Momo, a high school student, is devastated by the sudden death of her best friend Sayo, who had been by her side since elementary school. Seeking solace, she reconnects with Sayo's younger brother Keichiro at the funeral, after eight long years of separation. Together, they revisit their favorite childhood spots - the secret base in the neighborhood, the twilight-drenched Ferris wheel, and the candy shop where they often hung out. They chase these memories in a bid to recapture the joy of their youth and search for the sights they had planned to share with Sayo during the summer that slipped away. As they go, they leave behind a bittersweet summer story, painted in blue by the hearts of the bereaved boys and girls.