Out of nowhere Christie's Author Avatar, Ariadne Oliver, calls Poirot and summons him to her presence. Mrs. Oliver has an instinctive feeling that something wrong is happening, that someone is plotting something evil. Mrs. Oliver is at Nasse House in Devon, where the lord of the manor, Sir George Stubbs, is hosting a grand fete for the village. Mrs. Oliver has been charged with organizing a "murder hunt", a sort of spin on a scavenger hunt in which villagers will hunt around looking for clues for a fake murder. A cheerful teenager, one Marlene Tucker, will play the corpse in the murder hunt game...except that when Poirot and Mrs. Oliver go to check on young Marlene, they find that she is dead for real. Moreover, that same day of the fete, in fact at nearly the same time, Sir George's young wife, Lady Hattie Stubbs, has disappeared.
The cops decide that no one in particular would have wanted to murder a random 14-year-old village girl, and that Marlene must have seen something regarding the disappearance of Lady Stubbs that led the killer to eliminate Marlene as well. But who killed Lady Stubbs? Not her husband, who was at the fete in public view at the time. Then who? George's lovestruck secretary, Miss Brewis? Amy Folliat, whose family owned the estate until the deaths of all her menfolk in the war forced her to sell? Etienne de Sousa, Hattie's cousin who hasn't seen her in many years but showed up for a visit that very day? Either Alec or Sally Legge, the young couple renting a college on the estate? And is Hattie even really dead?
- And You Thought It Was a Game: Mrs. Oliver has been asked to organize a Murder Hunt as one of the activities in a village fete. She begins to feel that she is being manipulated via proxies into changing details of the fictional murder to fit someone else's script, and calls in Hercule Poirot because she feels a real murder might be on the cards. The girl playing the "dead body" is duly killed for real.
- Author Avatar: Ariadne Oliver for Agatha Christie. They're both genteel old English ladies who write mystery novels, sometimes while eating apples in the bathtub. Mrs. Oliver appeared with Poirot in several of the later novels, sometimes as The Watson, and always as a vehicle for Self-Deprecation from Christie.
- Blackmail Backfire: An iron law of Christie novels is that blackmailers always die. Even though poor Marlene Tucker was only blackmailing her target for minor trivialities like lipstick and scarves, she still knows too much about the death of the real Hattie Stubbs, so she has to die.
- Chekhov's Gun: Several characters remark how the titular "Folly", a sort of outdoor tea house done in Greek column style, is in a pointless place, in the middle of the woods in a spot where there's no view. In fact it was built there to conceal the burial place of the real Hattie Stubbs.
- Chekhov's Gunman: On the way to Nasse House, Poirot gives a ride to two young lady backpackers, who are headed back to the youth hostel that sits next to the mansion. One of those two backpackers, the Italian one who pretends to speak no English, is Sir George's real wife, and the murderer.
- The Dandy: Etienne De Sousa, the cousin of Hallie's who comes visiting the day she disappears, is described thusly: "polished elegance...sartorial perfection, the rich flowery smell of his brilliantine hair." This irritates Inspector Bland and, along with his foreignness, focuses suspicion on De Sousa.
- Dead Person Impersonation: The Twist Ending. George is a criminal and deserter from the Navy who bigamously married the real Hattie and then, after taking control of her fortune, murdered her immediately upon returning to England. His partner-in-crime and real wife then assumed Hattie's identity. The plan was probably to keep doing that forever, but the appearance of a cousin of Hattie's who could probably spot the imposter forced George and Fake Hattie to whip up a complicated plot.
- Foreshadowing: A very obvious bit of foreshadowing when Marlene Tucker says her grandpa "saw a body in the woods once"—this coming mere hours before Marlene is murdered. Of course her her grandpa did see a body in the woods, namely, the real Hattie Stubbs.
- Inspector Lestrade: The inspector, a very generic policeman who exists to ask questions, is named "Bland".
- Intro Dump: Poirot asks Mrs. Oliver who all is at Nasse House, and she gives him and the reader a rundown of all the characters in the story. Then a few pages later, when Poirot makes his way there, he's actually introduced to all of them at once.
- Karma Houdini: Presumably the discovery of the real Hattie Stubbs's body will send George to prison, but the unknown "girl of the underground criminal world", George's real wife and the one who strangled poor Marlene, got away clean. Poirot reflects that she had plenty of time to get away to London and find friends who can give her papers.
- Leaning on the Fourth Wall: The presence of Ariadne Oliver gave Christie plenty of chances for gags in which Mrs. Oliver talks about how mystery novels were constructed. In her first conversation with Poirot in this book, she complains about how people are always correcting her about how her mysteries should have been written.Mrs. Oliver: All right then, write it yourself if you want it that way!
- Right after that exchange, Poirot asks Mrs. Oliver to tell him who's at the mansion, saying "the cast of characters must be limited."
- Madame Fortune: Mrs. Legge plays "Madame Zuleika" the fortune-teller. It's just a game, and she's not really trying to fool anyone.
- Make It Look Like an Accident: The death of Merdell, the old boatman, is written off as him slipping off the pier after coming home drunk from the pub. Turns out it was murder, because he knew too much.
- Men Are the Expendable Gender: Lampshaded by Ariadne Oliver in one of her many Leaning on the Fourth Wall interludes. She reflects that female victims are more effective in stories because "Men get killed and nobody minds—I mean nobody except their wives and sweethearts and children and things like that."
- Non-Indicative Name: It's not a "dead man's folly"! Sir George, who built the folly, is very much alive.
- Oblivious to Love: Miss Brewlis, Sir George's secretary, is halfway through a rant about how Lady Hattie wasn't a fit wife for George, but in fact holds him back through her childishness, when Poirot realizes that she's in love with Sir George and he has probably never noticed.
- Pointless Civic Project: The titular "folly" is an ugly building built with the usual good taste associated with the Nouveau Riche. In this case, it was built with a purpose: to hide the corpse of the owner's real wife, with the second wife posing as her ever since they'd arrived.
- Rightforthe Wrong Reasons: Several characters state that the killer must be one of those foreigners staying at the nearby Hostel. the killer is staying at the hostel while also pretending to be Hattie but killed Marlene because she was married to George not because she was a foreigner.
- Scavenger Hunt: A variation called a murder hunt (following clues to various places leading to a staged murder scene) on the grounds of Nasse House. Naturally, there's a real murder.
- Shout-Out: When George starts ranting about the trespassers from the youth hostel creeping over the estate, an amused Sally Legge says "You sound like Betsey Trotwood campaigning against donkeys." Sir George asks who that is.
- Thicker Than Water: Mrs. Folliat knew about her son's crimes and was horrified by them, but still found herself unable to turn him in.I should have spoken out after Hatties death... But he was my son. How could I be the one to give him up?
- Trophy Wife: Seemingly played straight with Hattie Stubbs, the beautiful but simpleminded wife of filthy rich Sir George. Later averted when the ending reveals that actually Hattie was the rich one and Sir George was the Gold Digger—and that the Hattie Stubbs in the book is an impostor anyway.
- Two of Your Earth Minutes: By this point Poirot has lived in England for decades, but his foreignness is still emphasized by him describing "your Underground railways."