Set in a future-world vision of Tokyo where the police have been privatized and bitter self-mutilation is so casual that advertising is often specially geared to the "cutter" demographic, this is the story of samurai-sword-wielding Ruka and her mission to avenge her father's assassination. Ruka is a cop from a squad whose mission is to destroy homicidal mutant humans known as "engineers" possessing the ability to transform any injury to a weapon in and of itself.