May moves into a new apartment in Hong Kong with husband Ray and son Chi-lo. During the move in, Chi-lo and May see a hideously-deformed being. They told Ray what they had seen and he suggests moving out. Having already paid a lot for their new home, May decides that they should remain. One day, May takes Chi-lo to a neighbor's birthday party for fun. During this time, Chi-lo is kidnapped by the hideous being named Sharmaine, revealed to be an insane woman who mistakes the boy as her own son. After informing the police, May and Ray begin searching the complex. In the parking garage, Ray is stabbed by the deformed woman with a makeshift knife, critically wounding him and is later rushed to the hospital. With her neighbors refusing to help, May is forced to continue the search alone. While searching through the complex's air ducts and nearly being attacked by the deformed woman, she falls through a vent cover leading to the outside of the complex and is knocked out upon hitting the ground. A second attempt involves the help of a dog which quickly picks up the woman's scent, tracking her to a rooftop. The dog scares Chi-lo and is killed by the deformed woman but not before biting off one of her fingers. When May arrives, she ends up collecting the severed finger from the dog's mouth. May then brings the finger to the detective working on the case. Through fingerprint testing, they were able to identify the deformed woman as Chan Yim-hung, who had lived with her husband and son Wing-man, as well as Hong Kong's poorest residents, in the same area of the apartment complex years ago when it still a squatter area. When the city's government tried to reclaim the area, a riot ensures and Yim-hung's husband is killed in a subsequent explosion. Despite the squatters cleared out, Yim-hung returns there with Wing-man to continue their lives. After Wing-man is killed by falling debris while looking for food, Yim-hung leaves the area and would return sometime after the new apartment is completed, unaware that her son is dead and slowly becoming insane over what had occurred. May decides to lure out Yim-hung by printing copies of the latter's picture and throw them in the complex's air ducts. Yim-hung sees them and, attacks May in her apartment, ordering the latter to return her son. While May tries to tell her that her son died years ago, Chi-lo then arrives and Yim-hung takes him out of the apartment while fighting off May. Back in the complex's utility systems, Yim-hung sees a vision of her dead husband, begging her to let go of their "home" that no longer exists. Upon seeing a reflection of her deformed self in the glass door of an electrical grid, she smashes it, causing a blackout throughout the entire complex. Once May sees a cloth that belongs to Chi-lo floating down from the sky, she quickly realizes they are on the rooftop. After fighting through the police who were sent to investigate the blackout, she comes across them preparing to jump off. May quickly stops them and attacks Yim-hung for kidnapping and traumatizing Chi-lo. While May and her son embrace, Yim-hung jumps off the rooftops to her death, wanting to reunite with her family in the afterlife. As the screen goes black, Chi-lo can be heard asking, "Mom, would you abandon me?", as to when both May and Wan can be heard replying, "Of course not, not even if you abandon me."
