Futaba Ooki, a shy girl who had just moved from Tokyo to the beachside town of Shizuoka, catches the eye of her eccentric classmate, Hikari Kohinata, who instantly befriends her.

Noticing Futaba struggling with strength, Mato has her and Hikari go jogging to build up strength. Along the way, they come across some students from diving school, Ai and Makoto Ninomiya, who turn out to be their upperclassmen in the Diving Club. After a hectic first meeting, Ai holds a meeting to discuss how Futaba will be aiming for an Open Water Diver Certification, which somehow ends in a chalk drawing session.

The Open Diver course begins, with Futaba and Hikari paired up as buddies. Futaba loses one of her fins on the way to the descent point, but it gets picked up by another diver, turning her fears of the unknown world beneath her into excitement. Guided by the bubbles of the other divers, Futaba reaches the bottom of the ocean floor, wowed by the scenery and sealife spread out before her. Clearing all the tasks before her, Futaba passes the course with flying colors and becomes a fully-fledged Open Diver. Afterwards, Futaba and Hikari thank each other for everything that's happened since they became 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.

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.

It's finally time for Teko's advanced certification test, with a promise of a night-diving celebration if she passes!