The hyperactive teenage scuba diver, Hikari Kohinata, lives in a small town near the sea. On her first day of high school she meets the shy transfer student, Futaba Oki, who is struggling to find a place after moving away from Tokyo. Hikari takes the lead, dragging Futaba into her hijinx, and the two soon become close friends
After helping out at Kino's beach house during a lunch rush, Futaba decides to return there the next day while Hikari goes scuba diving with the others. While there, Hikari accidentally goes into an outdoor hot spring without her bikini top, putting her at risk of being exposed before the other girls come to save her. Later, after Hikari holds a barbecue party for everyone, Futaba worries about what she'd do if she has to part ways with Hikari, but Hikari encourages her to enjoy every moment they spend together.
While Futaba is away for a memorial at her country home, Hikari befriends a girl by the sea who is fascinated by a mother octopus guarding her young - Futaba is determined to befriend her. Meanwhile, Futaba works hard as a volunteer helping to set up the local fireworks festival, working hard enough to earn front row seats in the VIP section of the festival for her friends.
Futaba takes the underwater navigation tutorial but is brought crashing back to reality by the realization that she may be holding Hikari back - but is able to successfully complete it after she remembers all the lessons she had been taught by her friends. After watching the twins go on a night time dive, Futaba wrestles with the feeling that she may be holding Hikari back.
Futaba and Hikari spend an autumn day apart, and they both find autumn wonders of their own.
Futaba goes diving in a drysuit, as the gang looks to encounter the legendary Jet-Black Mermaid.
It's Halloween, and the Diving Club members find themselves in the midst of a special event in an amusement park. Hikari's in danger, and it's up to Futaba to be her knight in shining armor?
The Yumegaoka High Dream Festival arrives, and the night before, the entire student body is busy preparing for opening day. This night would end up becoming a very special one for Ai in particular, as she has a very special encounter.
After meeting Peter in an encounter straight out of fantasy, Ai now gets transported to a fantasy world, where she meets someone she never expected to meet.
Ai can't accept how Peter parted with Mato-chan and her, and so she decides to head back to Neverland, only this time she has company.
Teko is invited to bring in the new year at Amanchu-ya, and she finds herself in the company of the whole gang plus a few extras. As they watch the first sunrise, a new kind of emotion takes over her and Pikari.
Pikari ends up with a cold on her birthday, so Teko decides to visit her at home. She finds someone on the way, which leads to plans for another fun event once Pikari recovers.
It's finally time for Teko's advanced certification test, with a promise of a night-diving celebration if she passes!
