When Miss Marple learns of the deaths of businessman Rex Fortescue, his young wife Adele and their housemaid Gladys, the circumstances vividly recall the nursery rhyme "Sing a Song of Sixpence." Since Miss Marple had trained Gladys herself, she has a very personal reason to investigate.
The opulent Fortescue household is run under the watchful eye of housekeeper Mary Dove, but Miss Marple soon sees it is a fractured home — Rex's son Percival is desperately trying to save the family company while neglecting his wife Jennifer, and a bitter sibling rivalry has reignited since the return of younger son Lance with his aristocratic new wife Pat.
With the help of Inspector Neele and Sergeant Pickford, Miss Marple questions why Rex's daughter Elaine and the debonair Vivian Dubois should be so evasive about their whereabouts at the time of the murders, and she begins to understand the significance of the blackbirds that have been making unexpected appearances. Miss Marple discovers vital clues that lead to the mysterious Mrs. MacKenzie and her missing family, who loom out of Rex's shadowy past to provide revelations that allow Miss Marple to make sense of the rhyme and at last reveal the truth.
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
Financier Rex Fortescue drops dead at his London office from taxine, a poison derived from yew berries, which suggests the name of his home: Yew Tree Lodge. Oddly, he has a pocket full of rye grain.
Arriving at the lodge to investigate, Inspector Neele and Sergeant Pickford find a house full of suspects, all claiming that they "didn't do it."
Mary Dove, the housekeeper, is a fount of knowledge on the family's secrets, although it develops that she may not be who she claims. Meanwhile, Gladys, the parlor maid, is good-hearted but unusually nervous during questioning. Interestingly, she was formerly employed be Miss Marple.
Adele is Rex's widow and trophy wife, some 30 years younger than he. Her constant companion is Vivian Dubois, a very attentive golf instructor.
Rex has three grown children by his first wife. The oldest is Percy, his partner in the family business. Percy is married to Jennifer, a former nurse (adept at handing drugs, as a member of the house staff notes). She feels neglected by her husband's workaholic habits.
Next is Lance, Rex's estranged son in Kenya, who has recently married Pat, twice widowed on account of war and suicide. Elaine, the youngest of Rex's children, is single, but not by choice: her intended was rejected by Rex for having communistic tendencies. Elaine is a vegetarian.
Amid this atmosphere of sleuthing and suspicion, afternoon tea is served — except Gladys is not there to pour it, since she's dead in the garden with a clothes peg attached to her nose. Shortly after, Adele, alone in the sitting room, expires from her cup of tea. Coincidentally, Lance has just arrived from Kenya.
And soon Miss Marple arrives, attracted by news of the death of her former servant Gladys. She is intrigued by the "Sing a Song of Sixpence" theme being followed by the killer: the king, Rex, in the counting house, counting out his money (and also having a pocket full of rye); the queen, Adele, in the parlor, eating bread and honey; and the maid, Gladys, in the garden, hanging out the clothes, with the peg symbolizing a blackbird pecking off her nose.
Things start adding up when Marple learns that several months earlier Rex received a blackbird pie as an anonymous prank, and that the malicious reference and others like it were clearly meant to remind him of an old investment gone bad: the Blackbird gold mine in Africa.
Rex's partner in the failed enterprise was Mr. MacKenzie, who died in Africa, leaving behind a resentful wife and two small children. Marple tracks down Mrs. MacKenzie to an old-age home and discovers that she bears an undying grudge against Rex, whom she blames for her husband's death, and that at least one of her children, a daughter named Ruby, is still alive and grown up.
Inspector Neele is keen on the MacKenzie angle and the prospect that one of the younger generation has returned to do in Rex. Furthermore, when confronted, Mary Dove does not deny that her real name might be Ruby MacKenzie.
But Marple has concluded that the nursery rhyme is a red herring, and she lays out her conclusions to Neele. The real killer, she says, is...Lance.
Hearing about the blackbird pie prank the previous summer, Lance had figured that Ruby McKenzie was established in the house and a perfect scapegoat for the surreptitious poisoning of Rex. Lance's motive? The inheritance before the increasingly demented Rex lost it in the market. Lance was especially interested in the Blackbird mine, since he suspected it was rich in undiscovered uranium.
Inspired by his father's name (rex, being Latin for king), Lance worked out a sequence of murders that made it seem as if a nursery rhyme was driving the carnage, rather than Lance's self-interest.
Gladys was the unwitting key to his plot. Using the alias Albert Evans, Lance courted her and convinced her to doctor Rex's morning marmalade with a supposedly harmless "truth drug," which was in fact deadly taxine (derived from the abundant yew on the estate). She also put the rye in his pocket. Lance later lured Gladys to the garden, where he strangled her with a stocking, disposing of the only witness to his scheme. He then dispatched Adele, the first in line for the inheritance, with potassium cyanide.
Marple makes one last revelation: Jennifer, Percy's wife, is the real Ruby McKenzie, who insinuated herself into the family looking for her chance for revenge, which she never took apart from the blackbird pranks. She paid Mary Dove to divert suspicion from herself.
With the case closed, Marple returns home, where she receives a misdirected letter from Gladys, sent before the murder plot was put into motion. Poignantly, Gladys asks for advice about a young man she has just met, one who says he loves her and talks of a mysterious truth drug.