Following a recession, a totalitarian Japanese government passes an act to curb juvenile delinquency, sending a random junior high school class to participate in a game called Battle Royale. As junior high schooler Shuya Nanahara struggles to process his father's suicide, his friend Yoshitoki Kuninobu stabs their teacher Kitano, who subsequently resigns. Shuya's classmate Noriko Nakagawa secretly keeps Kuninobu's knife.
One year later, Shuya's class awaken on a remote island. Kitano explains that they were chosen to participate in the annual Battle Royale; they have three days to fight to the death until a victor emerges. Explosive collars will kill uncooperative students or those within prohibited areas. Each student is provided rations, a map, supplies, and a random weapon. Kitano then kills two of the students, including Kuninobu. Twelve students die within the first six hours, four by suicide. Mysterious, abused loner Mitsuko Souma and psychopathic high schooler Kazuo Kiriyama become the most ruthless players. Transfer student Shogo Kawada spares Shuya after killing one student, while Shuya accidentally kills another. Basketball player Shinji Mimura recruits others to help him hack into the computer system to disrupt the program.
Amid shifting loyalties and violent confrontations, Shuya promises to keep Noriko safe, as Kuninobu had feelings for her. They are eventually rescued from Mitsuko by Kawada, who reveals that he won a previous Battle Royale at the cost of his girlfriend, who sacrificed herself when they were the final two remaining. He volunteered for the current Battle Royale game to avenge her by winning and killing those in charge. Kiriyama eventually attacks and wounds Shuya with his Uzi but is saved by Sugimura.
Shuya awakens in the island's lighthouse, bandaged by Yukie Utsumi. Five other girls are hiding there, having made a pact to not participate in the game. Yuko Sakaki, having witnessed Shuya accidentally kill Tatsumichi Oki, attempts to poison his food. However, another girl, Yuka, eats it, leading to a confrontation between the group which results in a shootout. All but Yuko are killed; guilt-ridden, she jumps to her death. Shuya reunites with Noriko and Kawada and the trio set out to find Mimura. Later, Sugimura is killed by Kotohiri and professes his love for her before he dies. Mitsuko kills her and then engages in a brawl with Kiriyama before she is killed.
Mimura and two others successfully infiltrate the computer system, which Kitano manually resets. Kiriyama arrives and kills them, but Mimura uses his homemade bomb to blow up the base, injuring and blinding Kiriyama. When the trio arrives at the base, Kawada engages in a shootout with Kiriyama. Kawada is injured by Kiriyama's Uzi, while Kawada manages to detonate Kiriyama's collar.
On the final day, the trio awakens on the shore. Kawada, aware of the collars' internal microphones, feigns killing Shuya and Noriko. Believing Kawada has won, Kitano brings him into the base, but realizes that Kawada hacked the system months beforehand and had disabled Shuya and Noriko's tracking devices. Shuya and Noriko enter the control room and Kitano unveils a painting of the massacred class, depicting Noriko as the victor. He explains that he was unable to bear the hostility from his students, having been rejected by his own daughter, and that, because Noriko never disrespected him, he views her as a daughter. He asks her to kill him, but Shuya shoots him after he threatens them. After a final phone conversation with his daughter, Kitano succumbs to his wounds. The trio leaves the island on a boat, but Kawada dies from his injuries. Some time later, Noriko gives Shuya the knife that Kuninobu had used to injure Kitano. Shuya and Noriko are declared fugitives by the Japanese government, last seen on the run toward Shibuya Station.
In an epilogue, Shuya dreams of Kuninobu, who tells him to move on with his life and that everything will be okay. Noriko reflects on the time she met Kitano for ice cream after the knife attack. When she tells him she has Kuninobu's knife in her possession, Kitano responds, "In this moment, what should an adult say to a kid?"
