A family is walking down a country lane in Devon, England—a mother, Gabrielle, and her three children, eight-year-old Jessica, six-year-old Joanna, and a baby, Joseph. Suddenly, a knife-wielding young man appears out of nowhere and kicks the family’s dog into the nearby wheat field. He stabs Gabrielle, Jessica, and Joseph to death. Joanna escapes, running into the wheat field. Later that night, the police find and rescue her.
Thirty years later, on a December evening, “Security Consultant” and private detective Jackson Brodie visits a remote village in the Yorkshire Dales. He sees a little boy, Nathan, playing football on the village green. Jackson believes Nathan is his son, although his ex-girlfriend, Julia, denies this. He finds a chance to ruffle the boy’s hair in greeting, stealing and keeping a strand as he does so. That evening, he catches a crowded train, believing he’s headed home to London, but he belatedly discovers that he’s boarded a northbound train to Edinburgh. On the outskirts of Edinburgh, the train derails and violently crashes. While pulling an injured fellow passenger to safety, Jackson falls out of the train car and down an embankment. He realizes he’s badly injured and is dying himself.
In Edinburgh, Scotland, 16-year-old Reggie Chase works as “mother’s help” for Joanna Hunter, the woman from earlier in the story whose mother and two siblings were murdered when she was six. Now an adult, Joanna works as a doctor. Reggie, whose mother recently died, lives in a poor neighborhood and makes a long daily bus commute to care for Joanna’s baby son, Gabriel Joseph. Though she’s dropped out of school, she is avidly studying Greek and Latin classics in preparation for university. Reggie is devoted to the Hunters and regards them as her family.
One December evening, Detective Louise Monroe visits Joanna. She warns Joanna that her family’s killer, Andrew Decker, has been released from prison, and there might be a media circus. Joanna doesn’t tell Reggie, who knows nothing about Dr. Hunter’s childhood tragedy.
Louise Monroe recently married a cheerful, well-off widower named Patrick Brennan. Though Patrick is a good husband, Louise feels trapped in her marriage and like an impostor pretending to be a good wife. Louise is obsessed with cases concerning women who’ve run from killers, like Joanna Hunter. She’s lately discovered that Joanna’s husband, Neil Hunter, whose businesses have fallen on hard times, might be mixed up with drug dealers in Glasgow. He’s also suspected of burning down one of his arcades for insurance money.
Reggie has a weekly dinner with Ms. MacDonald, her former classics teacher, who’s now retired and is dying of cancer. In exchange for tutoring, Reggie runs errands for Ms. MacDonald and watches her elderly terrier, Banjo, while Ms. MacDonald goes to prayer meetings. This evening, she’s watching television with Banjo when she hears a terrifying crash. When she learns about the train wreck nearby, she immediately goes to help.
On the scene of the train wreck, Reggie, who learned first aid from Dr. Hunter, finds Jackson and saves his life. Afterward, the police inform Reggie that Ms. MacDonald caused the crash by driving off the road and into the train’s path. Jackson fades in and out of consciousness in the hospital for the next day or two. When he wakes up, he learns that he’s survived a severed artery and a concussion. Based on a wallet that was found in his pocket, the hospital has identified him as Andrew Decker, the killer of Joanna’s family. That name doesn’t sound right to Jackson, but he can’t remember his identity or the accident right away. Later, he remembers his real name and the name of his wife, Tessa, whom he married just two months ago.
The morning after the train crash, Neil Hunter calls Reggie and tells her not to come to work that day, since Joanna and the baby have gone to visit Joanna’s sick, elderly aunt in Yorkshire. Reggie is confused by Dr. Hunter’s uncharacteristically sudden departure. She’s also disappointed not to be able to tell her about last night’s events. Reggie goes home to her flat and finds that it’s been completely trashed by thugs, searching for her drug-dealing brother, Billy.
Reggie goes to the Hunters’ to walk their dog, Sadie. Neil, looking disheveled, suggests that she take Sadie home with her while Joanna is gone. While at the Hunters’, Reggie dials Dr. Hunter and thinks she hears her mobile ringing somewhere inside the house. Sadie also finds the baby’s beloved blanket, which looks bloodstained. Reggie fears that something bad has happened to Dr. Hunter. She meets with Louise Monroe to warn her, but Louise figures that Reggie has an overheated imagination. When Reggie shows her a postcard she’d salvaged from Jackson Brodie the night before, though, Louise rushes to the hospital. She and Jackson worked together and had a close relationship in the past, and although they’re both married now, they still have feelings for each other. Jackson is hazy while she’s there, and later he’s unsure if he just dreamed about her visit. Later, Reggie finds Jackson in the hospital and introduces herself as the girl who saved his life.
The next day, Louise returns to the Hunters’ house and finds Joanna’s abandoned cell phone. She’s beginning to think that Reggie is right and even to suspect Neil’s involvement, especially when she discovers that Joanna and her “sick aunt,” Agnes Barker, didn’t call each another recently, and that Joanna’s Prius is still in the garage. Later, Reggie lets herself in to the Hunters’ in search of Joanna’s phone, too. While she’s there, she narrowly avoids being found by Neil and two of his shady business associates. She overhears the men saying that Joanna and the baby have been kidnapped. She also finds Joanna’s purse left behind, which contains, among other things, her aunt’s address. She begs Jackson to help her find Joanna. Jackson is eager to escape the hospital, so he finally agrees.
Meanwhile, also Louise drives to Yorkshire with her young assistant, Marcus, to track down Joanna’s aunt. They learn that Agnes Barker recently died, so Joanna and/or Neil have been lying about Joanna’s whereabouts. They also learn that “Andrew Decker” has been arrested and taken to the hospital after a car accident. When they reach the hospital, they discover that “Andrew” is Jackson, who used Decker’s license to rent a car, then caused a minor wreck while driving on painkillers. He and Reggie had been attempting to find Joanna and her alleged aunt, too. Louise, Marcus, Jackson, Reggie, and Sadie head back to Edinburgh, with Louise and Jackson occasionally sniping at or flirting with one another. They also discuss the case, unable to figure out how Andrew Decker’s ID was seemingly swapped with Jackson’s and whether Decker’s disappearance has anything to do with Joanna’s own. Reggie finally hears the truth about Joanna’s past.
After Louise drops Jackson and Reggie off at Ms. MacDonald’s house for the night, they find Billy there. He threatens Reggie and leaves after being attacked by Sadie. He has trashed Ms. MacDonald’s Loeb Classics editions, and that night Reggie discovers that Billy has been hollowing out the books and smuggling heroin inside them.
Early the next morning, unable to sleep, Jackson, Reggie, and Sadie continue their search for Joanna. A Nissan is parked at the Hunters’, the same one driven by the men who’d threatened Neil. They find a spare key and steal Joanna’s Prius to follow the Nissan. They tail it down a country road outside of Edinburgh. When they reach a remote house, the men in the Nissan look inside and quickly flee. When Jackson approaches the house, Joanna emerges from the bushes, covered in blood. Neil’s associates had been holding her and the baby hostage until Neil signed over his businesses to them. Joanna found an opportunity and stabbed both men to death. She asks Jackson for help, and he burns down the crime scene. He drops off Joanna, Reggie, the baby, and Sadie near the Hunters’, and they sneak inside to bathe and change. The police, including Louise, are there to question Neil, and they are dumbfounded when Joanna appears, claiming to have suffered amnesia and to have no idea what happened to her over the past few days.
When Jackson returns to London, he finds Andrew Decker dead in his apartment, apparently having shot himself. The mystery of the swapped identities is never resolved. Jackson also learns that he’s been conned by his wife, Tessa, who’d assumed a fake identity and cleaned out his bank accounts. Though depressed, Jackson decides to make a fresh start. He almost throws away the lock of hair he took from Nathan, but he holds onto it. He’s encouraged to keep going when he remembers finding Joanna the first time—it turns out that he was the young soldier who found six-year-old Joanna lost in the wheat field after her family was killed. He figures he will continue being a “crusader.”
Louise, though pregnant with Patrick’s baby, decides she will leave him after Christmas. She interviews Joanna several times but never draws a conclusive connection among Joanna, Decker’s death, and with the burned-out bodies in the remote house. Reggie, meanwhile, moves in with Joanna, who is now estranged from Neil. She finally has a happy family life, and she never asks what happened the night Joanna escaped.
