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.

On her way to school, Hikari races against her homeroom teacher, Mato Katori, who she discovers to be a fellow scuba diver. Later that day, Hikari drags Futaba to the school's Diving Club, where they try on some drysuits and take a dip in the pool. Moved by Hikari's high spirits, Futaba decides to join her in diving.

Hikari leads Futaba to a road filled with cherry blossom trees to cheer her up. The next day, Hikari and Futaba sign up for the Diving Club, where Mato, the club's advisor, teaches Futaba about air pressure and ear equalization.

Futaba comes to observe a scuba diving class run by Hikari's grandmother, Kino. The next day, Futaba has her first experience diving in the school's pool, struggling to keep water out of her eye mask and losing confidence. However, with Hikari's guiding her as her buddy, she begins to get used to being underwater, managing to go to the deepest parts of the pool.

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.

On a rainy day, Mato decides to follow Hikari as she boards a train, discovering that her only reason for doing so was to witness some blooming hydrangeas for a brief instant. Later, on a particularly hot day, Mato has the club play red light green light as punishment for skipping on their exam studies.

Makoto becomes concerned when Ai receives what appears to be a love letter from her classmate, only to later find it was addressed to another girl. Later, as Futaba and Hikari take part in various fitness exams, Hikari helps Futaba improve her self-confidence during the 1000m run.

Futaba, who always takes photos to remember each moment, becomes shocked when her phone's memory has become full, requiring her to delete or move her old photos, including those of her friends from Tokyo. As Futaba finds herself unable to delete any of her old photos to make room for new memories, Hikari and the others buy her a digital photo frame to store all of her photos, both old and new.

Futaba and Hikari invite Ai to hang out with them as they go shopping for logbooks and swimsuits. Afterwards, the girls go to the beach, where Futaba tells Kino about all the things she has learned since meeting Hikari. Futaba expresses her concerns about whether she can become a suitable buddy for Hikari, though Hikari expresses that she's had times when she lost her way as well, saying that it is a chance to have more fun. Afterwards, Futaba finally manages to pass the pool course, allowing her to move on to the Open Diver course with everyone.

Futaba and Hikari follow the club's mascot, Cha, leading them to take a path that neither of them has been down before. Along the way, they come across a small kitten that appears to have been separated from her mother. With neither girl able to take the kitten in, they decide to ask around at school for a potential owner, eventually getting found by Mato, who reminds them of responsibility involved in taking care of a life. Hearing Futaba wants to help the kitten like she helped her, Hikari convinces her mother to let the kitten stay at her house until they can find an owner. Just then, the school's principal, who is Cha's owner, decides to take in the kitten, who is given the name Ohime.

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.