Sasamoto Riko has something that she treasures very much and that is the memories of her bittersweet unrequited love during high school. She thought that her treasure will remain as just a treasure but who would have thought that after 10 years, she would meet her unrequited love, Shirai again and that they would even start dating! Together with Shirai who's more mature now, she experiences her first date, her first kiss and then her first... Riko's heart is about to burst from the many "first" that keeps coming one after another!