Based on the childrens' book Little Polar Bear by Dutch author Hans de Beer.

Ninja... Ninja were... merciless demi-gods, ruling Japan with their karate in the age of ninja tranquillity. But, "some" committed a forbidden form of hara-kiri storing their souls at Kinkaku Temple for future resurrection. Their lost history was falsified and concealed and the truth about these ninja was long forgotten. Now, in the future where the universalization of cybernetic technology and electronic networks are God, suddenly, sinister ninja souls, resurrected from thousands of years past are unleashed on the dark shadows of Neo-Saitama. Fujikido Kenji, is a salaryman whose wife and child were killed in a ninja turf war. In a brush with his own death, Fujikido is possessed by an enigmatic ninja soul. Fujikido cheats death and becomes Ninja Slayer. A Grim Reaper destined to kill evil ninja, committed to a personal war of vengeance. Set in the dystopian underworld of Neo-Saitama, Ninja Slayer takes on Soukai Syndicate ninjas in mortal combat.

In the past, humanoid bears coexisted with humans. However, a meteor shower that fell onto Earth had a strange effect on bears throughout the world: they suddenly became violent and hungry for human flesh, spurring an endless cycle of bloodshed in which bear ate man and man shot bear, forgetting the lively relationship they once had. The "Wall of Severance" was thus built, separating the two civilizations and keeping peace. Kureha Tsubaki and Sumika Izumino are two lovers attending Arashigaoka Academy, who, upon the arrival of two bears that have sneaked through the Wall of Severance and infiltrated the academy, find their relationship under a grave threat. The hungering yet affectionate bears, Ginko Yurishiro and Lulu Yurigasaki, seem to see the bear-hating Kureha as more than just another meal, and in getting closer to her, trigger an unraveling of secrets that Kureha may not be able to bear. When their relationships provoke the Invisible Storm, a group that keeps order within the ideological school, the girls must stand on trial with their love, embarking on a journey of self-discovery en route to attaining true love's "promised kiss."

Hoichiro Ohma works in an office. Everybody knows him as a silent, humble man. Even his girlfriend, Nanase, doesn't suspect that he could be something more, but he is. When a person dies as a victim of murder; when someone kills himself with a curse on his lips; when someone's death needs to be judged, he is there, for he is a Judge Of Darkness. Following the Laws Of Darkness, with a book made of human skin and an unusual parrot, he pronounces judgement over living criminals that would otherwise go free.