This is an extremely graphic manga, containing frequent scenes of violence, murder, and death. There is also a lot of nudity, 'fanservice' and explicit sex, supposedly of 9th grade students.
The plot is easy to summarize. In a dystopian near-future Japan, a fascist government has created a gladatorial comabt in which a 9th grade class is abducted each year and forced to kill each other until there is only one survvivor. The students are given different weapons, ranging from Uzis to forks and boomerangs. The manga follows the 42 students of one class as they battle each other. The flavor is of a teen slasher horror movie where Freddie and Jason are your classmates.
Chapter arcs alternate between scenes of combat of the students trapped on the island arena and flashbacks to the normal life they led before "the Program". Several protagonist groups develop of students trying to avoid "playing the game", in some cases trying to actively destroy the Program itself. Several other major characters are students that embrace the amorality of the Program, and seek to win by killing off weaker classmates.
Still, even with three major good groups, two major antagonists, oodles of supporting characters, and flashbacks of everyone, the manga is padded with very repetitive philosophical conversations. The major group of protagonists does basically nothing but sit and talk until practically everyone else is killed. As in operas where characters take fifteen minutes to sing a final aria while mortally wounded, characters continue to ponder their existence and reminisce for pages and pages after their brains have been blown out.
And blown out they have been, in lyrical, explicit detail. The art is exceptionally graphic, going to pornographic at times, in the level of detail lavished on violence and harm to the body. At the same time, the sex scenes continue to dance around the idiosyncratic rules of Japan on the display of the genitals. (You'll also see a strong resemblance between one of the main characters and the style of the great Osamu Tezuka.) But the art is really focused on a fetish for bullets leaving gun barrels, brains, blood, and eyeballs not right in their sockets. Most characters display an excess of bodily fluids in the form of tears, mucus, blood, semen, and the ubiquitous "sweat drop".
Most characters are one dimensional, and wind up as cannon fodder to the two most evil and sociopathic of the students. The fate of some other students is study in paranoia. The male Big Bad has a pile of unfair advantages (genius, can learn any physical skill if he sees it once, etc.) on top of being a sociopath and drawing an Uzi. He is, of course, supernaturally hard to kill, and is drawn with a cape that hovers around his body no matter what fantastic martial arts moves he is pulling.
After lavishing over 100 chapters of attention to violence, the manga ends on a note of "love and hope and faith in human nature can conquer all" which even the characters realize is unrealistic. I am reminded of the moral ending tacked onto "Golden Lotus", the great erotic novel of China.
In the language of tvtropes.com (a great site), Battle Royale is high octane nightmare fuel from beginning to end. If you liked 1984, Lord of the Flies or A Clockwork Orange, you'll find the descent into paranoia, animalism, and ultraviolence familiar. It is up to each reader to decide if the author and illustrator are indulging themselves or using a legitimate device to explore the human condition. I vote for indulgence.