This is a recap of the novel version of Battle Royale. Note that while the novel, manga, and film all follow the same basic plot, elements between the stories may differ between each version.
The year is 1997, and the Greater East Asian Republic, a fascist police state based in an alternate-history Japan following World War II, is dealing with a problem concerning delinquency and student protests. In order to curb rising resentment by their youth, the government has instituted the "Battle Experiment No.68 Program": on the surface, a military research program into survival tactics and combat preparedness; but in truth, a means to sow distrust among the citizenry to curtail any possible thoughts of rebelling...
So it is that a group of students from Shiroiwa Junior High School, including aspiring rock star Shuya Nanahara and his friend Noriko Nakagawa, are on their way to a field trip...or so they believe. Unfortunately for them, the bus they are riding on is filled with knockout gas. When the students awaken, they find themselves in a classroom of an old school on a remote island, with metallic collars placed on their necks. They are approached by Kinpatsu Sakamochi, a government-sanctioned teacher who explains that their class was selected for "The Program": a contest in which the students must kill one another until only one remains. Each student is given a duffel bag with three days worth of food and water, some supplies, and a randomly-chosen weapon with which to accomplish this goal. Their compliance would be mandated by the collars, which contain a small yet powerful explosive charge; any attempt to escape or rebel, as well as remaining in areas that are randomly chosen as out-of-bounds, would detonate the charge and blow the student's head off. Additionally, if more than one student was still alive 72 hours after the contest began, all of the students would be killed.
Once the contest begins, Shuya and Noriko team up to survive until the end of the game. Shuya's motivations are spurred on by his hatred for the government, both for killing his parents for suspected dissension when he was a child, and for killing his best friend, Yoshitoki Kuninobu, during the pre-game briefing. Kuninobu had previously confided in Shuya that he had feelings for Noriko; thus, Shuya decides to honor his friend's memory by protecting her.
Other students go about the game in their own ways. Some opt to commit suicide rather than play. Others team up in the hopes of surviving to the end, or even fighting back against Sakamochi. Many decide they will play to win, hunting down their fellow students; some out of fear of their own mortality, but two in particular out of a desire to kill. One is Kazuo Kiriyama, a transfer student who is mentally unbalanced and takes to killing as well as he had painting or playing music in the past. Another is Mitsuko Souma, a victim of sexual abuse whose mind had been twisted by the horrors she endured, leaving her a viscous and murderous sociopath.
During their struggle to survive, Shuya and Noriko make an unlikely ally in Shogo Kawada, another transfer student who also survived a previous experience in "The Program". Kawada tells them that there may be a way out for all three of them, but because the collars also act as recording devices, he cannot tell them until they are the only three left, which means surviving until the bitter end.
Over the next few days, the students die one by one. After surviving an encounter with, and outliving, Kiriyama, Shuya, Noriko, Kawada are the last three students remaining. However, Kawada claims he only cooperated with them to ensure he would win as he had last time.
There are two gunshots, and the collars on Shuya and Noriko show no life signs. Kawada has won "The Program".
As is stipulated in the rules, victors of "The Program" are to receive a large cash prize and other awards, in adtion to safe passage off of the island. Sakamochi, however, knows that Kawada had found a way to disable the collars and plans to kill him. Kawada manages to kill Sakamochi before he, Shuya, and Noriko hijack a boat off of the island. In the escape, however, Kawada is mortally wounded; but he dies happy in the company of Shuya and Noriko, the two best friends he ever had, as they prepare to escape to America.