After barbarically slaughtering his parents, 16 year old Ryo is sent to a prison for boys. Continually tortured by his fellow inmates, Ryo takes up karate training after a savage beating leaves him almost dead. Returning to society after 3 years, Ryo catches the attention of the local triads for his natural ferocity and dedication to destruction.

Convinced to enter underground, no rules fighting tournaments, Ryo easily demolishes the competition. Seemingly unstoppable, Ryo's inevitable confrontation with champion Sugawara (Masato) may be his only chance for respect by the society that has so completely rejected him.

Based on the manga by Izo Hashimoto, Shamo tells the tale of juvenile delinquent Ryo Narushima (Shawn Yue), who's sentenced to a correctional institute after he snaps and murders his parents one fine spring morning. A thin, mousy boy, Ryo actually seems quite okay, but that doesn't matter to his new pals in the slammer. He's treated to some extreme physical affection in the men's room by his fellow inmates, turning him into a desperate, broken young man looking for an exit - even if it's self-induced. Help arrives in the form of Kenji Kurokawa (Francis Ng), a karate master who's assigned to instruct the inmates in the ways of extreme bodily harm. Teaching dangerous, sometimes deranged criminals to hone their bodies into lethal weapons?