At Enderby Hall, a sprawling Victorian Gothic mansion, the elderly butler Lanscombe raises the blinds after the funeral of Richard Abernethie. The Abernethie fortune was built on Coral Cornplasters, a pharmaceutical product. Richard, the eldest surviving son, raised his younger siblings after their father's death. His son Mortimer recently died of polio, leaving no direct heir and prompting a new will.
Mr. Entwhistle, the 72-year-old family solicitor and executor, surveys the relatives at the post-funeral luncheon: Helen Abernethie, widow of Richard's favorite brother Leo; Maude Abernethie, wife of Richard's chronically ill brother Timothy; George Crossfield, a solicitor and son of a late sister; Rosamund Shane, an actress married to the actor Michael Shane; Susan Banks, recently married to Gregory Banks, a chemist's assistant; and Cora Lansquenet, Richard's youngest sister, estranged from the family for 25 years after marrying a French painter.
Mr. Entwhistle reads the will, dividing the estate into six portions among these relatives. Cora is openly delighted. Then she tilts her head birdlike and asks, "But he was murdered, wasn't he?" (17). The family stares in dismay, and Cora quickly retreats, stammering that she thought so from something Richard said.
On the train home, Mr. Entwhistle reflects that Cora's embarrassing remarks were always rooted in truth and resolves to visit her discreetly. Helen, alone at Enderby, senses something was wrong at the moment Cora spoke, something about someone's face, but cannot identify it.
The next day, Cora is found murdered at her cottage in Lytchett St Mary, killed with a hatchet while sleeping. Her companion, Miss Gilchrist, discovers the body upon returning from an errand. Inspector Morton notes that trinkets were taken but discarded nearby, suggesting a staged robbery. Miss Gilchrist reveals that Richard visited Cora three weeks before his death, after which Cora remarked that Richard had fancies about persecution and someone poisoning him. Mr. Entwhistle knows Richard was not senile and suspects Cora understood more than she admitted.
Mr. Entwhistle visits each family member, probing their movements on the day of Cora's murder. Every alibi proves weak: George claims he was at the races, but a supposed winning horse did not even place; Rosamund and Michael give vague accounts; Greg claims the telephone was out of order, conveniently explaining why Entwhistle could not reach them; and Maude had a car breakdown and stayed overnight at an inn, leaving Timothy alone.
Deeply troubled, Mr. Entwhistle consults Hercule Poirot, a celebrated Belgian private detective. Poirot agrees that Cora believed Richard was murdered and that her death the next day is unlikely coincidence. He reasons a family member must be responsible, since Cora would not have hushed things up otherwise. Richard's doctor, Dr. Larraby, admits he cannot rule out poisoning, though cremation left no proof.
Susan drives to Lytchett St Mary for the inquest. At the cottage, Miss Gilchrist finds wedding cake in a parcel, which that night makes her gravely ill from arsenic poisoning. Susan also discovers a letter from Richard asking Cora not to tell anyone what he confided. Poirot's investigator, Mr. Goby, reports that none of the suspects' alibis holds up. Goby also reveals Greg's troubling history: He was once a voluntary patient in a psychiatric facility after deliberately giving a near-lethal dose of medication to a customer.
Posing as M. Pontarlier, a representative of a fictitious organization seeking to purchase Enderby, Poirot installs himself at the estate with Helen's cooperation. He confirms that Richard's medications could have been tampered with. When he mentions Greg's history to Helen, she drops a Victorian bouquet of wax flowers under a glass dome, shattering it. Poirot notes her reaction carefully.
Helen invites the family to Enderby under the pretext of choosing furniture before the house is auctioned. Miss Gilchrist accompanies Timothy and Maude, having been sent to care for them after Maude broke her ankle. During the evening, Poirot steers conversation toward nuns and mirror images. Miss Gilchrist casually remarks how nice the wax flowers looked on the malachite table. Poirot notes this keenly: The wax flowers were broken and removed before Miss Gilchrist arrived at Enderby as herself, meaning she could only have seen them if she was present on the day of the funeral.
Rosamund reveals that M. Pontarlier is actually Hercule Poirot, whom she recognized from a restaurant. That night, Poirot's mind churns through fragments: the smell of oil paint, nuns, the wax flowers, mirror images. He wakes knowing the truth. Simultaneously, Helen, reflecting on the discussion of mirror images, realizes what was wrong at the funeral. She phones Mr. Entwhistle at dawn but is struck on the head before she can explain.
Poirot arranges Helen's transfer to a guarded nursing home and sends Entwhistle to retrieve a specific painting from Miss Gilchrist's room: a canvas depicting Polflexan Harbour. In the summer-house, Poirot receives a series of revelations. Miss Gilchrist admits she overheard Richard telling Cora that a family member was trying to harm him, saying, "I can't do that. Not when it's a question of my own niece" (262), and asking Cora not to go to the police. Greg falsely confesses to killing Richard, driven by a psychological need for punishment and resentment that Richard despised him. Susan admits she drove to the cottage that day to question Cora but found no answer at the door.
A telegram from art critic Mr. Guthrie confirms that beneath the hasty overpainting of Polflexan Harbour lies "Definitely a Vermeer" (296), a work by the celebrated Dutch Old Master worth thousands of pounds.
Poirot assembles the family and Inspector Morton. He announces that there is no evidence Richard was murdered, and this is precisely the point: The entire case rests on Cora's words at the funeral. He asks how well any of them actually knew Cora, given that none of the younger generation had seen her in decades. Poirot reveals that the woman at the funeral was not Cora but Miss Gilchrist in disguise. Cora is drugged in her morning tea and kept unconscious at the cottage while Miss Gilchrist, wearing Cora's clothes and a false fringe of hair, travels to Enderby armed with years of Cora's reminiscences. She delivers the line about murder to create a false motive: If Cora appeared to have been silenced for accusing the family, the investigation would focus on the heirs rather than the companion.
Miss Gilchrist makes two critical mistakes. She practices Cora's characteristic rightward head tilt in a mirror, producing a leftward tilt instead. This was the wrongness Helen sensed. She also mentions the wax flowers, which she could only have seen while impersonating Cora. The true motive was a genuine Vermeer hidden among Cora's cheap picture-sale purchases. Miss Gilchrist, whose father was a painter, recognizes the painting's value when Cora did not. With Mr. Guthrie's visit imminent, she paints over the Vermeer, kills Cora with a hatchet, stages the burglary, sends poisoned wedding cake to herself to divert suspicion, and later attacks Helen.
Miss Gilchrist breaks down and confirms everything, raging that Cora could not recognize a Vermeer. The money would have funded her dream tea-shop. She goes quietly with Inspector Morton. In the epilogue, Helen confides to Poirot that Richard's money will allow her to provide for a secret child born of a wartime affair. Mr. Entwhistle reports that Miss Gilchrist was found guilty but will likely be sent to Broadmoor, a high-security psychiatric hospital, where she spends her time happily planning an elaborate chain of imaginary tea-shops.