An intense action anime thriller comes to life in Oni. As an elite member of the tech crimes task force, Konoko is an agent on a mission to fight the evil Syndicate. But things are not always as they seem... Konoko is haunted by unknown demons of her past, and the truth threatens to send her over the edge.

The events of Oni take place in or after the year 2032. The game world is a dystopia, an Earth so polluted that little of it remains habitable. To solve international economic crises, all nations have combined into a single entity, the World Coalition Government. The government is Orwellian, telling the populace that what are actually dangerously toxic regions are wilderness preserves, and using its police forces, known publicly as the Technological Crimes Task Force (TCTF), to suppress opposition. The player character, code-named Konoko (voiced by Amanda Winn-Lee), full name later given as Mai Hasegawa, begins the game working for the TCTF. Soon, she learns her employers have been keeping secrets about her past from her. She turns against them as she embarks on a quest of self-discovery. The player learns more about her family and origins while battling both the TCTF and its greatest enemy, the equally monolithic criminal organization called the Syndicate. In the game's climax, Konoko discovers a Syndicate plan to cause the Atmospheric Conversion Centers, air-treatment plants necessary to keep most of the world's population alive, to catastrophically malfunction. She is partially successful in thwarting the plot, saving a portion of humanity.