Twenty years before the novel begins, six-year-old Arden Maynor was swept away during a flash flood while sleepwalking near her home in Widow Hills, Kentucky. She spent three days trapped underground before a man named Sean Coleman found her clinging to a drainage grate. The story became a national sensation. After the 10-year anniversary rekindled public scrutiny, she legally changed her name to Olivia Meyer and disappeared.
Now 26, Olivia lives alone in Central Valley, North Carolina, working as an administrator at the local hospital. She has told no one about her past. Her closest relationships are with Bennett Shaw, a nurses' shift supervisor, and Elyse Ferano, a newer nurse. Her neighbor Rick Aimes, a retired contractor who sold her the house, watches over her.
The novel begins when Olivia receives a box of her deceased mother's personal effects. A county office had informed her months earlier that Laurel Maynor died of an overdose; the two had been estranged for years. Among the contents is a ballet slipper bracelet Laurel always wore. Holding it triggers a flash of buried memory: cold rocks, stagnant water, darkness. The box also contains an old flip phone. Olivia stores everything in her bedroom closet.
Two nights later, Rick finds Olivia standing barefoot in her yard at three in the morning, disoriented. She has not sleepwalked since childhood. The next morning, a man approaches her outside a store and says her name. Olivia does not recognize him and drives away.
Bennett catches Olivia searching the hospital medicine room for sleep aids and sends her to a specialist, who recommends a study Olivia declines. At a bar on Friday evening, the bartender mentions someone asked for Olivia by name. She returns home, texts Jonah Lowell, a former professor and ex-boyfriend, drinks some wine, and falls asleep. She wakes in the dark near the tree line, trips over a body, and finds her hands covered in blood. Rick checks on the body and calls 911. The man is dead.
Detective Nina Rigby takes Olivia's statement. Olivia omits the sleepwalking, claiming a ringing phone woke her. Rick supports her account. Elyse reports that the murder weapon was apparently a box cutter. Back home, Olivia checks her kitchen drawer and confirms her own box cutter is missing.
Over the following days, Bennett finds the ballet slipper bracelet under the couch cushions, far from the closet where Olivia stored it. She cannot explain how it moved. Rigby identifies the victim as Sean Coleman, 52, from Kentucky, the man who rescued Arden Maynor and the same man who approached Olivia at the store. Olivia is forced to reveal her true identity. Rigby warns Olivia about Rick, whose wife's death a decade earlier was ruled a suicide but widely doubted. Bennett discovers Olivia's secret online and confronts her; hurt, he nevertheless offers help, including his sister Mackenzie Shaw, a lawyer.
Elyse quits abruptly, leaving only an email and her badge. Olivia visits her apartment and finds it unlocked and chaotic, but Elyse is gone. Olivia later connects medications going missing from the hospital to an inventory investigation at Elyse's former employer.
Sean's son, Nathan Coleman, arrives in town. Olivia shows him where his father died, and they bond over shared grief for estranged parents. She finds a letter from Sean in her mailbox, dated before his death, urging her to contact him. While at Rick's house, Olivia discovers her missing box cutter under his sink, wiped with bleach. Rick admits he found it at the crime scene and cleaned it to protect her, revealing he has known her identity since she bought the house. Olivia later disposes of the box cutter in a sharps container at the hospital.
Bennett calls at four a.m. to report that Elyse has been found dead of an overdose at a campground cabin. An article exposes Olivia as Arden Maynor, quoting an anonymous professor who calls her a gifted liar. Olivia is placed on administrative leave.
Visiting Nathan at his hotel, Olivia discovers a hidden file in his luggage containing transcripts, threatening letters, photos of her old house, and a map of Widow Hills. Nathan has known her identity all along. She flees, certain Sean came to warn her about Nathan.
Olivia drives to Kentucky and meets Emma Lyons, the journalist whose career was built on the rescue story. Emma reveals that Nathan approached her a decade earlier with alleged inconsistencies: Sean walked the same route daily and Olivia was not at the grate until the final day; a doctor said her injury was unusual and she was not dehydrated enough for three days underground. Emma also reveals Laurel had been dismissed from nursing jobs for prescription infractions and implies the rescue may have been staged, but warns Olivia to let it go.
Olivia visits the rescue site. Nathan follows, having tracked her car. He reveals he wrote to Laurel posing as his father, and Laurel paid him, believing she was buying Sean's silence. Nathan has been blackmailing Laurel for years. He claims Laurel injured Olivia, hid her in a cinder-block room, and staged the disappearance. He grabs Olivia and tries to drag her into the woods, but police arrive after an anonymous 911 call and arrest him.
Back in Central Valley, Olivia discovers a newly opened bottle of wine tastes wrong and realizes someone has been drugging her, explaining the sleepwalking episodes. She looks up Erin Mills, a nurse who filed an incident report against her, and recognizes the woman's features. On impulse, she calls back the number that informed her of Laurel's death. A phone rings from inside the box in her bedroom closet: The old flip phone is on. Her mother is alive. Laurel faked her death, sent the box to lure Olivia back into the story, and has been living as Erin Mills at the hospital.
Olivia calls 911. Minutes later, Laurel enters through the back door. She admits she killed Sean Coleman, believing he was the blackmailer. She also confesses to manipulating Elyse, who was in recovery from addiction, using her to divert drugs from the hospital. When Elyse tried to flee, Laurel's involvement led to her fatal overdose. Laurel then reveals the original story was fabricated: She accidentally injured Olivia's arm on the steps, hid her in a cinder-block room, and staged the disappearance. When the search escalated, she moved Olivia to a drainage access point, but Olivia wandered through the pipes and was found by Sean.
Laurel cuts the power and tries to force Olivia to drink hot chocolate laced with sedatives. When sirens approach, she corners Olivia in the attic with a shard of glass. Olivia pushes her through a window onto a decorative balcony. When Laurel rises, Olivia pushes again. The railing gives way, and Laurel falls to her death.
In the aftermath, Rigby collects the tampered wine and hot chocolate as evidence. Nathan remains in custody on stalking and assault charges. Olivia chooses not to reveal the full truth about her mother's staging of the original rescue. With Laurel and Sean both dead and Nathan discredited, she is the only living witness to what really happened. She resolves that the story, for the first time, belongs only to her.