High school senior Natsuki is having the worst summer ever: he just got dumped by his girlfriend, and his college entrance exam scores are a total disaster. Desperate to save his grades, Natsuki's older sister recruits her charming college boyfriend, Sota, to be his private tutor. At first, Natsuki finds Sota's Mr. Perfect persona incredibly annoying. But as they spend hot summer days studying together, a sudden smile and a gentle pat on the head from Sota send Natsuki's heart racing. He's my sister's boyfriend. I'm supposed to be studying. So why can't I look away? Caught between exam stress and an impossible crush, Natsuki faces a summer of bittersweet self-discovery under the bright blue sky.