Detective Constable Mackenzie "Mac" Stone is plagued by blackouts regarding the night his wife, Lucia, vanished three months prior. Struggling with his heavy drinking, he becomes consumed by the search for her. All he's certain of is that shortly after she disappeared, she drained their joint bank account and made one last call to him from Paris.
Meanwhile, John Dean is grappling with a different battle. After suffering from amnesia due to a car accident five years earlier, he has no recollection of his past. Though he receives an offer of £7,000 to participate in a trial at a Cambridge lab that promises to restore his memory, he declines, content with his new life and marriage to Jenna. He manages the finances for her newly-established yacht design company.
In his relentless quest for Lucia, Stone becomes convinced that Dean is actually the missing IT professional, Paul West. His partner and trusted friend, Detective Constable Ian Reid, agrees to support him in the investigation. Initially, it appears unlikely that Dean is West, although there are slight physical resemblances. Paul West is believed to have committed suicide after the tragic loss of his wife and stepson in a house fire ruled accidental. However, since West’s body was never recovered, Stone finds enough evidence to suspect foul play, leading him to believe Dean is faking his amnesia.
The plot thickens when a serious incident occurs at Jenna’s company, prompting Stone to rush to the hospital, mistakenly accusing Dean of attempted murder. Ironically, it is Dean who suffers injuries while protecting Jenna. Following this, when Dean confides in Reid, it becomes evident that Stone's behavior has been unpredictable since Lucia’s disappearance. Reid also uncovers that only Stone holds the belief that Dean is West, the man responsible for a family tragedy.
Stone’s mental state continues to deteriorate as he experiences waking nightmares, seeing visions of Lucia around their home. An anonymous letter addressed to him reads, “Why did you do it?”, which he hands over to Reid for investigation. Haunted by fragmentary memories of that night, Stone worries he may have harmed Lucia but can’t recall any details. The discovery that the letter was printed using Stone's own machine leads Reid to question Stone’s mental clarity. Another mysterious letter arrives, boldly asking, “Why did you kill her?”, which Stone chooses to keep hidden from Reid.
As tensions build in Dean and Jenna's marriage due to financial difficulties and an unexpected pregnancy, Dean reluctantly decides to enroll in the Cambridge trial. The results reveal no signs of physical damage to his brain, suggesting if he ever suffered from amnesia, it is no longer in play, a truth he withholds from Jenna.
Reid receives yet another anonymous missive, this time recommending him to examine Stone's joint account. Convinced that Stone may have taken Lucia's life in a fit of jealousy after discovering her infidelity, Reid faces a moral dilemma as he admits to being involved with Lucia while they were together. With his boss, Detective Inspector Brennan, urging a thorough investigation of Stone, Reid finds no evidence supporting that Lucia traveled to Paris or withdrew money after her disappearance. As Reid voices his suspicions that the letters are manifestations of Stone’s repressed guilt, the tension escalates.
The mounting pressures lead Jenna to contemplate terminating her pregnancy, but she ultimately decides against it. As Stone digs deeper into Dean's past, he becomes convinced that Dean has previously committed other crimes, employing various aliases and taking the lives of women who discover his dark secrets or threaten to expose him.
An illegal search by Brennan turns up a large bloodstain hidden beneath Stone’s carpet, culminating in his arrest for Lucia's murder. Upon leading the police to the last location he remembers seeing Lucia, a stunning twist reveals she is alive. Lucia confirms Stone’s reality; she had been nearby, gathering the courage to confront their tumultuous relationship and her affair with Reid. She admits to emptying their joint account, booking her flight, and indeed, that Stone struck her the night she left, which explains the blood evidence. It's unveiled that Dean manipulated the evidence to implicate Stone in her murder.
As the narrative unfolds, Dean persuades Jenna to go on vacation aboard a client's yacht, only for Jenna to uncover a concealed package filled with cash and new identities intended for Dean’s next life. Just as Dean attempts to eliminate Jenna, Stone and Brennan intervene. The ensuing clash leads to dramatic consequences, with Dean taunting Stone before plunging into the depths of the water, leaving his fate uncertain.
The film concludes on a chilling note, revealing Dean at an undisclosed resort, ensnaring yet another woman into his web of deceit, introducing himself as “Mac Stone”.
