At the Coopers Chase retirement village, Ibrahim Arif, a thoughtful psychiatrist, Ron Ritchie, a former trade union leader, and the enigmatic Elizabeth Best are the core members of the Thursday Murder Club (TMC), a tight-knit group that meets weekly to revisit old cold cases. They are soon joined by Joyce, a retired nurse and new resident whose medical knowledge proves unexpectedly useful. The club draws inspiration from a case once handled by Detective Inspector Penny Gray in 1973, now comatose in hospice, involving the unsolved murder of Angela Hughes. Meanwhile, Donna De Freitas, a Police Constable recently transferred from London, visits the village and quickly forms a bond with the TMC, hinting that her own experience might help crack the mysteries they chase.
A redevelopment dispute over Coopers Chase pits the village against itself. One of the owners, Ian Ventham pushes the project forward, while his partner Tony Curran opposes it and vows to block the sale. Soon after, Tony is murdered in his own home, his house ransacked and the weapon missing. The TMC resolve to investigate, and with Donna’s help they uncover that local crime boss Bobby Tanner is secretly involved in the scheme and has long been thought dead after disappearing years earlier. They also learn that [Ron] Ritchie’s son Jason once worked with Tony Curran. Initial suspicion falls on Ian, but he is quickly ruled out. During a residents’ protest, Ian collapses and dies from a fentanyl overdose, leaving Tanner as the sole owner. Jason is arrested, though the evidence against him proves weak.
Bogdan, a Polish handyman, forms a cautious friendship with [Elizabeth Best], and he discovers a hidden grave containing a skeleton, later identified as Peter Mercer, the man who witnessed Angela Hughes’s death decades earlier. Elizabeth also begins receiving threatening messages, warning her to leave the matter alone.
When Jason provides an alibi—an affair with Ian’s estranged wife Gemma—the TMC pivot back toward Tanner. Elizabeth and Donna uncover that Tanner has been trafficking migrants and seizing their passports, and, to their shock, Tony Curran was his partner. Elizabeth confronts him and strikes a bargain: he must keep Coopers Chase intact in return for her silence. She then realizes that [Bogdan] was responsible for Tony’s death; Bogdan, while playing chess with Elizabeth’s husband Stephen, confesses that he killed Tony under pressure but did so unintentionally. His confession is secretly, and accidentally, recorded by Stephen, and Bogdan is arrested. He reveals he merely wanted his passport back so he could return to Poland to care for his sick mother.
The truth about Angela Hughes’s murder also emerges. The TMC deduce that Mercer killed Angela, but that [Penny Gray] killed Mercer and concealed his body. As Ian’s redevelopment plans threaten to reveal Mercer's body and implicate Penny, her husband John murders Mercer to protect her. Both Penny and John die soon after in hospice care, the implication being a fentanyl-fueled murder–suicide.
At their funeral, Elizabeth presents Penny’s club necklace to Joyce, formally welcoming her as a full member of the TMC. With Coopers Chase once more up for sale, Joanna, Joyce’s daughter, considers buying it. Life in the village quietly resumes, with old friendships renewed and the club ready for whatever mystery may come next.
