In 1988, a mysterious outbreak in Philadelphia causes a series of baffling fatalities, as several individuals simultaneously suffer from severe hemorrhaging. Among those tasked with unraveling this horrifying mystery is police officer Thomas Lockhart (Boyd Holbrook) and his loyal partner Maddox (Bokeem Woodbine), alongside Lockhart's brother-in-law, Detective Holt (Michael C. Hall). Their investigation leads them to discover strikingly similar puncture wounds on each victim, which hints at a more sinister connection. Toxicology results reveal traces of an unstable isotope that had been injected directly into the victims' vertebrae.
Their pursuit leads them to an assault survivor, who bears the same daunting injuries. Unfortunately, her life is cut short, but not before she provides a haunting account of her attacker—a black woman with a wound on her hand. When Lockhart and Maddox chase this suspect into the depths of a subway system, a startling confrontation ensues. As they grapple, the suspect divulges striking personal details about Lockhart's life, even foretelling his wife's impending childbirth, only to meet her end by an oncoming train during a struggle. The police, faced with numerous unanswered questions—including the presence of a bullet lodged in the woman’s hand, which was fired from a police revolver—ultimately deem the case closed. Tragedy strikes Lockhart when he loses his wife during childbirth, leaving him hollow with grief.
Fast forward to 1997, where a new wave of gruesome murders erupts, seemingly echoing the patterns of the previous killings. Lockhart, now a detective raising his young daughter Amy (Quincy Kirkwood), grapples with connecting the threads of this new case. An unexpected discovery of keys from the previous victim leads him to the revelation of an aircraft manufactured long after the initial slayings. Disturbingly, footage of the recent attacks hints that the woman from 1988 has returned, unchanged by time.
Lockhart encounters Naveen Rao (Rudi Dharmalingam), a physicist who argues that this may be evidence of time travel. However, Lockhart and Maddox remain skeptical. As tension escalates, they finally track the culprit to an airfield where Lockhart is confronted again by the ageless suspect. In a shocking turn of events, she reveals even more intimate knowledge about Lockhart's life and, during an ignoble confrontation, unintentionally kills Maddox before vanishing as quickly as she appeared.
By 2006, Lockhart has transitioned into a private investigator, his obsession with the case deepening over the years. He learns about an unrecorded victim from 1988, and a conversation with the victim's widow, Arlene (Philippa Domville), sheds light on a white nationalist militia group that the victim was involved with. Lockhart begins to suspect a chilling connection between this group and the subsequent murders. Despite warning from Holt to seek help, he forges ahead, impersonating an officer to unearth truths hidden beneath layers of deception. In a nail-biting chase, Lockhart manages to wound the suspect, but she escapes into a time-traveling device concealed within a sewer.
As the narrative advances to 2015, Lockhart is found waiting on a beach for the elusive woman. During this period, he receives a voice message from his daughter Amy, beckoning him to join her for the birth of her child. In a twist of fate, he is then kidnapped by Rao, who divulges the controversial technology enabling the woman to execute her missions across time. Rao asserts that her actions are preemptive strikes against individuals whose future actions could lead to catastrophic events. Lockhart, still resolute in stopping her, breaks free and confronts her once more.
To his astonishment, she presents him with a charm bracelet belonging to his granddaughter, Rya (Cleopatra Coleman), illustrating a tragic connection. Rya reveals that she is on a critical mission to avert a future catastrophe precipitated by a series of events leading to widespread death and destruction. Having traveled back in time, she confronts the weight of their shared history—Lockhart's guilt over her fate and his prior actions in 1988.
As their emotional confrontation unfolds, Rya expresses that her mission is a one-way trip, cognizant of the risks involved. Lockhart makes the harrowing decision to allow her to fulfill her purpose, believing in the righteousness of her cause. Their poignant goodbye leaves an indelible mark on Lockhart, who is left holding the charm bracelet representing both loss and hope.
Finally, in 2042, a now-elderly Rao executes the lethal injections, resulting in Rya's intended consequences of erasing her timeline. In a voiceover, Rya recounts the chilling catalyst for the violent civil unrest that followed, linking it back to a tragedy that could have altered the course of history. Yet, through the sacrifices made across generations, Rya’s efforts ensure the bombs never detonate, altering the future for the better.
Back in 2015, Lockhart experiences a moment of warmth and reunion as he holds his granddaughter Rya in his arms, embracing hope and a fresh start, echoing the comforting words he once shared with his own daughter Amy during her birth—"everything is going to be okay."
