"Ninja Slayer Kills" is a manga series set in a dystopian future where cyber-enhanced ninjas, known as "Neo-Sōkai," dominate the criminal underworld. The story centers on Kenji Fujikido, a salaryman whose life is shattered when his family is killed by ninjas. After surviving the attack, Kenji becomes possessed by the spirit of a legendary ninja assassin, Naraku Ninja, transforming him into the vengeful Ninja Slayer. Driven by a singular goal to exterminate all ninjas, Kenji embarks on a relentless quest for justice, navigating a world where corporate greed, cybernetic enhancements, and ancient ninja traditions collide.

The narrative unfolds in a chaotic, neon-lit urban landscape where ninja clans, such as the Soukai Syndicate, wield immense power. These clans are often tied to corrupt corporations, using their ninja operatives to maintain control through fear and violence. Ninja Slayer, armed with the skills and ruthlessness of Naraku Ninja, systematically targets these clans, engaging in brutal battles that blend traditional martial arts with cyberpunk aesthetics. His journey is marked by encounters with various ninja adversaries, each with unique abilities and motivations, as well as occasional allies who aid his mission.

Key characters include Yamoto Koki, a former ninja who becomes a reluctant ally to Ninja Slayer, and Laomoto Khan, a powerful ninja leader with a personal vendetta against Naraku Ninja. The series explores themes of revenge, identity, and the moral ambiguity of violence, as Kenji struggles to reconcile his humanity with the destructive power of Naraku Ninja. The plot is punctuated by intense action sequences, philosophical musings on the nature of ninja honor, and the consequences of unchecked technological advancement.

"Ninja Slayer Kills" is characterized by its fast-paced storytelling, blending dark humor with graphic violence and a distinctive retro-futuristic aesthetic. The manga’s world-building emphasizes the clash between ancient ninja traditions and a hyper-modern, cybernetic society, creating a unique backdrop for its exploration of vengeance and justice.