Hayama Kao yearns for an existence that would make people notice her. Now in high school, she is working as a temporary manager of her school's boys volleyball club against her will. Kao's older brother, an alumnus, currently a teacher and the advisor of the club, placed the responsibility on her shoulders to recruit Kuze Nagisa for the volleyball club if she wanted to quit being the manager. Then one day, she notices something that took her breath away. It was bluer than the sky, even bluer than the sea; it was Kuze Nagisa practicing volleyball all on his own. The blue days start from here!