東京の下町に位置する川沿いの町・六月町にひとりで暮らす少年・桐山零。幼い頃に家族を事故で亡くし、父の友人であるプロ棋士の家に内弟子として引き取られ育った彼は、生きるために将棋に没頭し、史上5人目の中学生プロ棋士となる。義理の姉に言われた「家も無い」「家族も無い」「学校にも行って無い」「友達も居無い」「アナタの居場所なんてこの世の何処にも無い」という言葉を否定することができず、自身の居場所を得るためにプロになることを志したのだ。他人の家で育ったことから、自身を「カッコウのひな」にたとえ、心に深い孤独を抱きながら人とのかかわりを極力避けて過ごしていた。
17歳になり、1年遅れで高校に通いながらプロ棋士として活動する桐山零が、ひょんなことから出会った三月町の川本あかりら三姉妹と交流することで、ゆっくりとその孤独を融かしていく。
Rei Kiriyama is a boy who lives alone in Rokugatsu-cho, a riverside town in Tokyo’s downtown area. He lost his family in an accident when he was young, and was taken in as an apprentice by his father’s friend, a professional shogi player. To survive, he devoted himself to shogi and became the fifth professional shogi player in history to play shogi while still in middle school. He could not deny his sister-in-law’s words, “You have no home,” “no family,” “you don’t go to school,” “you don’t have friends,” and “there is no place for you anywhere in this world,” and so he aspired to become a professional in order to find a place for himself. Having grown up in someone else’s house, he likens himself to a cuckoo chick and avoids contact with people as much as possible, harboring a deep loneliness in his heart.
At the age of 17, Rei Kiriyama is attending high school a year late and working as a professional shogi player. By interacting with three sisters from March Town, including Akari Kawamoto, whom he happens to meet, Rei slowly melts away his loneliness.