The story takes place on Kisejima, a small island in the middle of the ocean. It is said that the power slumbering within the local demon spirits can be unleashed by challenging them to trials, where ancient demons once defeated by Momotaro are called to battle. However, few have ever returned alive from Kisejima.

Kuuta, the protagonist, is the sole survivor of the Demon King's army that faced Momotaro in battle. He teams up with Kazemaru, a mysterious spirit residing on Kisejima, to take on challenges of all kinds and sizes.

While Kisejima may not be large, the colorful artwork by KENEI DESIGN, with its vibrant green meadows, towering forests, and plains of floating megaliths, creates a unique emotional impact unlike any other action game. Its sole creator has crafted a mini-world where each location is marked by its own unforgettable charm. ONI: Road to be the Mightiest Oni invites players to enjoy an experience in a small world for a change!.