The film was a mainstream domestic blockbuster, becoming one of the ten highest-grossing films in Japan, and was released in 22 countries worldwide. It received global audience and critical acclaim and is often regarded as one of Japan's most famous films, as well as one of Fukasaku's best films. Fukasaku started working on a sequel, Battle Royale II: Requiem, but he died of prostate cancer on January 12, 2003 after shooting only one scene with Takeshi Kitano. His son, Kenta Fukasaku, completed the film in 2003 and dedicated it to his father.

Japanese middle school student Shuya Nanahara copes with life after his father's suicide. Meanwhile, schoolmate Noriko Nakagawa is the only student attending class 3-B. Their teacher, Kitano, resigns after being impulsively attacked by student Yoshitoki Kuninobu.

One year later, class 3-B takes a field trip, but they are gassed, fitted with electronic collars, and sent to a "briefing room" on a remote island. Kitano explains that the class has been chosen to participate in the annual Battle Royale as a result of the BR Act, which was passed after 800,000 students walked out of school. An oddly cheerful orientation video instructs the class they have three days to kill each other until only one remains. The explosive collars will kill any uncooperative students or those within daily "danger zones". Kitano kills a girl Fumiyo Fujiyoshi for whispering to her friends during the video. Kuninobu confronts Kitano and the teacher slashes him with a knife (in the back of leg – the same area Kuninobu attacked Kitano two years earlier) before detonating his collar. Each student is provided a bag of food and water, map of the island, compass, and a "weapon" ranging in efficiency from firearms to a paper fan.

The program's first six hours see twelve deaths, two by suicide. Brazen, mute transfer student Kazuo Kiriyama and hazardous classmate Mitsuko Souma soon become the most dangerous players, while another transfer student, Shogo Kawada, seems somewhat more merciful. Shuya promises Kuninobu to keep Noriko safe, because his friend Kuninobu secretly loved her. Other students have their own goals: Shinji Mimura and his friends plot to hack into the military's computer system and destroy their base; Hiroki Sugimura searches for his best friend Takako Chigusa and love interest Kayoko Kotohiki. Takako is approached by Kazushi Niida who has been in love with her for a long time. He offers her to team up with him which she refuses. Getting increasingly angry and violent, Kazushi attempts to force himself on her, but Takako manages to resist his attack and kills him with her provided jack knife. Mitsuko witnesses the entire scene and shoots Takako four times who gets fatally injured. Shuya carries Noriko to a clinic after she collapses, where they encounter Kawada, who reveals that he won a previous Battle Royale at the cost of his girlfriend, Keiko, whose death he seeks to avenge. When Kiriyama attacks Shuya entrusts Kawada to protect Noriko, and runs off as a distraction. He is saved by Sugimura, who is using a GPS tracking device to track the location of every student.

Shuya awakens bandaged by Yukie Utsumi, the female class representative who's been secretly in love with him, in the island's lighthouse, where she fills him in on the past 14 hours. Five other girls from her clique have also been hiding out in the building. Utsumi gathers the girls, but Yuko Sakaki sparks a bloody massacre by accidentally poisoning Yuka Nakagawa, and she is the only survivor. She apologises to Shuya before jumping from the top of the lighthouse. Shuya returns to Noriko and Kawada, and they set out to find Mimura's group.

Sugimura tracks Kotohiki to a small warehouse, but she panics and shoots him. Sugimura professes his love before dying. Kotohiki cries in despair, and is found and killed by Mitsuko. Watching from the rafters, Kiriyama then guns down and kills Mitsuko.

Of the seven students remaining, all except Kiriyama are attempting or willing to subvert the game. Mimura, Keita Iijima, and Yutaka Seto infiltrate the military's computer system as they prepare to destroy the perimeter using a truck rigged with explosives. However, they are found by Kiriyama, who kills them all, but Mimura manages to detonate the truck, blinding the killer. When Kawada, Noriko and Shuya arrive at the hackers' burning base, Kawada confronts and kills Kiriyama.

On the morning of the final day, Kawada, aware of the collars' internal microphones, takes Shuya and Noriko aside and fakes their deaths. Suspicious, Kitano ends the game and dismisses the troops, intent on personally killing the supposed victor. Kitano realizes that Kawada had hacked into the game's system months beforehand, and has now disabled Shuya and Noriko's tracking devices. The three survivors confront Kitano in the headquarters, and he unveils a disturbing homemade painting of the massacred class, depicting Noriko as sole survivor. He reveals that he was unable to bear the hatred between him and his students, having been rejected by his daughter, Shiori. He confesses that he always thought of Noriko as a daughter, and asks her to kill him, but is shot by Shuya after he threatens her with a gun. As he falls, Kitano shoots, revealing that it is a water-gun. Suddenly, his phone rings, and Kitano sits down to answer it, telling Shiori that "if you hate someone, you take the consequences" before shooting the phone with a real gun and finally dying. Shuya, Noriko and Kawada leave the island on a boat, but Kawada dies from injuries sustained in his gunfight with Kiriyama – "glad" that in the end, he "found true friends."

Shuya and Noriko are declared fugitive murderers, and are last seen on the run in the direction of Tokyo's Shibuya train station.