Plot

Miss Marple aids a young mother and son by spiriting them away to the estate of an eccentric botanist, not knowing that it will soon become the scene of murder.

When old family friend Louisa Oxley visits Miss Marple one stormy night seeking help, Miss Marple decides to send her and her son, Archie, to safety at the labyrinthine estate of Greenshaw's Folly, owned by Miss Marple's good friend Katherine Greenshaw, an eccentric botanist and the last surviving Greenshaw. Louisa becomes Miss Greenshaw's secretary, and quickly attracts the attention of the gardener Alfred Pollock and actor Nat Fletcher. Things turn sinister when the Folly's faithful butler, Walter Cracken, is killed in what appears to be a tragic accident. Then a guest at the house, Horace Bindler, disappears without a trace. Miss Marple is convinced all is not well, and her suspicions are confirmed when Miss Greenshaw herself is brutally murdered. Suspects are everywhere, but none can imagine the secrets, both past and present, which Miss Marple uncovers.—shanty_sleuth

Miss Marple comes to the aid of Louisa Oxley and her young son, Archie. They end up at Greenshaw's Folly, an estate owned by a friend of Miss Marple, Katherine Greenshaw. However, their stay there is hardly uneventful. Soon after their arrival, the long-serving butler dies after falling off a ladder. Miss Marple suspects that it may not be an accident but the police are not impressed. Then Ms Greenshaw dies too, from an arrow through the neck. Now the police are interested...—grantss

Louisa Oxley and her son, Archie, visit Miss Marple on a stormy night seeking help. Miss Marple decides to send them to safety at the labyrinthine estate of Greenshaw's Folly, owned by Miss Marple's good friend Katherine Greenshaw, an eccentric botanist and the last surviving Greenshaw. Louisa is to be Miss Greenshaw's secretary.

Greenshaw needs help with her writing as she is nearly blind. Archie, exploring, runs across gardener Alfred Pollock who is covered in blood. Mrs. Cresswell, an employee, Greenshaw's nephew, actor Nat Fletcher, and Horace Bindler, a historian, are also at the house. Bindler explains to Miss Marple that Decimus Greenshaw was a mysterious man, with a laboratory in the house which is kept locked. The butler, Walter Cracken, tells Archie of a ghost that roams the Folly.

Things turn sinister when Cracken is pushed from a ladder. It is assumed to be an accident, but Archie saw what he believed to be the ghost that night. Meanwhile, Bindler snoops around the Folly and Pollock teaches Archie to shoot a bow, much to his mother's chagrin.

The next morning, Bindler has disappeared without a trace. He had refused to have a fire lit in his room, and Miss Marple finds a box hidden in the chimney. Father Brophy, who runs a local orphanage, visits to ask Miss Greenshaw for money, but she refuses, pointing out his numerous bar bills and gambling debts.

Thinking they have seen Oxley's husband in the window, Louisa and Archie also make a run for it, but their car breaks down. They are rescued by Pollock, and Fletcher returns them to the Folly.

Miss Marple consults with a local friend, Cicely, on Bindler's box, but she steals an item from the box after claiming to not understand a reference to the "Great Race". The orphanage's records for the years 1900-1910 are missing. Bindler's car is found at the bottom of a lake. A photograph is found of the orphans from many years ago, with several of the orphans having X's over their faces. Archie, still exploring, finds Mr. Greenshaw's laboratory unlocked. As he looks around, the door is locked behind him. Simultaneously, Louisa is locked in her study and Miss Greenshaw is shot with Pollock's bow. She calls Miss Marple at the orphanage, mentioning, according to the message, "A Heap of Haddock". Cicely turns up at the house and drags Archie outside, calling him "Edwin", and saying "Greenshaw won't get away with it this time". Archie's father is just arriving. When the police arrive, Archie has disappeared and Miss Greenshaw is dead.

The police find Archie at Pollock's cottage, where has refused to allow Archie's father to take him. The police find evidence that Pollock had been stealing and was about to run. As the suspects gather in the library, Cicely is found in the laboratory. Edwin was her dead brother; they both resided at the orphanage. Miss Marple finds Bindler's body in the dumbwaiter. Archie's father makes another attempt to snatch him and is again thwarted by Pollock.

When the inspector returns, he explains that Nat Fletcher will inherit the estate as Miss Greenshaw had died intestate. Fingerprint evidence points to Pollock and the police attempt to arrest him, but Miss Marple reveals all: The old Dr. Greenshaw had used the orphans as experimental subjects for his polio vaccine experiments; Father Brophy had been doing the stealing; Miss Greenshaw had asked for "A Pile of Carp" rather than a "Heap of Haddock", actually Pilocarpine, an antidote for nightshade. Nat Fletcher is shown to be an impostor and the son of Mrs. Cresswell - they had plotted to inherit the estate. The plan had been discovered by Cracken and also Bindler - who was actually a reporter - necessitating their removals. Pollock is revealed to be Mrs. Greenshaw's illegitimate son - finding this out, Cresswell and Fletcher had attempted to frame him for his own mother's murder.

Pollock inherits the estate and asks the Oxwells to stay on with him.