Andy is alone in his family's apartment when the zombies begin to attack and infect people outside. When he inspects the apartment hallway, a neighbor walks in. He begs to stay, but becomes a zombie and attacks Andy. Andy pushes her out and locks the door, then sees a large zombie attacking her neighbor through a peephole. Andy then receives a message from his parents that they are safe.
Andy locks his front door to the fridge and posts a rescue request on social media. By connecting his phone to a drone, he explores his neighborhood and realizes it's over. Although emergency power remains on, Andy gradually loses access to the phone network, the Internet, and running water.
One night, Andy sees the corpse being taken out by a police officer. He yells at them, causing a zombie to break into the apartment and knock on the fridge, destroying the food. Joon-woo lures him to fall from the balcony. He resorts to drinking alcohol to survive, but hallucinates about his family due to hunger and thirst. When the phone network is momentarily restored, Andy receives a voicemail from his family saying their asylum has been destroyed. His anger causes him to leave the apartment and attack the zombies, but when a mob attacks, he runs away and barely manages to sneak back inside.
Andy attempts to hang himself, but stops when he sees the laser pointer light up. The hint comes from Susannah Walters, a survivor living in the apartment building opposite. Walters tries to set up a zip line between their apartments by tying a cable to their desk and throwing the other end to Andy, but it ends up in the street. Andy uses his drone to successfully establish another line, but an infected firefighter pulls on the first cable, causing Walter's table to fly into his apartment, knock him over, and break into the railing of his balcony. She goes. The zombie starts climbing the cable. Andy distracts him by sacrificing his drone, giving Walter enough time to wake up and kill the zombies.
Andy raids his neighbor's unit for food, clothing and a walkie-talkie, narrowly escaping from his zombified neighbor. He sends Walters a walkie-talkie, and the two talk and bond. Suddenly, the infected crowd becomes agitated and a panicked Walters accidentally knocks over a shelf. Due to the noise, the corpse gathers at his door. Andy distracts them by calling the unit adjacent to him.
Walters says that the 8th floor of Andy's apartment looks empty. The pair gear up, rappel from their balconies, and fight their way to the 8th floor, but the doors are locked and they are chased by zombies. Suddenly, a stranger emerges from the last suite on the floor and rescues them. He gives them food and water, but he is drunk, and Andy falls asleep. The stranger ties the pair's wrists and locks Walters in a bedroom with his restrained, infected wife. Andy wakes up and threatens the stranger, but he releases his wife's shackles from outside the bedroom. Once Walters is silenced, the stranger opens the bedroom door, only to find that Walters has managed to wrap the infected wife's head in a cloth. The infected wife attacked her husband and Walters shot them both.
Gunshots attract more zombies. Walters tells Andy to kill him before he goes in. As he hesitates, they hear the helicopter outside. The pair chases the infected and runs on the roof. Before they are overwhelmed, an army helicopter captures the corpse and escorts the pair to a safe location. A news anchor's voiceover suggests that the wireless network is being restored and the military is rescuing survivors who made social media posts.
Andy later changed his social media post from #tryingtostayalive to #alive.
