put in a nursing home for her own protection.
Poirot then realizes that an aviator, whose trip has been followed by the media, is lost at sea, and that Nick was engaged to him. In the interim, his uncle has died -- meaning that Nick has probably come into a huge fortune. With the motive for the attempts on her life, Poirot sets out to find the will and the killer. He uncovers a lot more than that.
Really excellent, entertaining story, lushly produced, with Suchet perfection as Poirot. Hastings is pretty dumb in this one. Miss Lemon, as played by Pauline Moran, is a delight.
I figured this story out for the most part, but it's because my subconscious remembered it. It is a very intriguing mystery and you will be entertained by it.