After the death of his parents and sister, the introverted high school student Cha Hyun-soo is left all alone, and he has to move into a small apartment in the run-down Green Home apartment complex. With no reason to continue living, the young man decides to take his own life, but suddenly, some of the residents begin to transform into aggressive monsters. Hyun-soo must overcome his fear and get to know his new neighbors in order to confront the unexpected threat.