The Queen of Poisons

The story opens with Suzie Harris attending a Marlow town council planning meeting, hoping to scout allies for a future planning application for a pod hotel she wants to build in her back garden. As she observes the committee members from the viewing gallery, she notices a man wearing blue catering gloves serving in the kitchenette before slipping out through a fire door. When the meeting begins, Geoffrey Lushington, the beloved mayor of Marlow, makes himself a cup of coffee using the Nespresso machine, accepts sugar cubes from fellow committee member Marcus Percival, takes a sip, and collapses dead within seconds.

Suzie immediately phones her friend Judith Potts, who arrives with their third companion, Becks Starling, the resourceful wife of the local vicar, Colin. Detective Inspector Tanika Malik, recently promoted, officially enlists the three women as civilian advisers on the case. A postmortem reveals that Geoffrey died from ingesting aconite, a plant so lethal it is known as the Queen of Poisons. The poison is found in the residue of his coffee, along with sugar, yet no sugar bowl is recovered from the scene.

The friends set up a makeshift incident room in Judith's sprawling Arts and Crafts mansion on the Thames, pinning index cards for each suspect to the wall. They begin interviewing the four other committee members. Sophia De Castro, a wealthy wellness podcaster, shows them her private "Poison Garden," which includes a thriving aconite plant. She insists she could never harm Geoffrey because he once saved her from professional ruin after she mismanaged graveyard plot assignments on a council subcommittee, resulting in families being buried in the wrong graves. When the friends observe Sophia's husband, Paul De Castro, arrive home and ignore her coldly, they note the troubled marriage. Marcus Percival, a charming estate agent, insists Geoffrey's death must have been accidental. Debbie Bell, an accountant who works for Paul's wealth management firm, reveals she overheard Geoffrey and the committee's architect, Jeremy Wessel, arguing after a recent meeting, with Geoffrey telling Jeremy, "I'm not giving it back to you." Jeremy claims the dispute was over a bottle of Japanese whisky, but the friends remain skeptical.

Judith receives an anonymous phone call from a distorted voice instructing her to "follow the money." Forensic updates from Tanika deepen the mystery: the security camera cable in the council corridor was cut on the morning of the murder, a provocative photo of a woman with a diamond-shaped birthmark was found on Geoffrey's phone, and Debbie's fingerprints appear on the poisoned coffee capsule alongside Geoffrey's. Debbie explains she simply cleared a jammed capsule from the machine before making her own coffee.

A critical discovery emerges when the friends learn that Alec Miller, the retired birdwatcher who normally serves tea at council meetings, was told not to come in that night. When Becks plays a clip of Sophia's podcast, Alec identifies her voice as the caller who canceled the meeting. Using a methodical questioning technique, Suzie reconstructs a description of the mystery man she saw in the kitchenette: average height and build, short blond hair, dark glasses, and a blond goatee beard. The description matches none of the known suspects.

At the murder scene, Judith finds the missing container of sugar cubes, a glass Kilner jar, hidden in a filing cabinet. The jar bears Geoffrey's and Marcus's fingerprints but tests negative for aconite. A second anonymous call tells Judith to "check the petty cash." Using a webcam on the church spire installed to monitor nesting kestrels, Becks captures footage of the anonymous caller: a significantly overweight man on a mobility scooter. Sophia identifies him as Dave Butler, a freelance IT technician who once worked in her podcast studio.

When someone breaks into Geoffrey's house, the friends discover he had recently installed new security measures. Inside a locked filing box, they find a blackmail letter assembled from cut-out magazine letters. Forensics trace fingerprints on the letter to both Geoffrey and Marcus, who confesses he was the blackmail's recipient, not its sender. He had brought the letter to Geoffrey for safekeeping and admits to breaking into the house to retrieve it before police found it. Suzie later identifies that letters in the notes were cut from the masthead of Architectural Digest, implicating Jeremy, the only architect in the case. Jeremy admits he sent the letters out of longstanding resentment, having discovered that Marcus ran a viciously bigoted anonymous Twitter account. Geoffrey had deduced Jeremy's involvement, which was the true subject of their argument, but Jeremy insists he never went near Geoffrey's coffee, a claim Suzie corroborates.

Becks's painstaking review of council financial records yields a breakthrough when Judith spots duplicate receipt amounts from a hardware store that had already closed. Debbie confesses to embezzling roughly £2,000 per year by submitting fake petty cash receipts, initially to fund private treatment for debilitating menopause symptoms and later to purchase collectible figurines.

The friends gain entry to Dave Butler's house during a grocery delivery and find him living in squalor. Dave reveals that Geoffrey had recently reconnected with him, apologized for rejecting his science fiction novel years earlier, and offered to publish it. Dave claims he tipped Judith off about the embezzlement to help catch the killer but insists he does not know who committed the murder.

Evidence increasingly points to Sophia. Judith finds one of Sophia's earrings under Geoffrey's bed, and Paul confirms that Sophia has the distinctive birthmark from the photo on Geoffrey's phone. Sophia confesses she had been romantically obsessed with Geoffrey and was humiliated when he rejected her advances. She let herself into his house to photograph herself and sent the image to him, prompting his new security measures. The friends also discover that Sophia's supposed tire puncture on the night of the murder was merely a deflated tire, leaving her with an unaccounted hour. In a nearby toilet block, Becks finds a hidden carrier bag containing a blond wig, a goatee beard, and bandaging, all bearing Sophia's fingerprints and hair.

Yet Judith senses something does not fit. Sophia lacks a compelling motive, since Geoffrey had a lifelong pattern of keeping others' secrets. A small detail breaks the case open: Judith recalls that Dave once described Debbie as "sinister," a word that also means "left-handed" in Latin. Debbie is indeed left-handed, something Dave could only know from watching committee meetings remotely. Judith deduces that Dave installed a hidden camera in the debating chamber and, over the past year, lost a massive amount of weight in seclusion while padding his clothing to maintain the appearance of obesity in his rare public appearances. This allowed him to commit the murder disguised as the average-sized blond server while remaining above suspicion.

Dave's motive traces back to his deepest wound. His mother died while Sophia was mismanaging the council's graveyards, and her remains were among those permanently lost. When Geoffrey told Dave the full story during their reconciliation, Dave resolved to punish them both: Geoffrey for enabling the cover-up, and Sophia by framing her for murder. His anonymous phone calls to Judith were never genuine attempts to help but served as an insurance policy, allowing him to point to his cooperation if suspicion ever turned his way.

To catch Dave, Judith stages an elaborate trap. She arranges a reenactment at the debating chamber, knowing Dave will watch through his hidden camera. During the session, Sophia states, following Judith's script, that she has found documents in her boathouse showing the true burial locations. Judith then sets fire to her entire decades-old newspaper archive in her garden, a hoard she collected after her abusive husband's death, creating a blaze visible across Marlow. When Tanika receives a staged call about the fire, Dave seizes the apparent distraction to break into Sophia's boathouse loft. Judith, hiding inside, confronts him as he removes his balaclava and walks him through every step of his plan. Tanika arrives with officers to arrest him.

In the aftermath, Judith watches the remnants of her bonfire with her friends, feeling lighter for having finally let go of the archive. She reveals one last scheme: she arranged for Matthew Cartwright, a widowed old friend who had been pursuing a reunion with her, to encounter Marian Starling, Colin's difficult mother-in-law who has been living at the vicarage and making Becks's life miserable. Marian, instantly charmed by the wealthy and handsome man, takes the bait, offering Becks a potential escape. The three friends walk back into Marlow together, content in their friendship and unchanged in their essential natures.