Episodes 01

In September 2020, Cha Hyun-Soo is a soldiers’ target in what appears to be a no-man’s land. One month earlier, he moved into the apartment complex Green Home where a variety of people live. Depressed and suicidal, he avoids going outside and relies on food deliveries. One night, he realizes that his ramyeon delivery was opened by his neighbor and witnesses a woman holding the head of a dead cat in the apartment next door. He rushes into his apartment but is followed by his neighbor asking for help before turning into a “glutton” monster. Meanwhile, on the ground floor, residents cannot go out of the building which is on lockdown and one of them dies in the hands of a “vampire” monster.

Episodes 02

Asleep and guarded by his upstairs neighbor Ji-soo, hikikomori Hyun-soo relives the last moments before his parents’ and sister’s deaths in a car crash as well as their funeral where he blames them for not leaving him enough money to survive. Sang-wook kills the “glutton” monster. Ji-soo and Jae-heon encounter the “lotus root” monster. On the ground floor, conflicts start to arise between residents as they learn more about their situation and how people turn into monsters. Hyun-Soo saves two orphan siblings (whose father was killed by the “eyeball” monster) and then is saved in return with the help of his disabled neighbor, Du-sik, but soon starts experiencing symptoms.

Episodes 03

Hyun-Soo encounters the monster side of himself, who tries to convince him to embrace his evil side. The “Protein” Monster manages to infiltrate the building and is currently chasing the orphans Kim Su-Yeong and Kim Yeong-Su. Getting cornered by the “protein” monster, Im Myung-Sook gets in the way and unexpectedly changes into a monster and helps to stop the monster. Hyun-Soo escorts the children to Han Du-Sik, while Ji-Su attends to Im Myung-Sook who heals herself and reverts to human. Jae-Heon tricks the “protein” monster to fall off the building. Yi-Kung goes to the basement and tries to fix the building’s electricity, only to be attacked by the “Tentacle” monster, a spider-like creature which webs her to the ceiling. Meanwhile, the survivors on the ground floor watch the national broadcast and witness in horror as the president turns into a monster, causing panic amongst the survivors. The next morning, the desperate mother Cha Jin-Ok opens the gate of the building and tries to rescue her daughter. The soldier Lee Su-Ung rushes to save her, only for the “Vampire” monster to impale them both, killing them.

Episodes 04

Yi-kung manages to escape from her bonds and escapes from the ‘tentacle’ monster. An official government announcement details the current incident to the survivors. Hyun-soo, Jae-Heon, and Ji-Su decide to leave for the ground floor to check-up with the other survivors, while Du-Sik stays in his room with Myung-Sook and the children. Suddenly, something attacks Hyun-Soo, making him free-fall from the 9th floor to the ground. Moments later, he wakes and is approached by Eun-Hyeok and Eun-Yu. Now that Hyun-soo’s monster identity is revealed, the survivors discuss whether or not Hyun-soo should be kicked out. Eun-Hyeok suggests they hold a poll, but the convenience store owner Seok-Hyeon insists that Hyun-soo must be kicked out. To everyone’s surprise, the result of the poll is a tie, and Seok-Hyeon is furious. He suddenly gets a nosebleed, showing symptoms of turning into a monster. Thus, Hyun-soo is allowed to vote, and have himself isolated along with Seok-Hyeon. Sang-wook catches up with Yoon-Jae and beats him again, leading Yoon-Jae to seek refuge with the isolated ones. Jae-Heon prevents Sang-wook from beating him even further. Meanwhile, back upstairs, Du-Sik discovers that Myung-Sook has turned into a monster, a fetus-like shape which heart still beats and hums a song. Du-Sik makes a promise that should she turn into a monster, he will kill her – but he can’t do it.

Episodes 05

Eun-hyuk comes to Hyun-Soo and asks him to retrieve Du-Sik upstairs, and says the priority is the old man rather than the children. Yi-kung dislikes the plan of using Hyun-Soo to do dangerous tasks, although Eun-hyuk has his reasons, as well as showing her something that sparks her memory with her fiancé. Sang-wook has his own agenda for going upstairs, having secretly taken Yoon-Jae’s key. In a burning hallway, Sang-Wook is suddenly snatched by the “Reach” monster, while Hyun-Soo is attacked by the “Speed” monster. Luckily, the monster is scared by An Gil-Seob and Park Yu-Ri. They discover that Sang-Wook is only knocked unconscious. As they retrieve him, through the diary beside the dead monster, it is revealed that the guy lost his son because he failed to save him, and that he can’t let go of the guilt so that he succumbs to the monster. As Sang-Wook recovers, he continues going upstairs and reaches the locked room. Inside, he discovers the room to be a red room, finding photos of abused children – indicating that Yoon-Jae is a pedophile. Confronting Yoon-Jae downstairs, Sang-Wook goes all out on him and finally beats him to death. He takes his body outside and comes back with the body of Su-Ung and Jin-Ok’s daughter. Although he wants to isolate himself outside, Jae-Heon takes him back inside. The survivors build a funeral for the dead residents. Yi-Kyung discovers that she is pregnant. Hyun-Soo keeps on doing dangerous tasks for Eun-Hyeok.

Episodes 06

Through flashbacks, it is revealed that Sang-Wook’s father was caught in a fire accident, in which Sang-Wook barely survives with burn marks. He murders Ki-Hyuk, the arsonist convicted for the fire, who was given a lenient sentence. Years later, he was confronted by a father who begs him to rescue his daughter, in which he brought personal justice to Yoon-Jae in the present time. The survivors conduct a trial run in case of a monster attack, and they establish rules around the building to prevent any more casualties. Later, they all witness Seok-Hyeon has finally turned into a monster. His wife Seon-Yeon lands the killing blow to end his life. Eun-Yu starts to have feelings for Hyun-Soo, in which she makes a promise to him. Hyun-Soo is tasked to try find a functional car so they can use it to drive outside the building. But, after waiting for minutes, the others try to rescue him. Eager to find out more about the situation, Yi-Kyung decides to leave for her fiancé’s office to look for answers. She discovers that the government has been experimenting with people turning into monsters. Before she is about to leave, she is captured by the military.

Episodes 07

While being interrogated by the military, Yi-Kyung ends up revealing about Hyun-Soo. Hearing about this, the military decides to let her go, on condition that she will report anything about Hyun-Soo and leaves her with a tracker that will call the military to her location. The team tries to rescue Hyun-soo from the basement and then arrives in time to save Mr. Han from the spider monster. Ji-soo is discovered to have appendicitis and needs surgery so he dispatches the team heads out to get surgery tools on top of food. It is also revealed that Hyun-Soo was bullied in school. Hyun-Soo was trying to be a decent person, but things were not as he expected. Having grown fed up with everything, he tried to commit suicide by cutting his arm with a penknife. As soon as the dispatch team gets outside, their vehicle gets hit and turns upside down. The “Protein” monster is responsible for the attack.

Episodes 08

Following an attack that leaves Ji-soo heavily injured, Eun-hyuk performs surgery, even though he’s inexperienced at the job. Outside, Gil-seob and Hyun-Soo are cornered by the “Protein” monster. Gil-seob prepares to sacrifice himself to save Hyun-Soo, but Yi-Kyung suddenly comes with a firetruck and rams the “Protein” monster into the wall, and later pushes it down the cliff for it to be impaled by metal rebars. Inside, the surgery proves to be a success, and Ji-soo wakes up and has a heart to heart with Jae-Heon, him promising to teach her how to use his sword. Suddenly, the survivors discover someone who appears to be working with the military. With his walkie-talkie transmitting signals before it is destroyed, Yi-Kyung orders for the help sign to be dropped down. After taking down the sign, Hyun-Soo and Sang-Wook spot the infected security guard carrying a grass cutter with bugs swarming his entire face. Jae-Heon loses one of his arms in the process of keeping the Security Guard Monster occupied while the others are too fearful of fighting it. Jae-Heon then sacrifices his own life to have Eun-hyuk throw a molotov cocktail, killing him along with the monster, much to everyone’s shock and horror.

Episodes 09

Ji-Su is devastated to learn that Jae-Heon has died, and Seon-Yeon has shown signs of turning into a monster. The group is attacked by a group of outlaws, but luckily Yi-Kyung and Sang-Wook stay hidden from them. The mercenaries torture the survivors, gathering them in a boundary circle, and shoot them if they cross the line. They tell the story of how they encountered a chapel full of dead bodies, as a result of the people in there killing each other, showing how humans can be scarier than monsters. Seon-Yeong appears to have already succumbed to be a monster, and Hyun-Soo is forced to kill her. Through enough preparation, Yi-Kyung and Sang-Wook ambush the mercenaries, leading the survivors to overpower the latter. Yu-Ri is terrified at her killing a person in self-defense. Du-sik starts to show signs of nosebleeds, indicating that he is infected. On the rooftop, the team leader is about to kill Hyun-Soo, when one of the team members suddenly stabs and throws him off the building. He then murders the rest of the team, sparing only one. He introduces himself as Ui-myeong, revealing himself to be a monster just like Hyun-Soo. He reveals that he is the sole survivor from the church, and that joining the mercenaries was to disguise himself. Soon, the military sends pamphlets that ask the survivors to report any infected so that they will be granted safety. Ui-myeong asks Hyun-Soo which side he will be on, once the survivors learn of the information on the pamphlet.

Episodes 10

As the elevator opens, Hyun-Soo introduces Ui-myeong to the rest of the survivors and states that he is just like himself, which causes the group to stand down. Upon burning and killing the goo monster, Ui-myeong believes humans and monsters cannot coexist. When Sang-wook attempts to take Yu-ri outside after finding her having an asthma attack, he is stopped by Ui-myeong, who won’t let them go knowing that they will be eliminated by the military during Operation Golden Hour, which would eliminate everyone and everything. Hyun-Soo allows Sang-wook to leave but this causes Ui-myeong to open fire and kill two survivors, it is only after Hyun-Soo turns into a monster that he is stopped. Du-Sik sacrifices himself to calm Hyun-Soo down. In the other room, Gil-seob peacefully dies in his sleep and is buried. However, it reveals a secret tunnel. After the bodies are buried, winter arrives, as well as the military who say the survivors will be transferred to a safety camp as well as the infected should surrender. Hyun-Soo goes to confront the military while the apartment building starts to collapse as a result of a surviving outlaw causing an explosion and Eun-hyeok is presumably killed during the collapse just as his nose starts to bleed. The survivors make it above ground after entering the tunnels and are transferred to a safety camp. Yi-Kyung decides to join the military, and advises the survivors to stay alive. Hyun-Soo wakes up in an armored vehicle after confronting the military and is greeted by Sang-wook.