Long ago, demon-like creatures known as Oni butchered humans and brought on destruction solely for their own delight. A lone man named Momotarou opposed these gruesome, near-invincible monsters, but the war against them was just beginning. Thousands of years in the future, temperamental gun enthusiast Shiki Ichinose leads a carefree life. However, after his expulsion from school, everything changes. Shiki is suddenly attacked by a man and is rescued by his adoptive father, Tsuyoshi. As the two flee, Tsuyoshi reveals that Shiki is actually a descendant of the Oni, while both he and their pursuer are from the Momotarou lineage. The two Momotarou soon clash in a fierce battle, and Tsuyoshi sacrifices himself to protect Shiki. Devastated, Shiki awakens to his Oni blood that grants him immense strength at the cost of going berserk. Although the enemy escapes, Naito Mudano, an Oni teacher at Rasetsu Academy, takes an interest in Shiki. Under Mudano's guidance, Shiki must learn to control the Oni blood—if he ever hopes to avenge his father.

The daily life of high school student Naoto Watari revolves around his little sister Suzushiro. As their parents passed away two years prior, the siblings have been passed from one relative to another, resulting in Naoto taking the most responsibility for her. Although things have not been easy for the two, Naoto is now content with living at their aunt's place. This newfound peace is suddenly turned on its head when Satsuki Tachibana enrolls at Naoto's school. The two used to be good friends, but after Satsuki completely destroyed his family's garden, she disappeared from Naoto's life. Despite what happened, Satsuki seems to be making advances toward Naoto now that they have reconnected. Coupled with other people around him that harbor secrets, Naoto's desire for an idyllic life may be at jeopardy.

It is the Edo period. Ichitarou, the young master of Nagasakiya, one of the largest stores in Nihonbashi, has been in poor health since birth and is unable to go out. However, Ichitarou is always protected by demons such as Shirasawa and Inugami, who serve him. One night, Ichitarou goes out secretly and witnesses a murder. From that day on, bizarre murders begin to occur one after another in Edo... With the help of the demons, Ichitarou's search for the culprit begins!