With the introduction of strict new morality laws, Japan has become a nation cleansed of all that is obscene and impure. By monitoring citizens using special devices worn around their necks, authorities have taken extreme measures to ensure that society remains chaste. In this world of sexual suppression, Tanukichi Okuma—son of an infamous terrorist who opposed the chastity laws—has just entered high school, offering his help to the student council in order to get close to president Anna Nishikinomiya, his childhood friend and crush. Little does he know that the vice president Ayame Kajou has a secret identity: Blue Snow, a masked criminal dedicated to spreading lewd material amongst the sheltered public—and Tanukichi has caught the girl's interest due to his father's notoriety. Soon, Tanukichi is dragged into joining her organization called SOX, where he is forced to spread obscene propaganda, helping to launch an assault against the government's oppressive rule. With their school set as the first point of attack, Tanukichi will have to do the unthinkable when he realizes that their primary target is the person he admires most. [Written by MAL Rewrite]

In Tokyo's downtown district of Ikebukuro, amidst many strange rumors and warnings of anonymous gangs and dangerous occupants, one urban legend stands out above the rest—the existence of a headless "Black Rider" who is said to be seen driving a jet-black motorcycle through the city streets. Mikado Ryuugamine has always longed for the excitement of the city life, and an invitation from a childhood friend convinces him to move to Tokyo. Witnessing the Black Rider on his first day in the city, his wishes already seem to have been granted. But as supernatural events begin to occur, ordinary citizens like himself, along with Ikebukuro's most colorful inhabitants, are mixed up in the commotion breaking out in their city.

Unaired episode released with the seventh volume of the Blu-ray & DVD. It consists of three short episodes newly written by the manga author Fujiwara Cocoa. Fujiwara also designed a new character called Hana-chan for the episode.

The series is set in a near-future world where the boundary between human being and AI is vague. A rapid biological evolution, similar to "Cambrian Explosion," is caused by one incident. This incident turns into the genesis of a new jostle for supremacy, and the influence of the evolution goes beyond biological framework and spreads to various kinds of materials and technologies. The era of massive evolution involving diverse existences, such as human, machines, and biotechnologies, is coming. Characters fight an existential war in such a world. (Source: ANN)