Hiromi Katayama, 17, leads the Yokohama biker gang Crazy Babies and commands respect throughout the Kanagawa alliance for his formidable combat skills and strong moral principles. He values integrity and champions peacekeeping within Yokohama's gang environment. To achieve this, he imposes a one-month moratorium on violence, focusing instead on training new recruits and fostering better community relations. This initiative is partly driven by his desire to improve his relationship with classmate Asami, who shares his passion for motorcycling.

Hiromi's leadership faces immediate challenge when new recruit Toshi Ikeda violates the probationary rule against fighting. Toshi's actions inadvertently ignite conflict with the Nagoya-based Dragon Star Crew, a group aggressively seeking to expand its influence into the Kanto region and threatening Yokohama's stability. Simultaneously, Hiromi contends with rivalry from the Tokyo-based Mad Emperor biker gang, whose leader actively opposes Hiromi's non-violence stance and seeks to undermine his authority. The escalating actions of both the Dragon Star Crew and Mad Emperor disrupt the peace Hiromi strives to maintain, forcing him into a defensive stance to protect his gang and his city.

Hiromi's background includes uniting disparate Yokohama biker factions at significant personal risk, consolidating them under the Crazy Babies banner to reduce chaos and impose order. His deep commitment to this role intertwines with his personal life, particularly his protective and caring dynamic toward Asami, though their relationship remains secondary to his gang responsibilities. Throughout these events, his development centers on navigating the complex balance between his moral ideals and the pragmatic demands of gang leadership amid relentless external threats.