Karas tells the story of Yousuke Otoha, a former yakuza, living in a fictional version of Shinjuku, Tokyo populated by humans and yōkai (Japanese spirits). He is one of the titular karas, humans appointed as superpowered agents of the land. Able to transform into a car, an aircraft, and an armored crusader; the skilled swordsman is to stop his corrupt predecessor, Eko, from taking over Tokyo. Supporting characters such as Eko's former henchman, Nue; the yōkai; and Homura, the karas of another city, help Otoha in his quest. A concurrent side story focuses on humans affected by Eko's scheme.