In a quiet family apartment, Joon-woo finds himself utterly alone as chaos and an escalating zombie attack loom just outside. A neighbor bursts in, pleading to stay, but is transformed into a threat that he must swiftly push out and lock the door against. Through the peephole, he watches as a crowd of the undead swarms the hallway outside. Moments later, he receives a message from his parents, assuring him they are safe somewhere, somewhere else.
Determined to survive, he barricades the door with a heavy fridge and launches a desperate rescue plea on social media. With a swift, improvised plan, he attaches his phone to a drone to scout the surrounding neighborhood and realizes the world he's in has spiraled into an overrun nightmare. Even as emergency power remains, his access to the phone network, the internet, and running water begins to disappear, piece by piece.
One night, the outside world becomes louder and more dangerous. A zombie attack takes down a police officer nearby, and Joon-woo’s scream triggers a new danger: a zombie slips past, shatters the fridge, and tears into the food. He lures the creature toward the balcony and watches it tumble to the street. To endure, he turns to liquor, but the alcohol and thirst trigger vivid, haunted visions of his family. When the phone network flickers back on briefly, he finds a voicemail from his parents: their refuge has been overrun. Overcome with rage, he abandons the apartment to fight the undead, only to be forced back by a rising horde.
Desperation pushes him to a dangerous brink, and he tries to hang himself. A laser pointer’s light catches his attention, and he follows it to a beacon of hope: Kim Yoo-bin, a survivor in the building opposite. She attempts a perilous plan to connect their apartments with a zip line, tying a cable to her table and tossing the other end toward him, only for the line to land in the street. Undeterred, Joon-woo uses his drone to lash together a new line, but misfortune strikes when an infected firefighter yanks on the first cable, sending Yoo-bin’s table slamming into her balcony railing. A zombie climbs the line, and Joon-woo distracts the creature by sacrificing his drone, giving Yoo-bin the opening to wake up and kill the zombie.
In a tense raid, Joon-woo moves through his neighbor’s unit to gather food, clothing, and walkie-talkies, all while narrowly avoiding a zombified neighbor. He slips a walkie-talkie to Yoo-bin, and the two begin to communicate and form a fragile bond across the siege. As the infected hordes grow agitated, a misstep by Yoo-bin causes a shelf to topple, drawing more attention to their door. To survive, Joon-woo keeps the line of communication open with a frantic call to the unit next to hers, buying them precious time.
Yoo-bin reveals that the 8th floor appears to be empty, and the two teammates suit up for a daring escape. They rappel from their balconies and fight their way toward safety, but the doors to the upper floor remain locked and the zombies relentlessly follow. Just as hope teeters on a knife-edge, a stranger emerges from the last suite on the floor, offering food and water. The gesture seems lifesaving, but it is laced with danger: the supplies are drugged, and Joon-woo slips into unconsciousness.
When he awakens, the stranger’s grip tightens, and the restraints on Yoo-bin’s infected wife are released from outside the bedroom. A tense standoff unfolds: the stranger’s wife, now fully infected, breaks free and attacks her husband, prompting Yoo-bin to shoot both of them. The gunshots draw the hungry horde, and the pair is forced to sprint for the rooftop as the building erupts into chaos.
Above, an army helicopter appears, suppressing the growing swarm and lifting the two survivors to safety. A voiceover from a news anchor explains that wireless networks are gradually returning and that the military is rescuing survivors who managed to post messages on social media. The film closes with Joon-woo’s online post, originally tagged with “#StayAlive,” shifting in a quiet, hopeful moment to read simply “#Alive.” The escape marks a fragile reprieve from isolation, hinting at a broader recovery while underscoring the resilience it takes to stay human in an empty, dangerous world.
