Its about the country of Japan collapsing and the government imposing the Battle Royale Act because the kids are all rebelling...
The law basically states that every year, one random class will be chosen to battle it out until only one survives...
They get kidnapped and thrown into a remote island, given a random weapon (which ranges from excellent to practical to silly), and are given a time limit to survive and kill everyone else...
Ultra violent and hilarious, at times...