In present-day Sydney, Australia, Ella receives an early-morning call from Glenda, her mother’s neighbor in London, England, who tells her that Sally is in surgery after a serious fall. Glenda explains that Sally left the bath running and slipped in the water. The fall left her with a broken wrist and two broken fingers, facial bruising, and an infection.

Glenda tells Ella that she’s leaving in two days for her annual month-long trip to France, so she cannot provide the support and care that Sally needs. She urges Ella to come, noting that Sally has become increasingly isolated. Ella’s husband, Charlie, insists that she go to London. Ella resists, citing her demanding legal career and their eight-year-old daughter, Willow. Charlie points out that Ella barely sees Willow during the week and suggests that their life no longer makes her happy. He argues that the trip could help her resolve things with her mother and determine what she truly wants from her life, adding that it’s time for Ella to stop running away.

When Charlie says that Ella shouldn’t let her mother down, Ella gets angry and says that her mother “let [her] down” (13). Despite her reluctance, Ella knows she is eligible for a month’s paid leave and must return to England to care for Sally.

Upon arriving at her childhood home on Circus Street, Ella feels anxious. Inside, the grand Georgian house looks familiar but run down, with a foul smell from dying tulips in a vase. In the sitting room, she finds threadbare furniture and a pile of books including My Sister, the Serial Killer by Oyinkan Braithwaite. She feels nostalgic for her childhood love of reading—a habit that she abandoned after moving to Australia.

A large water stain on the ceiling reveals severe flood damage. She rushes upstairs and finds the first-floor bedrooms undamaged, but the door to what she remembers as a junk room is locked. She recalls her father installing interior locks for security but wonders why this room would be locked now. On the top floor, the damp smell intensifies. The landing carpet squelches, and the bathroom shows clear signs of flooding. Ella resolves to collect her mother before investigating the locked room.

At Lewisham Hospital, a nurse tells Ella that Sally is the most popular patient on the floor. Ella initially cannot locate her mother among the group of frail elderly patients until she realizes that Sally is reading to a woman named Judith. Ella notices her mother’s bruised cheek, cast, and taped fingers.

When Sally asks about Charlie and Ella’s daughter, Ella briefly fears that Sally has forgotten Willow’s name. She learns that her mother has a urinary infection that has affected her memory and caused confusion. As Sally says lengthy goodbyes to other patients, Ella grows impatient. While walking to the exit, Ella notices how frail Sally appears and feels an unexpected protective urge, but she also dreads being alone in her childhood home with this woman who feels like a stranger.

Arriving home, they see Nathan, a young man renting a room next door, sitting with a one-eyed, black-and-white cat. The cat, named Hadron, greets Sally warmly but growls at Ella. Nathan and Sally banter easily, and he mentions his mother’s financial troubles. Sally comforts him, offering support, and Ella grows suspicious of Nathan’s motives.

Inside, Sally reveals that Hadron is a stray she adopted. Ella feels envious that the cat has found a place to belong—a feeling she hasn’t been able to access for a long time. Sally explains that Nathan’s mother has fibromyalgia and that Nathan uses his student loans and works night shifts to help with her rent. Ella worries that Nathan is taking advantage of Sally and asks if she has given him money. Sally defends helping him as her duty. Bitterly, Ella wonders where Sally’s sense of duty was 21 years ago. The argument culminates with Ella alluding to past betrayals, saying that “even the ones you love the most” let you down and leaving the room in tears (38). Sally confesses to the empty room that she’s spent 21 years trying to earn Ella’s forgiveness.

After a quiet evening, Sally retires early. Helping Sally change is awkward for Ella. In Sally’s bedroom, her late father’s empty bedside table triggers painful memories of his death and her mother’s betrayal.

Downstairs, unable to find the television remote, Ella’s attention turns to the bookcase—her childhood happy place. She recalls alternating between reading classics and modern library books, a rule that Sally encouraged despite her father’s disapproval. When she picks up Jane Austen’s Pride and Prejudice, she’s transported by the old book’s smell. She remembers loving the novel at 15 even though her friend Verity found it confusing. Settling on the sofa, Ella allows herself, for the first time since leaving England, to get lost in reading.

The next morning, Sally mentions her university days with Ella’s father and a man named Andrew, but Ella changes the subject, making it clear that she doesn’t want to talk about Andrew. A woman named Pru, from Sally’s Library and Community Group, arrives, and Hadron growls at her just as he did at Ella. Pru mentions that others have tried contacting Sally, but she hasn’t returned any of their calls. She delivers the next book-club selection, Paulo Coelho’s The Alchemist, and says that she’ll return My Sister, the Serial Killer for Sally.

Sally appears confused and claims not to know the book. When Ella points to it, Sally recovers and says it was not to her taste. Ella asks why Sally stopped attending the book club, and Sally grows defensive, saying that life is too short to read unenjoyable books. When Ella voices concern about her mother’s memory and isolation, Sally snaps that she is getting old and that Ella has not been around to notice changes. Cold silence follows.

After an hour-long search, Sally finds the key to the junk room in a kitchen drawer. On the landing, the musty smell is overwhelming. Ella unlocks the door to discover a once beautifully decorated library, now completely ruined by water damage from the flood.

Shafts of sunlight illuminate the damage: a soaked velvet armchair, stained floral wallpaper with black mold, and a buckled bookcase under a collapsed cornice. Books lie in a sodden mess or bloat on shelves, liquid dripping onto standing water on the floor. Stunned, Ella asks what the room is. She turns to find her mother in the hallway, her body shuddering with heaving sobs.

Sally explains that the room was a library she created for Ella. She never stopped their birthday tradition—every year, she bought and inscribed a book, adding it to the library at midnight on Ella’s birthday. Ella is overcome with guilt.

Sally describes choosing each book with help from local booksellers. Each selection was meant to impart a lesson or reflect on her mistakes. The room became her sanctuary, where she felt connected to Ella through the books she curated for her. Sally admits that she stopped visiting the room last year because she lost motivation and couldn’t remember where she put the key.

To comfort her, Ella offers to salvage books but discovers that most of them are irreparably damaged—the pages are falling out or turning to pulp. She finds one birthday book, but the inscription has been washed away and is illegible. Ella realizes that her mother created this room of memories, now physically erased by water.

Focusing on practicalities, Ella discovers hundreds of unread emails on Sally’s laptop. Sally admits that she’s grown too overwhelmed to keep up with them. Ella finds Sally’s insurance policy but realizes that it’s expired. The broker tried contacting Sally multiple times, but emails and voicemails went unread. Sally explains that she ignored unknown numbers, fearing fraud. Ella feels pity, recognizing how much her once-capable mother is struggling.

Before leaving to buy lunch, Ella offers to check Sally’s phone messages, but Sally demands privacy. The word triggers a painful memory of her mother’s betrayal, prompting Ella to accuse Sally of never wanting a real relationship with her. Sally insists that she was giving Ella the space she thought she wanted. When Ella declares that she’s only there out of duty, Sally feels deeply wounded. Sally says that the library she built should prove how much she wanted Ella home. Unwilling to process this, Ella leaves.

The novel flashes back to nine years ago. Sally receives a rare call from Ella. Sally has resisted calling after Ella moved to Australia, respecting their unspoken rule that Ella initiates contact. On the call, Ella announces that she’s pregnant. Sally feels ecstatic but hurt when Ella reveals that she’s already 16 weeks along. Ella explains that they wanted to wait until they were in the safety zone, and Sally realizes that her surprised reaction was misinterpreted as judgment.

Sally offers to help with the baby since she is now retired. Ella declines, explaining that Charlie plans to be a stay-at-home father and adding that she knows how busy Sally is. The comment crushes Sally, who only stays busy to fill the void Ella that left. Sally feels that nothing in her life could be more important than being there for her daughter and grandchild. To hide her tears, she tells Ella that her doorbell is ringing and ends the call. After hanging up, she cries.