The story centers on Kim Do Yoon, a man who finds himself alone in his family's apartment as a sudden zombie attack unravels the world just outside the door. When a neighbor bursts in and begs to stay, panic turns to horror as the man becomes a rampaging zombie and attacks him through the door. Joon-woo pushes the threat out and watches from the peephole as a large horde swarms beyond, while hopeful messages from his parents appear on his phone, telling him they’re safe for the moment.
Desperation drives him to barricade the front door with a refrigerator and to broadcast a rescue plea via social media. He attaches his phone to a drone to scout the neighborhood, learning that the city has spiraled into chaos. The emergency power persists, but one by one, communication networks, the internet, and running water fade away, leaving him in a hollow, isolated survival chamber.
One night, a brutal scene unfolds outside: zombies overwhelm a police officer, and Joon-woo’s frantic scream draws a prowling zombie into the apartment. The creature knocks over the fridge, spoiling the scant food. He lures it toward the balcony and forces it off, a momentary victory that tastes like danger. Hunger gnaws at him, and liquor becomes a grim companion, driving him to hallucinations of his family as thirst gnaws at his resolve. When the phone network flickers back briefly, a voicemail from his family reveals that their refuge is also under siege. In a surge of rage, he bursts from the apartment to lash out at the undead, but a massive horde forces him to retreat, slipping back inside just in time.
A moment of despair nearly seals his fate as he attempts to hang himself, but a laser pointer’s light stops him in his tracks. The signal comes from Kim Yoo-bin, a survivor living in the building across from his. She tries to bridge the chasm between them by rigging a zip line—tacking a cable to her table and tossing the other end toward him. It lands in the street, though, forcing a new plan. Joon-woo uses his drone to engineer another line, only for an infected firefighter to yank the first cable taut. Yoo-bin’s table is hurled across her living space, knocked unconscious, and pinned against the balcony rail. The ensuing zombie climbs the line, but Joon-woo distracts it by sacrificing his drone, granting Yoo-bin a precious window to wake and deliver the killing blow.
Resourceful but careful, Joon-woo raids his neighbor’s unit for sustenance, clothing, and walkie-talkies, narrowly avoiding a zombified neighbor along the way. He passes a walkie-talkie to Yoo-bin, and a cautious bond begins to form as they exchange messages and hope. The undead become increasingly agitated, and a clattering shelf betrays their presence as the noise draws them toward Yoo-bin’s door. Joon-woo conducts a last-ditch distraction by calling the unit adjacent to hers, buying a little time.
Yoo-bin notes that the eighth floor of Joon-woo’s building appears empty, and the two teammates seal their plan to join forces. They rappel from their balconies, fighting through the carnage toward the 8th floor, only to find the doors locked and the horde dogging their every step. A stranger finally emerges from the final suite, offering food and water, but the relief is short-lived: the stranger’s offerings are tainted, and Joon-woo succumbs to sleep. When he wakes, the stranger’s grip loosens on his infected wife, and Yoo-bin finds herself locked in a bedroom with the wife restrained outside. A tense confrontation erupts: Joon-woo threatens, the stranger releases the restraint, and Yoo-bin discovers that the infected wife’s head is tangled in fabric. The wife retaliates, and Yoo-bin fires, killing both the husband and wife in a desperate bid for survival.
The gunfire triggers a further influx of zombies, and Yoo-bin implores Joon-woo to end her life if she fails to survive the onslaught. They are interrupted by distant helicopter rotors, and the two dash to the rooftop with the horde in pursuit. A military helicopter arrives, fending off the zombies and lifting the pair away to safety. In a closing voiceover, an anchor explains that wireless networks are being restored and that the military is rescuing survivors who managed to reach out via social media. The final shot centers on Joon-woo’s post, where the hashtag shifts from "#StayAlive" to "#Alive," signaling a fragile but hopeful new beginning.
