Crows: Episode 0 is set in an all boys Japanese high school, where instead of having educational classes and courses, what gets put on screen (I haven't read the manga obviously) happens to be an ecosystem of triad society split into different turfs according to grade levels, classes and reputation. Unification of all levels in the school is a challenge, and new boy Genji Takaya (Shun Oguri) throws down the gauntlet on the first day to take down reigning school gangster Tamao Serizawa (Takayuki Yamada), which he finds impossible given that the latter's picked up by the police.
But of course there are unwritten rules to follow in order to engage the top, and he enlists the help of a two-bit average Yakuza hoodlum Ken Katagiri (Kyosuke Yabe) to help plot his path of success. For the most parts, the story is simple to follow, as we shadow Genji in his quest to conquer the high school class by class, through sheer brute force, gaining of respect, or simply just friendship established. As his reputation grows, so does his threat towards Serizawa, which sets up the inevitable climatic showdown where the rival gangs gotta settle who's gonna rule the school. As the saying goes, one mountain cannot hide 2 tigers.