The time is Heian, and 13-year-old Masahiro is the grandson of the rare yin-yang master Abe no Seimei. Still only half a man, Masahiro is still training to surpass his grandfather, while being teased by his partner Mokkun, the Mononokai. Then, there is a commotion in the Inner Palace, and Masahiro and Mokkun start their own investigation, but a strange shadow lurks behind the scenes... Can the loser Onmyoji save the capital?