Following a major recession, the Japanese government has passed the “BR ACT” to control Japan’s unruly youth. Middle school student Shuya Nanahara copes with life after his father committed suicide. Noriko Nakagawa is the only student regularly attending class 3-B. Their teacher, Kitano, resigns after being wounded by Yoshitoki Kuninobu, Shuya’s best friend.
One year later, class 3-B takes a field trip, but they are gassed and taken to a remote island. Kitano reappears surrounded by JSDF soldiers, explaining to the class that they are chosen to participate in the annual Battle Royale as a result of the Act: they have three days to fight to the death until a victor emerges, while explosive collars will kill uncooperative students or those within daily “danger zones”. Each student is provided rations, water, a map of the island, compass, flashlight, and a random weapon. Kitano personally kills two of the students for disobedience, one of them being Kuninobu, who dies from collar detonation.
The first six hours see twelve deaths, four by suicide, and eight caused by the desperate, psychotic Mitsuko Souma and psychopathic volunteer Kazuo Kiriyama. Transfer student Shogo Kawada lets Shuya go after killing one student, while Shuya accidentally kills a student, Oki, which is witnessed by Yuko Sakaki, while basketball player Shinji Mimura plots to hack into the military’s computer system to disrupt the program. Amid shifting loyalties and violent confrontations, Shuya promises to keep Noriko safe as Yoshitoki secretly loved her. He carries her to a clinic, where Kawada reveals that he won a previous Battle Royale at the cost of his girlfriend, Keiko Onuki, whose death he seeks to avenge. When Kiriyama attacks, Shuya entrusts Kawada to protect Noriko and runs as a distraction. Shuya is wounded by Kiriyama’s Uzi. However, he is saved by Hiroki Sugimura, a martial artist who has recently had his friend Takako Chigusa die in his arms, and is on a personal mission to find his unrequited love, Kayoko Kotohiki.
Shuya awakens in the island’s lighthouse, bandaged by female class representative Yukie Utsumi, who has a crush on him. Five other girls are also hiding in the building, including Yuko, who attempts to poison Shuya out of fear of Shuya possibly killing them like he did Tatsumichi Oki. However, Yuka accidentally eats the food, leading to a shootout between the girls. Yuko is the only survivor; horrified and realizing her mistake, she apologizes to Shuya and commits suicide. Shuya finds Noriko and Kawada, and they set out to find Mimura.
Now only ten players left, Hiroki is killed by Kotohiki, who is then killed by Mitsuko. Kiriyama kills Mitsuko with her own weapon, making Noriko the last surviving girl. Mimura and two others, Yutaka Seto and Keita Iijima, infiltrate the military’s computer system, but Kiriyama kills them, but not before Mimura uses his homemade bomb to explode the base to hide all evidence. When Kawada, Noriko and Shuya arrive at the hackers’ burning base, Kawada confronts and kills Kiriyama, who had his eyes burned out by the explosion, by detonating the collar with his shotgun, but in turn is seriously injured by Kiriyama’s Uzi.
On the morning of the final day, Kawada, aware of the collars’ internal microphones, seemingly kills Shuya and Noriko by shooting them. Suspicious, Kitano ends the game and dismisses the troops, intent on personally killing the supposed victor. Kitano realizes that Kawada hacked the system months beforehand, and has disabled Shuya and Noriko’s tracking devices. The three survivors confront Kitano in the headquarters, and he unveils a homemade painting of the massacred class depicting Noriko as the sole survivor. He reveals that he was unable to bear the hatred between him and his students, having been rejected by his daughter, and confesses that he always thought of Noriko as a daughter. He asks her to kill him, but Shuya shoots him after he threatens her with a gun. As he falls, Kitano shoots, revealing the gun to be a water pistol. Kitano’s daughter calls him; after an argument, he shoots the phone with an actual gun before dying of his wounds.
Shuya, Noriko and Kawada leave the island on a boat, but Kawada dies from the injuries, happy that he found friendship. Shuya and Noriko are declared fugitive murderers, and are last seen on the run in the direction of Shibuya Station. Noriko gives Shuya the Seto Dragon Claw butterfly knife Kuninobu used to injure Kitano at the beginning of the film. They then run off together.