In a bustling Hong Kong fish market, a disturbed man named Doggie, Tony Leung, creates a tense standoff with the police, swinging a cleaver as the crowd watches. Mr. Tsui, Stanley Fung Shui-Fan, a compassionate psychiatrist who donates his time to help the mentally ill, arrives and calms the chaos enough for Doggie to be taken into custody. Miss Lau, Tina Lau, Deanie Yip Tak-Han, a perceptive Hong Kong journalist, observes the incident and becomes fascinated by Tsui’s work with people living on the streets.
Together, Mr. Tsui and Miss Lau visit Tsuen, Paul Chun Pui, a patient who claims to be rehabilitated. Tsuen explains that Castle Peak, the mental health facility, is pleasant, but once released, patients can easily forget their medications in a new environment. He has been out for a year, living on his own, but his wife has left him and he is granted only one supervised visit with his young son per month. Tsuen asks Mr. Tsui to help him spend more time with his son, yet Tsui reminds him that such decisions rest with the court.
Chung, Chow Yun-Fat, a hard-edged chainsmoker who hoards cigarette butts, flees when he spots Tsui. Tsui follows Chung back to a rundown shack where Chung’s daughter has measles. They bring her to the hospital, and Tsui presses Chung about his son. Chung insists he did not kill his son and then leads Tsui and the police to a woodland site where his son is buried. A confrontation emerges with Ah Ming, a doctor who prescribed hepatitis medicine to Chung without realizing it was meant for his child; Ah Ming explains that he witnessed the birth of Chung’s children and has always cared for them.
Tsuen attempts to take Ah Hei, Cheung Hei, on an unscheduled trip to the store for his birthday, but his ex-wife interferes and chastises Ah Hei’s teacher, Miss Li, for allowing it. A clash with his ex-wife’s new husband follows, and she warns that an injunction will be filed to limit Tsuen’s visitation rights. Tsuen returns home with a bump on his head and begins talking to himself, even biting into a chicken the way his mother brings, until she finally agrees to take him to the hospital when she finds him crouching in the shower.
At the hospital, Tsuen puts on a convincing performance before the doctor and is released, but outside he disappears for hours until Mr. Tsui locates him. Miss Lau, aiming to raise public awareness, writes a story about Tsuen’s bizarre behavior—his habit of breaking the necks of chickens—in an attempt to push for his admission to the hospital. The plan backfires, however, as neighbors mob the building and confront Tsuen, angering his mother in the process.
Tsuen heads to his son’s school, locking the door behind him as the mob gathers outside. Miss Li, Season Ma, tries to open a path of escape, but Tsuen kills her with a cleaver. A policeman breaks through a window, and Tsuen is shot when he kills the officer. Mr. Tsui contemplates resignation, but his supervisor refuses to accept it, since Tsui is the last social worker still backing his caseload. He receives news that Doggie’s parents have called to say he intends to commit suicide, prompting Tsui to search the fish market once more.
There, a flash from a photographer’s bulb unsettles Doggie, and he swings his cleaver at Mr. Tsui, killing him. The funeral that follows draws together Tsui’s patients and acquaintances, and Miss Lau continues to dedicate herself to caring for the mentally ill on the streets of Hong Kong.
