When her dad gets a job overseas, Konatsu Amano has to leave the Tokyo life she's always known and relocate to a small seaside town to stay with her aunt. The move also means starting a new school surrounded by complete strangers, and it's a lot to handle for a girl who has trouble with change. But on her first day in her new town, Konatsu is instantly drawn to Koyuki, an older girl who is the sole member of the Aquarium Club. Konatsu's introverted tendencies are hard for her to overcome, but maybe she's found something worth coming out of her shell for?

The Aquarium Club is finally coming alive after welcoming new members, but summer is nearing and Koyuki has started studying hard for exams. Working up her resolve, Koyuki invites Konatsu on a moonlit walk on the beach where she bares her heart about an important choice. Graduation is approaching and the time they have left together is more precious than ever, but how does Konatsu respond to hearing Koyuki's decision?