As the evening progresses at the medical school party, Zoe Parker, a medical student, finds herself reluctantly giving in to the peer pressure of her friends and indulging in the festivities. The night takes a harrowing turn when she ventures into the morgue to grab a keg of beer, only to encounter Max, a disturbed patient who has developed an unsettling fixation on her. What starts as a simple interaction quickly escalates into a nightmare as Max's obsessive behavior morphs into violent aggression, forcing Zoe to fight for her life against his assault. Just when Zoe's situation seems hopeless, a grotesque twist occurs: a corpse in the morgue comes back to life, attacking Max and compelling him to flee in terror. Seizing this unexpected opportunity, Zoe escapes, but chaos ensues as hordes of reanimated corpses, known as "rotters," emerge from the shadows, claiming the lives of everyone at the party except for her.
In the aftermath of this unexpected catastrophe, the survivors must find refuge in camps, leading Zoe to the High Rock Emergency Bunker, where she is monitored by Lieutenant Miguel Salazar. Five years go by with the group's connection to the outside world severed, plunging them into a state of isolation and uncertainty. Within the confines of the bunker, Zoe and her friend Elyse dedicate themselves to treating illnesses and searching rotter blood samples in hopes of discovering a cure. Though Miguel remains skeptical about the chances of finding a solution, he allows their efforts to continue, driven by a spark of hope that they might recover their world from the undead menace.
Zoe's relentless determination leads her to convince Miguel to permit a vital supply run to Whittendale University, where she hopes to acquire much-needed medication for Lily, a young girl battling bacterial pneumonia. As she bravely navigates the perilous university grounds, a shocking reminder of past horrors resurfaces: Max, now partially reanimated, has made his way onto the campus. Gripped by terror upon seeing her former tormentor, Zoe attempts to escape, unwittingly attracting the attention of a nearby gaggle of rotters. In the midst of this madness, her companions rush to rescue her, but not before Frank becomes a victim of the relentless undead.
With the weight of grief heavy upon them, Zoe and her group regroup to deliver critical medication to Lily, whose survival hangs by a thread. As underlying tensions grow, Frank's tragic loss escalates the animosity between Zoe and Miguel, creating a rift that could endanger their survival further. The lingering presence of Max serves as a terrifying reminder of the horrors they continually confront, leaving everyone uncertain of their ultimate fate.
During this desperate fight for survival, Zoe must confront her deepest fears and summon every ounce of courage she possesses. Will she find a way to utilize Max's blood to engineer a vaccine, or will the persistent onslaught of rotters finally consume them all? In a bold attempt to test the potential vaccine, Zoe and Elyse enlist Baca's help to open the gates and allow a single infected individual inside. What begins as a structured plan rapidly spirals out of control as chaos unfolds, resulting in the death of Thomas and the infection of Elyse. Amid the ensuing pandemonium, Miguel fights valiantly against the undead, sealing the gates but stoking Zoe's desperate pleas to use the vaccine on her ailing friend. In a heartbreaking turn, Miguel’s harsh disregard for Elyse’s fate leads to a fatal conclusion as he is forced to end her suffering, minimizing her agony.
As the truth begins to unravel, Miguel discloses to Baca that he had observed Zoe's name etched onto Max’s arm during their tumultuous journey. When confronted about this revelation, Zoe shares the harrowing narrative of Max's attempted assault, an experience that has left scars on her mind. Meanwhile, Max continues to taunt her, persistently declaring, “You are mine.” Despite his derision, Zoe manages to evade his grasp and finds herself being pursued by the rotters, leading to a dangerous confrontation with Lily's mother, who has suffered from Max’s insidious infection.
As the horde closes in, Zoe finds herself in a nail-biting chase, relentlessly pursued by Max. Baca bravely attempts to intervene but is thwarted by Miguel's calculated coldness, resulting in both of them getting bitten and left to die in the tumult. Concurrently, Lily runs for her life as Max seeks to claim her as well.
In a poignant climax, Zoe tracks Lily down to a greenhouse where she faces off against Max in a visceral showdown. With mud-encrusted hands, she exacts revenge on her tormentor by dismembering and decapitating him, putting an end to his persistent terror. As calm returns, Zoe and Lily head back to their stronghold, only to find Baca on the verge of despair, accepting his fate. Believing in the vaccine's potential to save him, Zoe administers the serum, restoring him to a semblance of normality.
In the aftermath of the chaos, as repairs commence at the High Rock Emergency Bunker, Zoe broadcasts a hopeful message to any potential survivors, announcing that they possess the life-saving vaccine. Yet, the ominous rumble of rotter growls from the nearby forest serves as a chilling reminder that their battle against the unyielding undead threat is far from over.
