In Chicago, a tragic disaster strikes when an explosion occurs at Dr. Martin Steiner's Ashburn Laboratories, resulting in numerous casualties. In the aftermath, ghostly remnants of the victims begin appearing, haunting the living as non-sentient beings that eerily mimic certain actions at regular intervals.
Fast forward ten years, and we meet Veronica 'Ronnie' Calder, who feels increasingly disturbed by her family's refusal to recognize the daily presence of her deceased father, Robert, at their breakfast table. Her unease deepens when she encounters a remnant of a young man, who she comes to understand is named Brian. Brian's lingering spirit mysteriously scrawls the word "run" on a mirror before disappearing, leading Ronnie to fear for her safety.
During a lesson on remnants led by Mr. Bittner, Ronnie suffers from a sudden earbleed, heightening her anxiety about Brian's intentions. Seeking answers, she seeks out Mr. Bittner at his home, but he dismisses her concerns as mere paranoia. Determined to uncover the truth, she recruits her new classmate Kirk to aid her investigation. Kirk's research unveils a dark past, revealing that Brian was connected to the disappearance and subsequent death of Pastor Greer's daughter, Mary. They discover that Brian took his own life the day after Mary's mysterious disappearance, a tragedy that haunts Ronnie further when she realizes their birthdays coincide.
As their inquiry deepens, Principal Pescatelli summons Kirk to discuss their findings. This meeting culminates in tragedy when Pescatelli falls through the ceiling during a school basketball game, prompting Kirk to believe their investigation has dire consequences. Ronnie and Kirk soon connect Brian's name to another pair of sorrowful cases involving Emma Shaw and Claire White. When Claire's sister, Kerry, reveals that Brian was previously her sister’s boyfriend, they boldly venture to Chicago's No-Go Zone, where Claire’s death transpired—revealing the chilling Ashburn Laboratories once more.
Upon arrival, they encounter a disheveled Dr. Steiner, who divulges that he was experimenting with the idea of using the deceased as vessels to bring back the dead, a risky project he believed Brian was trying to manipulate to redeem his tragic mistakes. In a misguided attempt to atone for his actions, Dr. Steiner ultimately takes his own life, leaving Ronnie and Kirk with the burdensome truth.
Faced with looming danger, Kirk is arrested when a gun is discovered in his locker, leading Ronnie to seek Mr. Bittner's assistance again. Here, she learns about Kirk's tumultuous past involving firearms and his expulsion from a previous school, but soon finds herself questioning his innocence. Bittner promises to protect her for the remaining 48 hours leading to her birthday, sensing that it may be when Brian seeks to enact his grim resurrection plan. With Bittner's guidance, they reinforce her home, securing a panic room designed to thwart the presence of remnants.
As the climax unfolds, Kirk uncovers that Bittner has a much darker motive tied to his daughter Eva's untimely demise. After knocking Kirk out, Bittner buries him alive, revealing his true intent: to murder young women who share his daughter's birthday to resurrect her. Meanwhile, Ronnie narrowly escapes to a frozen lake, where Bittner catches up with her. In a fight for her life, she breaks the ice, plunging them into the frigid waters below. Just as it seems hopeless, Mary Greer's ghost intervenes, leading to Bittner’s watery demise while Brian’s spirit saves Ronnie.
Brimming with memories of her father teaching her to ice skate at that very lake, Ronnie struggles to stay conscious. Thankfully, Kirk arrives just in time, having unearthed himself with the help of his father's spirit. The two face their shared burden as they visit Robert's grave, determined to honor the truth surrounding the tragic events of Chicago. As they leave, Brian’s ghost looks on, a reminder of the story they will now carry with them.
