Synopsis One night at his apartment, Sang-Hoon (Lee Sung-Min) hears a woman’s scream. He looks outside his apartment and sees Tae-Ho (Kwak Si-Yang) hitting a woman with a hammer. Sang-Hoon and Tae-Ho then make direct eye contact. Sang-Hoon does not call the police. The next day, that woman is found dead. Detective Jae-Yeob (Kim Sang-Ho) investigates the case. Sang-Hoon is still terrified and does not tell Detective Jae-Yeob about what he witnessed. Soon, another resident who witnessed the same murder is killed by Tae-Ho. Sang-Hoon tries to protect himself and his family from the murderer.