In 1988, Philadelphia is shaken as multiple individuals simultaneously succumb to massive hemorrhages, leaving the police baffled. Enter Police officer Thomas Lockhart and his partner Maddox, who, alongside Lockhart's brother-in-law, Detective Holt, delve into this mysterious case. Lockhart's keen observation of strikingly similar puncture wounds on the victims leads him to suspect a connection between them. Their investigation soon brings them to an assault victim with identical injuries, who, before meeting an untimely death, describes her attacker as a black woman with an injured hand.
The pursuit for answers intensifies as Lockhart and Maddox track the suspect into a subway, where chaos ensues. The suspect, however, manages to subdue Maddox and, in a tense confrontation, reveals intimate details about Lockhart's life, including the news that his wife is giving birth that very day. As the two struggle, tragedy strikes when a train collides with the suspect, ending her life. Confusion fills the air, but the police ultimately close the case, unaware that the darkest chapter of Lockhart's life is only beginning with the loss of his wife during childbirth.
Fast forward to 1997, Lockhart finds himself grappling with a new wave of terror as a copycat killer emerges, shattering any sense of normalcy he's built while raising his young daughter, Amy. With no clear motive linking the victims, Lockhart employs his detective skills to trace evidence back to keys found in 1988, pointing him to an aircraft from 1996. Enigmatic physicist Naveen Rao claims this indicates time travel, but Lockhart and Maddox remain skeptical. Their investigation leads them to a shocking reunion with the original suspect, who appears unchanged by time, and after a chaotic altercation that results in Maddox's accidental death, she once again slips away, leaving Lockhart reeling in disbelief.
Come 2006, Lockhart, now a private investigator, is consumed by his obsession with solving the case, convinced that it involves time travel and the now-missing Dr. Rao. Meanwhile, teenage Amy is living with Holt, while Lockhart's connection with her becomes increasingly distant. A breakthrough occurs when Lockhart discovers an unlisted victim linked to the 1988 murders. Meeting with the victim's widow unveils chilling connections to a white nationalist militia. Alongside Holt, Lockhart ventures to propose a radical theory—that the suspect is navigating back through time, eliminating members of this militia, only to be met with skepticism.
Desperate for answers, Lockhart impulsively borrows Holt's badge to investigate further, leading him to the murdered ex-girlfriend of the militia leader. In a gripping chase, Lockhart manages to wound the suspect just before she vanishes through a time machine, culminating in his arrest at Holt's hands while Rao observes from the shadows.
In 2015, while waiting on a beach to rendezvous with the woman from his past, Lockhart receives a voicemail from Amy, inviting him to her child's birth. Suddenly, Rao reveals himself, confessing to being the mind behind the technology used by the woman to commit her killings. He believes she is executing justice against those who incited unfathomable tragedy. After a harrowing escape from Rao, Lockhart confronts the woman, who unveils her true identity as Rya, his granddaughter.
Years later, in a critical turn of events, Lockhart persuades an adult Rya to pursue her mission, provoked by the militia's acts of terrorism which resulted in a civil war in 2024 when she was merely nine. Rya navigates the complexities of time travel, appearing every nine years to carry out her grim task using an injection that allows her to eliminate targets from the timeline. Unbeknownst to her, events from 2006 still lay ahead, and her injury has yet to occur. Overwhelmed with guilt as he confesses to Rya that he was responsible for her death in 1988, Lockhart ultimately stands by her resolve and permits her to fulfill her mission. Following her departure, an elder Rao activates the injections that eradicate Rya's targets, erasing the tragic events from history. In a poignant closure, Lockhart reunites with his family in 2015, embracing a flicker of hope for a future alongside his granddaughter.
