As the sole witness to a dying man's enigmatic last words, would-be young adventurer Bobby Attfield is determined to solve the riddle they posed. Attfield forms an unlikely alliance with the beautiful socialite Frankie Derwent and visiting family friend Miss Marple. When someone tries to kill Bobby, the trio realizes a murderer is at large, and a trail of clues leads them to Castle Savage and its strange inhabitants.
The trio charms their way into the castle and meets the other-worldly Sylvia Savage and her teenage children by the late, and loathed, Sir Jack Savage. While daughter Dorothy seems intent on snaring Bobby, son Tom haunts Frankie with all the stealth of his pet viper. Tom is not pleased when she catches the eye of handsome music teacher Roger Bassington.
Meanwhile, Miss Marple quietly watches from the distance. Drawing on her deep knowledge of human behavior, she carefully observes the people connected to the castle — the controlling psychiatrist Dr. Nicholson and his delicate young wife Moira; the eccentric Claud Evans with his hothouse full of exotic orchids; and Wilson, Sylvia's solemn, ever-present butler.
Realizing that Jack was also murdered, Bobby and Frankie must navigate a hotbed of stifled emotion, treachery and poisonous deceit to make any headway in the case. Their theories are tested when another death occurs and Commander Peters makes a decisive arrest. In a shocking climax, Miss Marple reveals the truth behind the murders, and in so doing is finally able to answer that most baffling question, "Why didn't they ask Evans?"
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Warning: Contains significant plot spoilers
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Warning: Contains significant plot spoilers
Below a cliff edge, a dying stranger's last words, "Why didn't they ask Evans?" send young Bobby Attfield on a search for the victim's identity, the reason for his death, and the answer to the question, who is Evans?
He is joined by budding love interest, Lady Frankie Derwent, who soon objects to the involvement of Bobby's old family friend Miss Marple.
Nonetheless, the three sleuths make an effective team and quickly establish that the victim is a Mr. Pritchard (later discovered to be John Carstairs), that he was involved with a local estate called Castle Savage, and that someone is trying to put an end to their snooping.
Frankie deliberately crashes her sports car at the gate of Castle Savage in order to insinuate her way inside. Marple later shows up disguised as Frankie's nanny and Bobby as her chauffeur.
Castle Savage, they discover, has an appropriately gothic cast of characters. The mistress is Lady Sylvia Savage, recently widowed from her second husband, Jack, who keeled over from a coronary the day after changing his will. Her first husband, George, Jack's older brother, died under mysterious circumstances in China.
Sylvia's daughter, Dorothy, is a wide-eyed realist about the family's dysfunctions and takes a keen interest in Bobby. Meanwhile, her brother, Tom, is obsessed with deadly vipers and the injustice of his father's will.
Then there is Dr. Nicholson, a psychiatrist and poison authority married to an ethereal beauty, Moira. Roger is Dorothy and Tom's debonair piano teacher, who seems to have eyes only for Moira — until Frankie comes along.
Sylvia's close friend Commander Peters is a former police detective and a foil for the undercover sleuths, although he doesn't realize they are several steps ahead in solving the accumulating crimes.
The butler, Wilson, is Sylvia's faithful dispenser of narcotics provided by Dr. Nicholson. And finally, there is Evans — Claud Evans — a fancier of orchids, including the toxic variety.
After several sinister incidents, including the fatal poisoning of Evans, Marple and the others burst into Sylvia's bedroom, finding her in the clutches of Moira and Roger, now revealed to be her long-lost children with George, returned from the past to kill her for abandoning them in China, when she took Jack as a lover after he had George killed.
Moira is poised to plunge a syringe with snake venom into Sylvia, but Marple holds her at bay by offering to tell what she has surmised about the siblings' conspiracy. Namely, that Moira killed Jack with snake venom, reproducing the symptoms of a heart attack, thanks to information innocently supplied by Dr. Nicholson.
As part of the plot, Roger impersonated Jack in order to change the will, requiring witnesses who would not notice the ruse. Carstairs, a friend of Jack's, later showed up and called the will into question, leading to his "fall" from the cliff.
The "Evans" of his last words was not Claud Evans (who Roger later killed to frame him), but the housemaid Florrie Evans, an obvious person to witness the will, until she was sent off because she would have recognized Roger. This explains Carstairs' unfinished question: Why didn't they ask Evans...to witness the will?
In the scuffle that follows Marple's expos´, Tom shoots Roger dead and Wilson turns the fatal syringe on Moira. Bobby and Frankie leave as lovers.