Mellow tells the story of Haijima Akira. He's on the run from Osaka gangsters for messing with the boss's daughter: To escape and stave off the amputation of a portion of his anatomy, he disguises himself as his sister, also named Haijima Akira, and takes her places as teacher of English at a high-school. Akira has to navigate the perils of being a (female) high-school educator, equipped only with a ferocious temper and a total lack of knowledge. Mayhem ensues.