Murder at Gulls Nest is set in the fading English seaside town of Gore-on-Sea, Kent. It follows a former nun who leaves her religious order to search for a missing friend and becomes entangled in a series of murders at a boardinghouse.
The novel opens with a prologue describing a dead woman in a dark underground pit with shell-encrusted walls, her neck broken and her hand outstretched in supplication.
Nora Breen, a middle-aged Irishwoman, arrives at Gulls Nest boardinghouse shortly after leaving the Carmelite monastery at High Dallow, where she spent thirty years as Sister Agnes of Christ. Adjusting to secular life, she relishes her freedom but feels unsteady without religious routine. The faded villa is run by Helena Wells, a polished but anxious landlady, and her surly housekeeper, Irene Rawlings. Nora takes a first-floor room whose previous occupant left unexpectedly in August. She meets Helena's daughter, Dinah Wells, an eight-year-old mute child who hides in curtains, furniture, and a tree-house platform high in a garden oak.
At supper, Nora meets the boarders: Stella Atkins, a kind young clerk; her sullen husband, Teddy Atkins, who works at the local amusement park; Bill Carter, a retired navy chef and black-market dealer; and Professor Poppy, a charming elderly Punch and Judy showman with a small white dog named Toby. Another boarder, Karel Ježek, arrives late and asks Nora if she has taken "Miss Brogan's old room." Teddy erupts at the name and storms out.
Nora carries letters from Frieda Brogan, the room's previous occupant and a young orphan from County Sligo who entered the Carmelite order. As the infirmary nurse, Nora grew deeply attached to Frieda during her frequent hospitalizations for a serious heart and lung condition. The bond ran deeper: Frieda's features recalled the baby Nora had been forced to give up after being raped as a teenager. Though Frieda was born too late to be that child, Nora's heart recognized her. When Frieda's health worsened, she was sent to the coast and settled at Gulls Nest. She wrote weekly until the letters stopped at the end of August. Her final letter mentioned that the residents were concealing secrets and that she intended to uncover them. Alarmed, Nora obtained a dispensation from her vows and followed Frieda's trail, concealing her connection from the boarders.
Nora begins investigating. Bill tells her that on Frieda's last night, he saw her post a letter Nora never received. Local gossip suggests Frieda was involved with a married man. Finding no trace of Frieda through local institutions, Nora goes to the police station, where a confrontation with the desk officer lands her before Detective Inspector Rideout, a rumpled, war-scarred man with sharp instincts. He agrees to investigate.
On returning to Gulls Nest, Nora finds Teddy dead on the floor of Poppy's workshop. A coffee cup on the workbench smells of bitter almonds, indicating cyanide poisoning. One puppet is missing from the wall: Jack Ketch, the executioner. In Teddy's pocket is a handkerchief embroidered with the initials F.B., one Nora made for Frieda. Karel's attic room appears vacated. Poppy raises a troubling theory: The poisoned coffee was his, since everyone knew the coffeepot belonged to him and Teddy never drank coffee alone. Stella, who claims she spent that morning reading aloud to Poppy, describes hearing someone enter the kitchen while kitchen helper Rose was out buying milk.
Nora presses forward. She discovers Cymag, a rabbit pesticide containing sodium cyanide, in a locked garden hut and finds notes on cyanide-related chemicals in Karel's room. Among Dinah's treasures, she spots a crucifix identical to Frieda's. Stella reveals she is expecting a baby and accuses Karel of murder. A local photographer confirms Karel and Frieda were close. Following Irene on her day off, Nora discovers that Irene's son, George Rawlings, was severely injured by a land mine and lives in a convalescent home. He is Helena's partner and Dinah's father. Helena was already married to a London doctor when she had an affair with George; her husband paid her to disappear.
Karel returns and Rideout clears him with a travel alibi. Days later, Poppy is found dead in bed, his face frozen in terror, with a swazzle, the reed device that produces Mr. Punch's voice, lodged in his throat. Toby Dog is also dead. Karel is cleared of this death as well, having been beaten unconscious and hospitalized the night before.
Visiting Karel in the hospital, Nora finds a photograph labeled "September 1939, Theo and John, Cuddfan," the last word being Welsh for "hideaway." She recognizes the boys as Teddy and Karel, childhood friends. Through a Carmelite contact in Wales, she traces the photo to a farm and learns that a farmer's son named John befriended an evacuee named Theodore. They enlisted underage together, but only Theodore returned; John Morgan was reported killed in combat.
Meanwhile, Nora follows Dinah through tunnels in the garden shrubbery and emerges in the neighboring vicarage garden, where a rabbit falling into a hole reveals underground caverns beneath the properties. Police recover a body from the shell-encrusted chambers. Rideout shows Nora a beadwork purse the nuns made as a parting gift, and Nora confirms it belongs to Frieda. A fall of over twenty feet through an eroded iron grate broke her neck.
At the hospital, Karel finally speaks. His real name is John Morgan. He and Teddy grew up together on a Welsh farm, fell in love, and served together in the war. When Teddy married Stella, John faked his own death by swapping identity papers with a dead soldier and roamed Europe as Karel Ježek. He tracked Teddy to Gulls Nest, where they lived as strangers to hide their relationship. He confirms Stella arranged his beating through Percy Ladd, the son of a local magnate.
The final clues converge. Stella finds the missing Jack Ketch puppet tucked into her bed. Nora examines the detective novel Stella claimed to have been reading to Poppy on the morning of Teddy's death and finds far too few pages turned, suggesting Poppy was drugged and Stella slipped away. Searching Stella's room, Nora finds evidence that Stella is menstruating and concludes the pregnancy claim was a lie designed to support the idea that Teddy had no reason to die. Rose reveals that the protective stones atop the milk bottles were missing that morning, meaning someone removed them so birds would upset the bottles and force Rose out of the house.
Nora brings Rideout to Gulls Nest. In the garden, she finds Stella on the tree-house platform with a terrified Dinah beside a tea set laid with poisoned sugar. Stella confesses. She overheard John and Frieda discussing Teddy and John's secret love and, enraged by years of sacrifice for a husband who looked through her, planned her revenge. She removed the milk-bottle stones to force Rose out, drugged Poppy with sedatives so he fell asleep during their reading session, and prepared poisoned coffee for Teddy. She later revived Poppy with smelling salts so he would believe she had been with him the entire time. She steered Frieda over the weak grate that night and listened as she fell. She killed Poppy because he was beginning to realize she had slipped away that morning. When Stella tries to force Dinah to drink the poisoned tea, Dinah pours it out. Stella slaps the child, and Dinah screams, her first vocal sound. Stella lunges for Dinah, trips, and falls from the platform to her death.
In the aftermath, Rideout and Nora review Dinah's notebook, which contains drawings of everything the child witnessed. Dinah was the first to find Teddy dead; she tidied the cup, saucer, and spoon into a line and took the Jack Ketch puppet, later placing it in Stella's bed as her own silent accusation. Walking on the beach, Rideout asks Nora to stay in Gore-on-Sea. The novel ends with the two heading along the promenade, a favorable wind behind them.