Since ancient times, the ninja and the yakuza have been at war, spilling blood around the world as empires rise and fall. This centuries-old grudge is rekindled in the present day, with the ninja working as shadowy enforcers who banish evil, while the gokudo (yakuza) gather lonely outcasts to commit crimes. The story centers on two young men: Shinoha Tanaka, a ninja who has lost the ability to laugh, and Kiwami Kimura, a gangster following the path of the gokudo who hides his true identity behind the face of a calm office worker. When these two from opposite sides of a 300-year blood feud cross paths, they discover they share a surprising common interest, an obsessive love for a magical girl anime called Princess Flash, and an unexpected friendship begins to form.

The series is set in a violent, hyper-stylized version of modern Japan, where elite warriors from both factions possess superhuman abilities. Shinoha is a member of the Eight Imperial Ninja, a group of powerful enforcers who work for the government to eliminate evil. Kiwami belongs to the Eight Gokudo of Destruction, the yakuza's most formidable fighters, who have found a way to gain powers of their own. As the two men bond over their shared hobby, they remain completely unaware of each other's true allegiances, leading to a tense and ironic situation where their personal friendship grows even as the war between their organizations escalates into increasingly dangerous and bloody territory.

The narrative unfolds through several notable arcs. The story begins with their chance meeting, but the conflict quickly escalates in the Blood-Scented Flare of Akasaka arc, where Kiwami must maintain his composure when a fellow ninja launches a surprise attack on a gokudo meeting. This is followed by the Blazing Chivalry and Justice in Kabuchika arc, where Shinoha is forced into a deadly confrontation with a young, inexperienced gokudo member, testing his resolve. The war expands dramatically in the Great Burial of Love arc, a large-scale battle that sees a 30,000-strong biker gang led by the gokudo's bosozoku god, Hiroki Yajima, clash with Shinoha and his allies on the Teito Highway. The climax of the season is the Young Maniacs Death Game arc, where the conflict becomes personal when a powerful enemy, Gamute, storms the Prime Minister's residence and forces Shinoha and Kiwami into a deadly game, pushing their secret bond and their loyalties to a breaking point.