On a December morning in 1998, Sloane Price contemplates the unthinkable, reading a suicide note and preparing to end her life in the bathtub, when her Sloane's father—a verbally and physically abusive presence—calls for breakfast. A frantic, screaming woman at the door jolts the household into motion, and he urges them to leave. But as Sloane steps into the street, she witnesses zombies swarming the neighborhood, and one of the creatures breaks into the home through a shattered window. The brutal confrontation between her father and the undead ends with him being bitten in the left arm and, in a last desperate act, him choosing to kill a zombie by driving a shard of broken glass into its eye. Shocked by the violence, Sloane escapes, leaving her infected father behind.
In the chaos of the streets, Sloane climbs over a backyard door to safety and finds two of her high school classmates, Rhys Moreno and Cary Chen, in a nearby garden. The trio teams up to search for a secure refuge. They soon run into two more classmates, Grace Casper and Trace Casper, along with their mother, and the group begins to fracture under pressure. Tensions rise as Cary clashes with Mrs. Casper over how best to protect the children, and that night the teens huddle together in an upper-floor room, trying to stay alive.
Memory and grief echo through Sloane as she recalls her older sister, Lily Price, whose bond with Sloane was forged in easier times before their father’s abuse forced Lily to leave. A birthday envelope in Lily’s yellow letter becomes a poignant symbol of connection, the younger sister deciding to read it later and leaving the envelope in the glove compartment. In the present crisis, the group’s uneasy alliance persists as another zombie breaks in, forcing them to move.
The group makes it to the high school’s parking lot and tries to reach the building for safety. Inside, Mrs. Casper sacrifices herself by stepping between the teens and the pursuing crowd, urging them to run. Cary finds his way into the administration office and kills a zombified Principal Lively, after which the teens barricade themselves inside. In the gym, a tense confrontation erupts when Trace accuses Cary of being responsible for his mother’s death, and a brief fight ensues before the others intervene.
Seeking shelter and guidance, Sloane, Rhys, and Cary encounter their English teacher, Mr. Baxter, who is asleep and hidden in a closet. They seize his gun, and after Baxter awakens and is fed, his strange demeanor unsettles them. Although he denies infection, Baxter bites Cary on the arm before Trace and Rhys eject him outside. Cary stays in the nursing office, prepared for any sign of illness.
As they navigate the hallways again, Sloane discovers Baxter wounded and bleeding in a corridor. A zombified Baxter attacks Grace, who dies in Trace’s arms after he fires a fatal shot with Baxter’s gun, an act that leaves Trace overwhelmed with guilt and further embroiled in fear and blame. Blaming Sloane for interfering, Trace lashes out at her, but Cary and Rhys pull him away, vowing to protect Sloane.
With danger escalating, the trio decides they cannot remain inside the school. They retrieve a car and plan to head toward a radio-reported safe location. Cary volunteers to distract the zombies outside, and he dashes into the street, disappearing as the crowd closes in.
Back at home, Sloane and Rhys return to the house and confront a terrifying discovery: an infected Lily in the basement. Lily lunges at Sloane, who defends herself with a metal bar and kills her sister’s attacker. Lying next to the basement sofa, Sloane also discovers her father’s corpse, suggesting he may have attacked Lily before the outbreak spiraled further.
With the situation deteriorating, Sloane and Rhys steal the family car and drive toward the area the emergency radio has described as safe. As Rhys finalizes directions, Sloane retrieves the yellow envelope Lily left behind, warmed by her sister’s words. When Rhys starts the engine, Sloane sheds the suicide note and, together, they set off toward an uncertain but shared future.
