As high school student Eddie (Jacob Zachar) arrives at Lincoln High School’s after-school detention, he finds himself among a colorful array of classmates: Janet (Christa B. Allen), the bubbly cheerleader who unknowingly captures his heart; Brad (Jayson Blair), the arrogant jock known for his bullying antics and ROTC dreams; Jimmy (Max Adler), Brad’s accomplice and another football aficionado; and Ash (Justin Chon), the easy-going stoner with a constant smile. Eddie’s loyal friend Willow (Alexa Nikolas), a goth who shares his passion for zombie flicks, rounds out the group. Just as they settle into their seats, Ash makes a reckless move by trying to sell drugs to Mark ([Joseph Porter]), a quiet student whose troubled demeanor hides more than initially apparent.
Ash’s hasty retreat is triggered by Mark's sudden, bizarre behavior, which escalates to a shocking incident where he bites Mrs. Rumblethorp ([Michele Messmer]), sending the group into chaotic flight. Eddie desperately warns their teacher to stay away from Mark, but his plea falls on deaf ears. What follows is nothing less than a precursor to impending doom, as the group soon discovers a horror-filled sight: students transformed into relentless undead.
With escape routes quickly vanishing, they take shelter in the school library, believing this refuge will protect them from the chaos outside. However, their moment of safety is abruptly shattered when Mrs. Rumblethorp reanimates, forcing the group to restrain her and ultimately end her life with a paper cutter, wielded by Ash.
As they navigate the surreal circumstances, Brad begins to withdraw, troubled by a concealed bite wound. Tensions rise while the group scavenges for answers, leading to a harrowing encounter with the school librarian, who attacks and infects Jimmy before Brad is forced to kill her in self-defense.
A heated argument erupts between Eddie and Willow regarding Jimmy's fate; Eddie argues for his immediate termination to stave off the undead transformation, whereas Brad clings to hope that Jimmy may fight the infection. However, Jimmy’s acceptance of his grim fate leads him to jump out the window, turning into a meal for the very zombies they fear.
Anxiety mounts within the group, prompting Ash to suggest they partake in cannabis to ease their frayed nerves, surprisingly bonding them together amid the insanity. They share insights about their high school personas, breaking free from their old stereotypes: Eddie, the studious overachiever; Willow, the outsider; Janet, the self-centered cheerleader; Brad, the boastful jock; and Ash, the laid-back stoner. But their bonding is short-lived, as Eddie reveals he carries a gun to cope with his rejection from Harvard, and the group soon faces an onslaught of attacking undead.
With their situation continuously growing dire, Brad, Willow, and Ash opt for a risky escape through the school's ventilation system. Meanwhile, Eddie and Janet remain in the library, unaware of the dangers lurking close by. As the trio maneuver the perilous ducts, they find themselves confronted by zombified rodents indulging on Ash's severed limbs, only for the vent to cave in and isolate Willow and Brad in the twisting school corridors.
In the throes of chaos, Janet persuades Eddie to realize her long-held desires, yet their passionate moment is tragically interrupted by the surprised arrival of Willow and Brad. Inside the library, Willow is taken aback to discover Eddie’s intentions with Janet just as the group faces the relentless advance of zombies. In a desperate struggle for survival, Brad's infection overwhelms him, turning him into a threat to Janet, who manages to repel him before casting him out of the window.
As the remaining members battle their way through the school’s hallways to find an exit, they construct a makeshift cart from library supplies. The confrontation leads them to the gymnasium, where they begin their ascent to the roof. There, Janet faces her grim fate after being bitten, begging Willow to end her suffering in a moment of poignant solidarity. However, their silence is brutally interrupted by an invasion of zombies. In a selfless act, Janet shoves herself and the undead Brad off the roof, making the ultimate sacrifice to save her friends.
As the survivors grieve their losses, they are suddenly rescued by a military unit that descends upon the school, vanquishing the undead horde. In the aftermath, Eddie and Willow emerge as the remaining survivors, sharing their first kiss amidst the chaos. Just as they prepare to leave, a zombified Janet reappears but is swiftly dispatched by the military’s gunfire, saving Eddie from infection and allowing them to finally head home together in peace.
