Battle Royale takes place in a fictional fascist Japan in the year 1997. The state, known as the Republic of Greater East Asia (大東亜共和国 Dai Tōa Kyōwakoku), arose after an alternate World War 2, after a rebellion was put down by the combined military and police forces. The government controls everything, and anything "immoral", such as rock music, is banned, unless it beautifies the government, along with an unnamed dictator with a strong cult of personality able to bend the whims of the populace.
The government has established a military program, the Battle Experiment No. 68 Program (戦闘実験第六十八番プログラム Sentō Jikken Dai Rokujyuu Hachi Ban Puroguramu), wherein fifty randomly selected classes of third-year junior high school students are kidnapped, dropped into a remote location, and forced to kill one another until only one student of each class remains. Ostensibly, it is to help the government and its military research survival skills and battle readiness – in actuality, it is meant to instill terror and distrust in all of Japan's citizens to curb any attempts at rebellion, by showcasing the government's power and ability to target citizen's families and preying on the fear of being killed by a friend.
A group of students from Shiroiwa Junior High School (城岩中学校 Shiroiwa Chūgakkō), a junior high school in the fictional Kagawa Prefecture town of Shiroiwa, prepare for a field trip – among them are wannabe rock star Shuya Nanahara, whose father was killed by the regime, Noriko Nakagawa, the demure crush of Shuya's best friend, Shogo Kawada, a quiet, tough young transfer student, and psychopathic prodigy Kazuo Kiriyama. En route, they are gassed – the "field trip" was a ruse for the Program.
They awake in a classroom in a small, vacated island, surrounded by troops, and wearing metal collars around their necks. A teacher, psychopathic sadist Kinpatsu Sakamochi, briefs the students: the class has been chosen to participate in the Program. They have three days to kill each other until one stands as the victor, or the metal collars will detonate, killing them all (as Sakamochi gleefully demonstrates on one student). The students are issued survival packs and a random weapon/tool, and sent out onto the island one by one. While most of the students receive guns and knives, some acquire relatively useless items like boomerangs, dartboard darts, or a fork. Hiroki Sugimura finds a radar device that tracks nearby students, and Toshinori Oda receives a bulletproof vest.
To make sure the students obey the rules and kill each other, the metal collars around their necks track their positions and will explode if they attempt to remove the collars, or linger in "Forbidden Zones"; randomly chosen areas of the map that increase in number over time, re-sculpting and shrinking the battlefield and forcing the students to move around. The collars secretly transmit sound back to the organizers of the game, allowing them to hear the students' conversations, root out escape plans, and log their activities. The collars will explode if the students go a full day without anyone dying.
The students desperately fight amongst each other for survival, with mentally-ill bully Mitsuko Souma and Kiriyama killing many. Shuya takes Noriko under his wing after his best friend is killed, believing that he has a duty to honor his fallen friend by protecting his crush. Shogo – who was in a previous Battle Royale and hopes to put an end to the Program – avoids the fighting, joining with Shuya. Shuya's friend, athlete Shinji Mimura, attempts to hack the system running the Program and bomb the building where Sakamochi and the other personnel overseeing the Program are stationed, but is killed by Kiriyama.
Eventually, halfway through the third day, only Shogo, Shuya, Noriko, and Kiriyama remain, with Kiriyama dead set on hunting down the trio. After a frantic car chase, Kiriyama is finally gunned down – but Nanahara and Nakagawa are held at gunpoint by Shogo, who taunts them over being so naive as to trust anyone in the Program. The collars record gunshots and Shuya and Noriko flatlining.
Declared the winner by Sakamochi, Shogo is escorted to his transport off the island, surrounded by soldiers. Sakamochi, however, reveals that he knows Nanahara and Nakagawa are alive and that his supposed execution of Noriko and Shuya was a ruse after he found a way to disable their collars, and attempts to kill Shogo. Shogo kills him as a hidden Nanahara and Nakagawa hijack the ship and kill the soldiers on board. As the boat sails towards the mainland, Shogo succumbs to his wounds sustained during the hijacking, and dies, but not before thanking Shuya and Noriko for being his friends.
On the advice of Shogo, Shuya and Noriko escape to the mainland and plan to escape to a democratic America, pursued by the government.
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