Tribe Nine takes place in a dystopian future ("The Year 20XX") where disorganized youths form their own Tribes that soon turn violent and run rampant across the city of Neo-Tokyo. To contain the violence, the government of Neo-Tokyo implements the "XB Law", stating that all future conflicts between Tribes will be settled through games of "Extreme Baseball", overseen by robotic umpires. The games are similar to baseball except there are fewer restrictions on equipment, the "field" can be as big as an entire ward, and the only ways to get an Out require striking out the batter or knocking out a baserunner while holding the ball. Games can also end if the opposing pitcher is rendered unconscious during the game.