Dead Man's Folly
Poirot is summoned to Nasse House in Devon by Ariadne Oliver, who is staging a Murder Hunt as part of a summer fête the next day. At Nasse House, Mrs Oliver explains that small aspects of her plans for the Murder Hunt have been changed by requests from people in the house rather deviously, until a real murder would not surprise her.
The wealthy Sir George Stubbs owns Nasse House. His much younger wife is the beautiful Hattie, Lady Stubbs. She shows interest in fine clothes and jewellery only, appearing simple to all but her husband's secretary, Miss Brewis, who sees through Hattie's outward appearance but is herself conflicted because of her own feelings for her employer, Sir George. Hattie and George were introduced by Amy Folliat, the last of the family who had owned the estate for centuries. Widowed, Mrs Folliat lost her two sons during the War. With the death duties very high in the post-war period, she had to sell the ancestral home and grounds to keep it intact. She took on the orphaned Hattie, introducing her in society. Mrs Folliat rents the lodge on the estate. Michael Weyman, an architect, is on site to design a tennis court; he criticises the inappropriate location of a recently built folly. Sir George shouts at three young tourists who cross his private property; they are a Dutch woman, an Italian woman, and a man wearing a shirt decorated with turtles.
On the day of the fête, Hattie receives a letter from her cousin, Etienne de Sousa, who will visit that day; she appears very upset by his abrupt visit. A local Girl Guide, Marlene Tucker, waits in the boathouse to pose as the dead victim when a player finds the key to enter. Her first visitor is Miss Brewis with a tray of refreshments at tea time, at Hattie's request. With Mrs Oliver, Poirot discovers Marlene dead in the boathouse. Hattie cannot be found. Mrs Oliver produces an abundance of theories to explain the murder and the disappearance, while the police and Poirot narrow the field from all attending the fête, to those familiar with the Murder Hunt. The investigation focuses first on Etienne de Sousa and briefly on Amanda Brewis. Further confusion is added by the behaviour of the Legges, staying in a cottage on the estate and whose marriage is in trouble. After weeks of no progress, Poirot visits Devon again, learning that Hattie is still missing. Merdell, the old boatman, who drowned, was Marlene's grandfather. Poirot puts together several stray clues: Marlene's grandfather had seen a woman's body in the woods; Marlene received small sums of money used to make small purchases, now in her younger sister's possession. Merdell had told Poirot mischievously that there would "always be Folliats at Nasse House".
In the dénouement, Poirot explains that Sir George Stubbs is really Amy Folliat's younger son, James, a war deserter. Mrs Folliat paired him with the wealthy but naive Hattie, hoping that the marriage would be beneficial to both. But James fleeced Hattie of her money to establish his new identity and to purchase the old family home. Unknown to Mrs Folliat, James had married a young Italian woman after deserting the war. He killed the original Hattie shortly after entering into the bigamous marriage, and his Italian wife played the role of Hattie thereafter. Marlene Tucker had learned the true identity of George Stubbs from her grandfather. Both were murdered separately, although the old man's death has been presumed accidental. The day before the fête, the fake Hattie poses as an Italian tourist staying in the nearby hostel. She switches between the two roles frequently over a 24-hour period. The fake Hattie sends Miss Brewis to bring refreshments to Marlene shortly before the girl is murdered. She kills Marlene then changes to the tourist guise, tossing the large hat she wore as Hattie in the river. She then leaves the area as the Italian tourist carrying a rucksack.
The date of Marlene's murder had been selected to cast suspicion upon Etienne, who had written weeks earlier of his visit, as he told Inspector Bland. Having grown up with the real Hattie, Etienne would not have been fooled. Neither the arrests of the culprits nor legal charges against the despairing Mrs Folliat are mentioned. The novel concludes with the sounds of the police smashing up the folly to locate and exhume Hattie's body.