In 1988, a terrifying incident unfolds in Philadelphia as multiple individuals simultaneously suffer fatal hemorrhages, resulting in catastrophic blood loss and brain disintegration. Police officer Thomas Lockhart (Boyd Holbrook) teams up with his partner Maddox (Bokeem Woodbine) and his brother-in-law, Detective Holt (Michael C. Hall), to probe into this gruesome case. Lockhart identifies a troubling connection among the victims when he discovers identical puncture wounds on their bodies. Toxicology tests reveal the use of an unstable isotope injected into their vertebrae, deepening the mystery.
The investigation takes a darker turn when Lockhart and Maddox encounter an assault victim bearing the same distinctive injuries. Before succumbing to a similar hemorrhage, she describes her attacker as a black woman with a wounded hand. A thrilling chase ensues when they attempt to apprehend the suspect, who unexpectedly subdues Maddox. In a chilling confrontation, she reveals intimate details of Lockhart's life, including the fact that his wife is about to give birth. Just before a traumatic struggle leads to her death by train, she ominously predicts her fate.
In the aftermath, Lockhart struggles with the shocking confusion brought on by the unexplained circumstances of the case, especially since, despite firing upon the suspect, her bullet wound remains a mystery. Sadly, his life takes another tragic turn as his wife dies during childbirth. Fast forward to 1997, an eerie resurgence of gruesome killings mimics the earlier frenzy, and Lockhart, now a detective nurturing his young daughter, Amy (Quincy Kirkwood), faces a perplexing challenge as no clear motive or connection to the latest victims emerges.
As he retraces clues from the 1988 incidents leading to an aircraft built in 1996, shocking camera footage reveals the same woman from years past, who should have been gone forever. Naveen Rao ([Rudi Dharmalingam]), a physicist, charges Lockhart with the notion of time travel, yet his theories go unacknowledged. The story escalates when Lockhart discovers the suspect alive and unchanged at an airfield, leading to a dramatic event where she unintentionally kills Maddox and once again traps Lockhart.
By 2006, Lockhart, now a private investigator beyond obsessed with this time-twisting case, grows increasingly detached from Amy, who now lives with Holt. His relentless search ultimately unveils a previously unrecorded victim. Through this discovery, he encounters Arlene, the late victim's wife, who discloses that her husband was involved in a white nationalist militia, and Lockhart begins piecing together a chilling narrative that connects the murders from 1988 and 1997.
When he presents his theory to Holt about a time-traveling killer targeting members of the militia in reverse chronological order, he is met with skepticism. In desperate pursuit of justice, he impersonates a police officer, only to find yet another victim murdered by the same assailant. He manages to wound her during a chase, leading to a glimpse of a time machine shrouded beneath the surface.
Fast forward to 2015, Lockhart anxiously awaits a potential reappearance at the beach. Simultaneously, Rao confronts him, revealing his sinister technology that powers the fatal injections used by the woman, justifying the killings as a preventive measure against future calamities. Lockhart, however, remains firm in his belief that her actions must be stopped. A heart-stopping reunion occurs on that beach, where the woman reveals herself to be Rya (Cleopatra Coleman), Lockhart's granddaughter, and explains that she is on a mission to avert a catastrophic civil war triggered by her former allies.
This encounter becomes a heartbreaking confrontation, as Lockhart ultimately acknowledges his role in Rya's fate. Overwhelmed with sorrow, he attempts to convince her to abandon her mission, yet she adamantly reminds him of its one-way nature. Resolute in her mission's importance, she comforts him and departs, leave behind her charm bracelet as something to remember her by.
In 2042, Rao executes the injections, targeting Rya's former enemies, effectively erasing her existence. Rya’s voice resonates through the timeline as she recounts how her actions prevent a disaster that could have devastated thousands. Meanwhile, back in 2015, Lockhart finds redemption as he reunites with his family, cradling his granddaughter Rya, adorned with her charm bracelet, and comfortingly assures her that everything will be alright—words he once whispered to her mother, Amy, during her own birth.
