One night at his apartment, Sang Hoon hears a woman's scream. He looks outside his apartment and sees Tae Ho 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 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.