A young woman living in Philadelphia goes by the name Addison Hope, though she knows it is not her real name. Two years earlier, Ed Gordon, a long-haul truck driver, found her bleeding and disoriented on a highway in New Jersey with no identification, no phone, and no memory of who she was. Ed brought her home to his wife, Gigi, a nurse, who took her to a hospital where doctors diagnosed her with retrograde amnesia: the complete inability to recall events before the onset of the condition. The Gordons took Addison in, motivated in part by the murder of their own daughter, Beth, who was killed while hitchhiking years earlier. Ed used his trucking connections to obtain the birth certificate of a deceased infant and create a fraudulent identity so Addison could work and function in society. She bears jagged scars on both wrists that suggest a past suicide attempt she cannot remember.
Addison is engaged to Gabriel Oliver, who manages his family's art gallery in Philadelphia's Old City neighborhood. Gabriel accepted her amnesia without reservation, but his mother, Blythe Oliver, worries that Addison may already be married or may one day recover her past and leave Gabriel. Blythe secretly hires a private detective, Jim Fallow, who runs Addison's DNA through federal databases, checks missing persons reports, and conducts facial recognition searches. Nothing turns up.
In a parallel storyline set in Boston, Julian Hunter, a doctor, mourns his missing wife, Cassandra, who vanished two years earlier, leaving behind all her belongings. Julian tells their seven-year-old daughter, Valentina, stories about her mother and refuses to believe Cassandra is dead.
In Philadelphia, Addison struggles with disturbing flashbacks. One vision places her in a dark room reaching for a gun while a voice urges her to kill. Another triggers a horrific image of a woman's body on a floor, her face destroyed. Addison is terrified she may have killed someone. During a trip to Fort Lauderdale with Gabriel, a man at a restaurant insists he recognizes her from a strip club called the Blue Mirror, where she danced under the name Juniper. Ed investigates but hits a dead end: The club closed years ago, and its owner died under suspicious circumstances.
The storylines converge when Julian attends a medical conference in Philadelphia. He spots a poster for Addison's photography exhibition at the Oliver Gallery and recognizes the woman as his missing wife. He returns the next evening, approaches Addison, and shows her a photograph of herself with a little girl. She does not recognize him. When Gabriel intervenes, Julian announces that he is her husband. Ed insists Julian return to Boston and come back with proof. Julian does, bringing birth certificates, identification, and a passport bearing the name Cassandra Hunter. He explains that Cassandra had depression and anxiety rooted in childhood trauma, was on psychiatric medication, and that they used a surrogate to have Valentina. Despite Gabriel's objections, Addison decides she must return to Boston. She cannot abandon a child who may be hers.
Julian drives Addison to his home in Brookline, a suburb of Boston, where he gives her space in a guest room. Slowly, small memories surface. The scent of his cologne triggers a flash of a kiss. Hypnosis sessions with a therapist recover memories of their honeymoon. On their wedding anniversary, a song at a restaurant stirs another recollection. She resumes her relationship with Julian, cuts off contact with Gabriel and Gigi, and bonds deeply with Valentina, feeling an instinctive love for the child. Julian tells her she had a previous abusive marriage to a man named Zane Dryer before she married him.
Then things grow dark. Items begin disappearing around the house. Julian claims she has a history of stealing and hiding things due to poor impulse control. He reveals that after Valentina's birth, she became paranoid and jealous of the surrogate, Sonia, and shot her to death. He says he disposed of the body to protect the family. Cassandra recognizes the dead woman from her recurring flashback and believes herself a murderer. Her mental state deteriorates further. Alone one night, she hears a menacing voice telling her she is evil and will hurt Valentina. She wakes the next morning holding a butcher knife, approaching the sleeping child. Julian sends Valentina away for safety and tells Cassandra the voices are commanding her to harm her daughter. He says ending her own life is the only way to protect the girl. He dictates a suicide note, which she writes, and hands her a vial of pills.
As she prepares to swallow them, she spots a tiny camera and speaker mounted in the ceiling corner. Julian planted the voices through the speaker system. She is not losing her mind. She spits the pills at him. Julian attacks, revealing the truth: Her real name is Amelia. The real Cassandra is dead. Valentina is not her biological child.
Part III of the novel shifts to four years earlier and exposes Julian's crimes through his own narration. Julian had been the real Cassandra's psychiatrist before becoming her husband, using hypnosis and false memory implantation to convince her that her first husband, Zane Dryer, was abusive. When Cassandra later obtained Julian's clinical notes and discovered the manipulation, she confronted him and threatened to take Valentina and report him. Julian strangled her and buried her body in the backyard.
He then found a replacement: Amelia Foster, a patient he was treating after a suicide attempt. Amelia had survived a family massacre in which her father killed her mother, twin sister Shannon, and grandmother on Christmas Day. Over a summer, Julian used intensive hypnosis and drugs to erase Amelia's memories and implant new ones, convincing her she was Cassandra. He altered family photographs, obtained new identification, and used hypnosis on young Valentina to make the child accept the new woman as her mother. For years he kept Amelia medicated and isolated. Amelia fled two years earlier after discovering a hidden shrine to the real Cassandra and overhearing Julian confess to the murder. A truck driver picked her up and attempted to assault her; she fought him off but struck her head, triggering the total memory loss with which Ed found her on the highway.
In the present, Blythe's detective traces the Blue Mirror connection to Amelia's twin sister Shannon and uncovers a newspaper article about the Foster family murders. Blythe, Gabriel, Ed, and Gigi visit Zane Dryer, who confirms the woman in Gabriel's photograph is not the real Cassandra. The group races to Boston, arriving at Julian's house to find police on the scene. During the struggle, Amelia had stabbed Julian with a hidden knife and called 911 from a locked bathroom.
Gabriel begs Amelia to return to Philadelphia, but she refuses, insisting Valentina needs her and that she is the child's mother in every way that matters. Julian is convicted of second-degree murder and sentenced to 25 years. With a new therapist, Amelia recovers her real memories through hypnosis, including her childhood in Orlando and the discovery of her family's bodies. Julian consents to her custody of Valentina, and Amelia becomes the child's foster mother while pursuing adoption. Ed and Gigi serve as surrogate grandparents. In the epilogue, Amelia has moved to a small home in Lexington with Valentina. She learns Gabriel is returning from volunteer work in India. His sister, Hailey, invites her to a welcome-back party. Amelia considers attending but chooses to focus on her recovery and on being a mother. The novel closes with Amelia declaring that she finally recognizes the woman in the mirror and is no longer a stranger to herself.