The totalitarian Japanese government signs the BR Act into law which makes it easier for them to reign in what they see as out-of-control Japanese youth. Although he is not one of these members of society, the movie's protagonist, Shuya Nanahara, has quite a few friends who do fall foul of authorities, including his best friend, Yoshitoki Kuninobu, who wounds their class teacher Kitano, forcing her resignation.
A year later their class is taken to a remote island under the pretense of being taken on a field trip. They learn that they are to be participants in the annual Battle Royale, a three day fight to the death until the emergence of a victor - the last person standing, it you will. Any students who are considering play acting at the battle will be killed as malingering will be punished by the remote detonation of an explosive collar that every participant is forced to wear. The rules of the game are explained by their old teacher, Kitano, who demonstrates how the collars work by detonating a couple, killing a few of the students, Yoshitoki being one of them. Each student is given some basics to enable them to be competitive; a compass, map of the island, basic food and water rations and a weapon of some description allocated to them at random.
There are four suicides in the first three hours of the game, and eight deaths caused by two participants, Mitsuko Souma and Kazuo Kiriyama. Shuya kills a student by accident and is stricken with remorse. Meanwhile, a classmate, Shinji Mimura, begins to think outside the box and decides to try to hack into the military's computer system and disrupt, perhaps even stop, the game.
Shuya decides to become the protector of Yoshitoki's girlfriend, Noriko, in honor of his friend. He takes her to a clinic accompanied by a transfer student named Kawada, who had let Shuya escape from his clutches earlier in the day. He reveals that he had won a Battle Royale whilst he was attending another school but had to kill his girlfriend in order to do so. He is participating in this Battle to avenge her murder.
Shuya is injured and passes out, awakening later to find himself at the lighthouse on the island's coast. He is being bandaged by his class representative, Yukie Utsumi, who is secretly attracted to him. She and five girlfriends have been hiding in the lighthouse since the start of the game. A near-fatal poisoning results in a shoot out amongst the girls, and only Yuko, whom Shuya likes, is left. She is unable to deal with the guilt of what she has done and commits suicide. Shuya finds Kawada and Noriko, and the three set off to find Mimura and to see if he has managed to hack into the system and derail the battle.
With ten players remaining, participants start falling fast. It transpires that Noriko is the last woman standing after Mitsuko is killed with her own weapon. Mimura has enlisted the assistance of two other students, Yutaka Seto and Keita Iijima, but another student who is not working with them kills him. Before he dies Mimura manages to diffuse a home-made bomb that he has placed near the computer he is using so that the evidence of the hacking will be gone. Kawada realizes that the collars have internal microphones and so seems to kill Shuya and Noriko, which ends the game, but Kitano realizes that Kawada had hacked the system and disabled the tracking devices placed on Shuya and Noriko enabling them to get away along with Kiwada, on a little boat. Kiwada later dies from the injuries he sustained in the blast, leaving Noriko and Shuya to flee together.
