Plot

The rapid spread of an unknown infection has left an entire city in ungovernable chaos, but one survivor remains alive in isolation. It is his story.

Without warning, people in the streets of Seoul become extremely violent, cannibalistic, attacking passersby, behaving like zombies. A young man alone in his parents' 4th floor apartment becomes spectator to all this. There're no groceries left in the apartment. He foolishly opens the door and a neighbor forces himself in. He's infected but is finally forced out. TV informs him that the virus is spreading nationwide. As the days pass, the phone and internet stop working, then the water's gone. When all hope is gone, he sees a cute girl in the apartment across from his.—Scott Filtenborg

In Seul, the gamer Oh Joon-woo is sleeping when his mother leaves money for him to go shopping in the grocery. Out of the blue, infected people become violent, attacking other people on the street of his condominium. Soon he learns from the news that a virus is turning people into flesh-eaters zombies. Without any food, Joon-Woo survives locked in his parents'' apartment, but when the building runs out water, he decides to commit suicide hanging himself in the living room. However, a beam of laser calls his attention, and he sees a survivor in an apartment on the building in front of his. Using his drone, he contacts the beautiful Kim Yoo-bin, who is living alone in her building. She provides supplies to him, and he decides to scavenge his neighbor's apartment looking for water and food. He finds two walkie-talkies and now he can communicate with Kim. But the zombies are dangerous and threaten their lives.—Claudio Carvalho, Rio de Janeiro, Brazil

Oh Joon-woo (Yoo Ah-in) awakens and discovers he is alone at home. His parents left that morning in a hurry leaving him money for groceries. Oh Joon-woo is a gamer and is online with some of his gaming friends when he first hears about the strange disease that causes infected people to bite and even eat uninfected people thereby infecting them. When Oh Joon-woo turns on the television, he is surprised to see how chaotic the country is. As he reaches his balcony, Joon-woo sees a chaotic and panicked crowd fleeing and running for their lives. Pandemonium is breaking out. Traffic accidents are numerous. Oh Joon-woo notices a girl calling out for her mother. When her mother finds the girl, their embrace become a violent attack as the girl starts to devour her mother.

Back inside again, Oh Joon-woo hears similar noises in the hallway. As soon as he opens the door to the hallway, a neighbor named Lee Sang-chul (Lee Hyun-wook) from the next apartment barges in. Sang-chul begs Joon-woo to let him stay. Joon-woo allows him to use the bathroom. A TV report describes the transformation infected individuals experience including aggressive behavior and bleeding of the eyes that can ultimately lead to full out cannibalistic assaults. When the neighborhood exits the bathroom, Joon-woo notice a wound on his neighbor's hand. Shortly thereafter, Sang-chul's eyes turns red and his body convulses. Joon-woo pushes his neighbor back out of the apartment and into the hallway and locks the door to save his own life. Through the peephole, Joon-woo obverse a hideous murder in the hallway. Blood seeps under the door into his apartment. Joon-woo secures the front door by pushing his refrigerator in front of it.

Oh Joon-woo receives messages from his family assuring him that they are safe. He checks his food supply and regrets of not going to the grocery the day before. He dedicates himself to survival. As night has fallen, Joon-woo falls asleep on the couch. The next day, Oh Joon-woo is awakened by a TV discussion of professor Kim Yeon-jeong of the causes of the behavioral changed attributed to a mutated viral infection. The professor states that there seems to be a connection to the right brain which is the source the very aggressive behaviors observed.

The internet, as Joon-woo soon finds out, is filled with people asking about their loved ones and putting up addresses under the #Alive. Seeking his own rescue, Joon-woo puts up a similar post. He also comes across a disturbing video of a rather stupid young man, who is hanging outside the terrace of a multiple story building with his phone attached to a selfie stick in a bid to get a signal. The young man is excited when gets a signal only to lose it soon after. He then stretches on tiptoes, loses his footing and falls from his perch.

Joon-woo sends out his drone to get a brief glimpse of the terror going on in the apartments around him before his cell phone battery dies. Then he even loses the connection to his streaming video game service. He falls asleep but is awakened by a commotion outside. From his deck, Joon-woo observes a policewoman (Oh Hye-won) being torn to bits by fellow police officers now turned into zombies. Joon-woo shouts at the predators out of sympathy, but the policewoman is overwhelmed. Joon-woo's attempt to scare off the zombies nearly 100 meters away by yelling at them does nothing but to attract the attention of a zombie close to his home. Almost immediately, a zombie pushes his way past the refrigerator and into Joon-woo's apartment. Joon-woo is outside the railing hanging off his lanai when the zombie rushes him and falls several stories to the pavement below. Although Oh Joon-woo is unharmed, it seems that his encounter with the zombie has further reduced his rations. The little food in the fridge has all found its way to the bloody mess on the floor.

After two days alone, Joon-woo records a video of his nearly miraculous survival for his subscribers. By Day 7, Joon-woo has invaded his father's liquor cabinet. TV news further details that the virus seems to be only spread by contact with the blood. Avoiding bites and stabs is the best way to stay free of contamination. The infected get a superior sense of strength and sensory capabilities. Many still possess the ability to perform behaviors they had before infection such as opening doors and windows or performing their occupations. Joon-woo eats his last bowl of Ramen soup. Joon-woo spills his water bottle and then discovers there is no running water in his apartment.

By Day 10, Joon-woo is frustrated by the advisory on the television stating that survivors should simply wait it out at home. He refuses to leave the apartment and end up like the infected. He recalls his mother listened to internet radio shows. His attempt to listen to the radio on his phone is foiled when he cannot find any wired earphones to serve as an antenna. All his earphones are wireless.

By Day 15, Joon-woo hallucinates that he is reuniting with his family. Waking up from the hallucination, he is left very sad. He gets a call from his mother, but the signal is so bad he barely hears a thing. He then duplicates the stunt the stupid young man on the internet had pulled earlier using a selfie stick to get better reception. He does not fall, but he clearly hears that something is not quite right with his family.

In his anger, Oh Joon-woo breaks things in his home and goes outside. There, he sees a zombie that is surprisingly slow, and he takes out his anger on the zombie using a golf club. The violence urges others to come by. Not too long after he is nearly swamped in a sea of zombies. Joon-woo manages to get to his apartment alive when zombies exiting the elevator pile up against the zombies pursuing Joon-woo down the corridor. Then Joon-woo quietly walks behind the zombies until he arrives at his front door. Once Jun-woo is behind the locked door, the zombies gather outside.

By Day 20, Joon-woo decides that it is time to take his own life. As he dangles from the noose he has tied, a laser beam projects into his house from the apartment across the yard. The beam points to the word "NO" on the writing on the wall. He struggles with the noose until he is freed. Joon watches the beam then point to different alphabets on the writings on the wall to spell the word, IDIOT. (Someone is finally saying it). The source of the laser beam is a girl on the lanai opposite his. Kim Yoo-bin (Park Shin-hye), seems to have been watching Joon-woo with a pair of binoculars. When Joon-woo, now freed from the noose, sees her, he waves to her excitedly. She waves back and gestures that meet again at seven o'clock.

Unlike Oh Joon-woo, Kim Yoo-bin seems very organized and has prepared her house with a series of booby traps for zombies. Joon-woo wakes up at eleven o'clock, late for the seven o'clock rendezvous time they had previously agreed on. Joon-woo gestures an apology and then projects his name on his phone. She reciprocates. After their brief attempt to communicate Kim Yoo-bin hears a sound and attempts to find the source. They agree to meet at seven o'clock again. One of Kim Yoo-bin's traps has snared a zombie that made its way into her house. She proceeds to axe its head off.

The next morning, at the agreed upon time, Yoo-bin attempts to pass food over to Joon-woo when she sees that he is hungry. She attaches a rope to a baseball and throws it across hoping it gets into his home. The ball does not make it and it lands in the street. Oh Joon-woo then uses his drone to deliver a rope to Kim Yoo-bin. Using the rope bridge, she passes him a container of snacks and a bottle of water with the encouragement not to eat too fast. An infected firefighter pulls on the first cable, causing Yoo-bin's table to fly across her apartment, knocking her out, and lodge into her balcony railing. The zombie starts climbing the cable. Joon-woo distracts the zombie with his drone ultimately sacrificing it. Shortly thereafter, Yoo-bin wakes up and cuts off the hand of the infected man as her reaches her lanai. The zombie falls to his death. Joon-woo flashes a message to Yoo-bin on his phone to "Stay Strong".

Viewing his meager supplies and quite desperate, Joon-woo decides to "go farming". Leaving his apartment, he finds the key to his neighbor's door on a dead body. Joon-woo cleans out the refrigerator and finds other food, clothing, a headphone with a cord and walkie-talkies inside his neighbor's unit. While searching for other items, an infected sleeping zombie wake ups and attacks Joon-woo. Joon-woo manages to save himself and bring the essentials to his apartment.

After feasting on the peanut butter he found and unsuccessfully finding a radio station with his antenna cord, Joon-woo sends a walkie-talkie and some food to Yoo-bin on the zip line. The two communicate through it. With the talkies, they eventually start to communicate. Yoo-bin is not interested in discussing Joon-woo's suicide attempt. Instead, they decide to make noodle soup together. Later Yoo-bin admits she is scared of heights. Joon-woo finds it illogical that she has such a fear and was still able to cut off someone's hand. Yoo-bin says the monsters are no longer human which relieves her of any guilt or fear. She tells Joon-woo that she fell while climbing.

While they talk, Jun-woo asks Yoo-bin to bring out basins as he can hear heavy sounds and thinks it's raining. They look out the terraces, but it is not raining. Instead, they see a multitude of zombies running into both apartment complexes simultaneously. They both lie low and try to stay silent until Yoo-bin accidentally knocks over a shelf in her house. Joon-woo calls a neighboring unit on the phone to momentarily distract the zombies in front of Yoo-bin's apartment. Yoo-bin prepares to leave as she knows she would soon be overrun with zombies. She informs Joon-woo that her destination is the 8th floor of his apartment complex which, as she had earlier noticed, was empty. They both agree to meet up there.

Yoo-bin jumps out of the terrace of her home, repels down to the street and runs towards the apartment complex across the way swinging her axe to fight off numerous zombies. Joon-woo is very impressed by her bravery and runs out to help her. He knocks a zombie off the top of Yoo-bin with his golf club. They both narrowly miss being swamped by zombies as they enter the elevator to the 8th floor. They meet face to face for the first time.

As they cautiously exit the elevator, all is peaceful on the 8th floor. This lasts a few moments while they are checking for unlocked doors. Soon they are again chased by zombies. Joon-woo fights them off with a golf club while Yoo-bin finds an unlocked door. A man (Jaon Bae-soo) opens his door and quickly pulls them in before the Joon-woo and Yoo-bin are bitten and eaten. Once inside, they all share their stories of survival.

The man seems kind and offers them water and food. He says he has enough for them to survive until a rescue team comes around. They are overjoyed but it seems their joy is short lived as they soon discover that the food was laced with drugs. The man zip ties their wrists while they slumber. He locks Yoo-bin in a room with a sky painted on the ceiling.

Yoo-bin wakes up under a mobile in a locked room. The man is planning to offer Yoo-bin as a meal to his starving wife (Lee Chae-kyung) now turned zombie. Yoo-bin realizes that she's in danger and cries for help. Joon-woo wakes up and threatens the older man with a gun they previously had recovered outside. The man holds the wife's harness telling Joon-woo keeping his wife alive is something he must do. He offers to let Joon-woo survive of he just abandons Yoo-bin. When Joon-woo threatens to kill the man, he releases the tether holding back his wife. Joon-woo leaps toward the tether but it has mostly slips away under the door. Sounds of an attack are heard and then the room goes silent. The older man opens the door to see what is going on. Yoo-bin manages to save herself by pulling the fabric over the infected wife's head. The older man, submits himself to his wife attack. Yoo-bin shoots them both with the gun.

The gunfire only draws the attention of more zombies. Seeing the end so close, Joon-woo and Yoo-bin decide to shoot themselves instead of getting bit and become infected. Just before Joon-woo pulls the trigger on his partner, they hear the rescue helicopters. With hope rekindled in them, they struggle hard to make it to the rooftop of the apartment building. The use a push cart to clear a path through the zombies. Then Joon-woo holds off a hoard of zombies in the stair well with a bicycle while Yoo-bin escapes to the roof. Joon-woo eventually pushes the zombies backward giving himself enough time and distance to join Yoo-bin on the roof. The choppers, however, are no longer in sight.

Jun-woo screams at the sky, "There are survivors here! We are still alive!" The zombies find their way to the rooftop, push open the door and they gradually creep closer and closer to Joon-woo and Yoo-bin. Just when they are about to overwhelmed by the zombies, the military appear in a helicopter and shoot at the zombies while helping the survivors into the chopper. Once aboard and flying to safety, Joon-woo receives a message from his family assuring him of their safety. He looks towards Yoo-bin and they are both glad to be alive. A news report details how valuable posting on social media has been in locating survivors in the urban areas. The reference on Joon-woo 's social media posting changes from #I_MUST_SURVIVE to #ALIVE.