The protagonist opens his door to find himself face-to-face with a sadist...

"I've come to be made your pupil, like you promised!"

Kuzuryu Yaichi holds one of the highest titles in the shogi world, "Ryuo," at the age of 16. The girl who has shown up at his front door is Hinatsuru Ai, a nine-year-old in the third grade.

"Huh? Pupil? What?"
"Don't you remember?"

Though he doesn't remember her, the duo's life together thus begins. Eventually, Ai's honest enthusiasm helps Yaichi rediscover the passion he had begun to lose.

[Source: Crunchyroll]