Futaba Ooki and Hikari Kohinata are spending the day at Hikari's grandmother Kino Kohinata's diving services store Amanchu-ya. Hikari teaches Futaba the fundamental items needed on shore when preparing for a dive: a swimsuit, shampoo, a change of clothes including underwear, and a diving logbook. Kino, who has not seen Futaba since Futaba's first visit to the shore is surprised to learn that the two girls have become friends. Futaba wonders if it's fate that the woman who taught her such an important lesson is the grandmother of the girl who is helping her learn to expand her horizons.
Hikari then gets ready to teach Kino's customers. Futaba is surprised that Hikari is going to be an instructor, but Hikari tells her that she knows the area very well, and that becoming a professional diver for the area is her dream for the future. The girls meet the two teenagers who Hikari will be instructing - a pair of twins who are experienced divers from the local area. Hikari reminds the divers to do a thorough buddy check, and she teaches Futaba what that entails - checking whether the bouyancy device is fit properly, that the weights are easily accessible to remove immediately, that all equipment is tied properly, and that the air regulator is clear to breathe through. As a final preperation, the two visiting divers put on matching cold resistant diving hoods - the boy wears a kappa hood, and the girl wears a monkey hood. The group heads towards the water, leaving Futaba alone with Kino at the beach house.
Kino tells Futaba that during this time of year the water is murky due to phytoplankton growing in the colder water. This significantly decreases visibility for divers, and is called "springtime murkiness" - visibility is at best only around 5m. The group comes back to shore to change their air tanks, and is surprised to find that the group enjoyed themselves despite not being able to see anything. Kino tells Futaba that even though that can't see any of the sights, they still enjoy it because it's the season where life begins - because there are more nutrients in the water, creatures lay their eggs and it's the only time that it can be seen. The fish eat the phytoplankton, and those fish are eaten by bigger fish, and eventually the water will turn clear again. Kino hopes that Futaba will be able to dive by the time the water clears.
The group returns to shore again, and begins washing their gear with fresh water. Futaba thinks she sees her underwear sat on a post by the hose and is shocked. She goes to grab them, but the female diver was already going to grab them. Without speaking, Hikari frantically nods, trying to communicate that the underwear belongs to her. The girl smiles, finishes preparing to leave, and goes home. After they've gone Hikari goes into the shower, and afterwards goes to get dressed again, only to find that what she thought was her underwear isn't underwear at all, but the girl's monkey-shaped cold resistant hood. Both girls panic.