Promise Me Never

The novel opens with a brief prologue: an unnamed woman wakes bound in a dark basement, her last memory of sitting on a sofa with Bennett, sipping wine. Heavy footsteps descend the stairs, a knife appears, and she is killed.

The story introduces Savannah, an aspiring novelist working as a manuscript reader at the literary agency Margolis, Sutton & Klein in New York City. She has idolized Bennett Bryson, a bestselling horror author represented by the agency, for years and strategically positioned herself at the firm to gain proximity to him. Bennett's sales have declined, and his agent, Marion Sutton, has pressured him to headline an upcoming writers' conference to promote his new release, Don't Ask If She's Sorry. Bennett is recently widowed; his wife, Poppy, died in what was ruled an accidental drowning from their yacht. Savannah, who has been studying a dating guidebook about creating romantic mystery, sees the conference as her chance.

At the conference, Bennett meets Savannah after she impresses him during a bookstore talk. He is immediately captivated, feeling attraction for the first time since Poppy's death. Over the following days, he insists she join him for dinners, and they spend the night together in his hotel suite. Bennett extends his stay for a month.

Interspersed with Savannah's present-day narrative are first-person flashback chapters narrated by Poppy. Poppy recounts meeting Bennett, their rapid courtship, and her move to his secluded estate in Bedford, New York. A mixed-media collage artist, Poppy clashes immediately with Felicia, Bennett's loyal housekeeper and former childhood nanny. She befriends Henri, a young man at the local coffee shop, and Isabel, Bennett's lifelong family friend. Bennett proposes, and Poppy accepts despite misgivings about giving up her independence. She learns he was previously engaged to two women, Angela and Devon, who both vanished after supposedly texting him breakup messages.

In the present, Bennett and Savannah's month together deepens into love. When he prepares to return to Bedford, Savannah insists on a commitment before moving in. He proposes, and she manipulates the timeline, claiming religious convictions and ignoring his calls until he agrees to marry within weeks. Marion warns that Bennett is solitary and obsessive, but Savannah proceeds. A brief scene hints at a hidden identity: when someone at a restaurant calls her "Millie," she flees in panic.

Savannah marries Bennett and moves to the estate, where Poppy's art still fills the house and Bennett's office remains locked. His behavior oscillates between tenderness and control: He snaps at her for disturbing his work, gives contradictory feedback on her manuscript, and apologizes each time, establishing a pattern of isolation. Privately, Bennett maintains a password-protected folder of photos of women from his past, dwelling on intimate details of his last encounters with each. He reassures himself that Savannah is different but muses that if she discovers his secret and things do not work out, she too will have to go.

In Poppy's flashbacks, she grows suspicious of the locked office. She secretly duplicates the key, enters while Bennett is away, and discovers something deeply troubling on his computer. She plans to confront him on their yacht.

After Bennett leaves for a European book tour, Savannah begins receiving warnings. A note on the car reads: "You're in grave danger. Bennett Bryson isn't who you think he is." A woman at the dog park claims Bennett has killed three women. Bennett dismisses the note, and Felicia tears it up. Unknown to Savannah, Bennett has installed cameras throughout the house and monitors her remotely.

The mysterious woman reveals herself as Isabel and tells Savannah that Angela and Devon both vanished suspiciously, that Devon left behind her insulin, and that Henri believes Poppy's death was no accident. Savannah discovers an article about the unsolved murder of Louisa Matthison, Bennett's high school girlfriend, who drowned with head trauma after a party at his house. She visits Devon's father, Harrison Masterson, in Brooklyn, posing as Devon's old friend. Harrison is convinced someone took his daughter. Henri tells Savannah he believes Bennett killed Poppy.

Isabel urges Savannah to search the house. In Bennett's jacket pocket, Savannah finds the office key. She also discovers an Hermès scarf on the mudroom floor and matches it to a photo of Angela. Inside the office, she finds a gold box in a desk drawer containing small personal items, apparent trophies. She texts Isabel, who comes over and, while briefly left alone in the office, "discovers" two old cell phones on a high shelf. Savannah charges them and confirms they belong to Devon and Angela.

Savannah is about to call the police when she notices Isabel's necklace: gold add-a-beads with a distinctive pink bead, identical to one worn by Devon's mother in a photo at Harrison's house. The bead ties Isabel directly to Devon. Savannah realizes Isabel planted the phones and that Bennett is not the killer. Isabel, seeing Savannah's recognition, pulls out Savannah's own Glock. At gunpoint, she confesses: She has been in love with Bennett since she was 14. He slept with her intermittently but always relegated her to friend status. Each time he became involved with another woman, Isabel killed the rival. She murdered Louisa in high school, killed Bennett's sister Janie when Janie discovered the truth, and kidnapped and killed both Angela and Devon, dumping their bodies in a well on her father's Pennsylvania farm. Isabel forces Savannah to write a farewell letter framing Bennett, injects her with ketamine, and drives her to the farm.

Savannah wakes in a locked room. She taunts Isabel, saying Bennett will never love her and revealing she knows about Janie. When Isabel lunges in rage, Savannah seizes the gun. Bennett, who watched Isabel plant the phones on his home cameras from Rome, has flown back. He arrives at the farmhouse, and in a struggle with Isabel, the gun discharges and kills him. Isabel is arrested and later convicted on four counts of murder.

One year later, Savannah's novel Killer Twist, a fictionalized account of Isabel's crimes, debuts at number one on the New York Times bestseller list. At a book signing in Santa Fe, a woman approaches and says: "Hello, Millie. Make it out to Poppy." Savannah looks up to see her sister, alive.

The novel reveals that Savannah's real name is Millie Morningside, Poppy's fraternal twin. Their mother pitted them against each other through beauty pageants, calling Poppy the "golden child." Millie changed her name after college. When the sisters briefly reconnected, Poppy showed Millie her new boyfriend, Bennett, the author Millie had long idolized. Millie accused Poppy of stealing him and severed contact. After Poppy's supposed death, Millie saw her chance.

Poppy explains what happened on the yacht. She found printed evidence in Bennett's office that he had used artificial intelligence to plagiarize another author's bestselling novel, copying the voice and storyline while changing enough to avoid a lawsuit. When she confronted him, he pushed her over the railing. She survived because she routinely wore a concealed auto-inflating flotation device, having never learned to swim. A fisherman rescued her. Discovering she was pregnant, she chose to remain hidden rather than risk Bennett gaining custody and fled to New Mexico, where she gave birth to a daughter she named Millie.

The sisters log into Bennett's computer and find a folder containing Savannah's abandoned manuscript about a female vigilante, the premise he dismissed as too clichéd. He had changed the character names and planned to publish it as his own. Poppy suggests he may not have intended for Savannah to still be alive when the book came out. The sisters reconcile: They convert the Bedford estate into a writer retreat, and Savannah signs a three-book deal for the vigilante series. An epilogue in the form of a New York Times article confirms Isabel's conviction, the recovery of bodies from the well, and the sisters' reunion.