Adachi Asumu, unsure of what he wants to do with his life and unconfident about taking an entrance test to a well known band school goes on a trip with his mother one day. They travel to an island by boat and suddenly a young girl falls over the side of the boat. A man races to her rescue, jumping overboard himself, just in time to grab the girl. Asumu takes notices and thinks the man who saved the girl, Hibiki, is a bit odd. The two part ways as Asumu and his mother get to an island. There Asumu encounters a Makamou and witnesses Hibiki become an Oni.

Asumu and Hibiki meet up many more times at a sweets shop that the Takeshi organization owns and operates from. It is the meeting ground for many different Oni around Japan. Asumu ends up meeting the trumpet wielding Oni, Ibuki and Ibuki's supporter. Supporters are exactly what they sound like, people who support the Oni on their missions, and in many cases are training to be Oni themselves. Ibuki's supporter doesn't feel Asumu should be around Hibiki as he only drags Hibiki down. Eventually he ends up winning her trust and respect.

Later on the third and fourth series riders appear. Todoroki and his master Zanki, Zanki is an aging Oni who can't fight as well as he used to. Todoroki steps up to take his place as he becomes an Oni with the "ki" suffix, signifying that he is no longer the student. Todoroki is a rookie and still has much to learn and treats Ibuki and Hibiki as his superiors as they have more experience, even though Ibuki is younger than Todoroki himself.

Hibiki, Ibuki, Todoroki and Zanki make up the main Oni cast of the series. Many other Oni appear in the series, but they're in the series for an episode or two at the most. Over the course of the show other characters join the main cast, even a young boy who tries to get Hibiki to make him his apprentice.