Battle Royale takes place in an alternate time-line - the government of Japan is a police state, known as the Republic of Greater East Asia (大東亜共和国 Dai Tōa Kyōwakoku). Under the guise of a "study trip," a group of students from Shiroiwa Junior High School (城岩中学校 Shiroiwa Chūgakkō) in the fictional town of Shiroiwa (Kagawa Prefecture) are sleep-gassed on a bus. They awaken in the Okishima Island School on Okishima, an isolated, evacuated island south-west of Shodoshima, also in Kagawa Prefecture. They learn that they have been placed in an event called The Program, also known as Battle Royale. Officially a military research project, The Program is a means of terrorising the citizens, of causing such paranoia as to make organised insurgency impossible. According to the rules, every year since 1947, fifty third-year junior high school (fourteen to fifteen years old) classes are isolated, and the students required to fight to the death until one remains. Their movements are restricted by metal collars, later identified as Model Guadalcanal No. 22, around their necks which contain tracking and listening devices; if any student should attempt to escape The Program, or enter declared "danger zones", a bomb will be detonated in the collar, killing the wearer. If no student dies in any twenty-four-hour period, all collars will be detonated simultaneously.
After being briefed about The Program, the students are issued survival packs which include a map, compass, flash-light, food and water, and a random weapon or other item, which may be any thing from a gun to a paper fan. During the briefing, two students anger the supervisor, Kinpatsu Sakamochi, who kills both. As the students are released onto the island, they each react differently to their predicament; delinquent Mitsuko Souma murders those who stand in her way using deceiving tactics, Hiroki Sugimura attempts to find his best friend and his secret love, and Shinji Mimura makes a failed attempt to escape the Program.
In the end, four students remain: protagonist Shuya Nanahara, Noriko Nakagawa, Shogo Kawada - a survivor of a previous instance of the Program - and antagonist Kazuo Kiriyama. Following a car chase and shoot-out between Kazuo and the main characters, Noriko kills Kazuo by shooting him with a revolver. Shogo then takes his two partners to a hill. After telling Shuya and Noriko that he will kill them, Shogo shoots in the air twice, faking their deaths for the microphones planted on the collars. He then dismantles the collars. When Shogo is on the winner's ship, Shuya and Noriko board the ship. On the ship, Shogo kills Sakamochi and a soldier while Shuya kills the other soldiers on board. Shogo tells Shuya how to escape, succumbs to his wounds and dies. The two remaining students return to the main-land and find a clinic belonging to a friend of Shogo's father. From there, they make plans to escape to the U.S., facing an uncertain future as they run from the authorities.