On the first day of 2nd term at Yumegaoka High School, Futaba Ooki meets up with Hikari Kohinata in their classroom before class, where Hikari is cramming the last of her homework due that morning. After the first day of classes is over, they go down to the Diving Club room for the first time since before summer vacation. Nobody is there yet, but they hear a strange wail, so go outside to check it out.
The sound came from Ai Ninomiya and her brother Makoto, who have found holes in their equipment while checking them over for autumn use. The group debates among themselves whether they should buy their own made to measure dry suits, but because they cost upwards of 100,000 Yen (around $1000) they're prohibitively expensive and they usually rent them from a diving shop or use the school's equipment. Mato appears and reveals that she's found an online store which is holding a student discount sale. The students all call their parents to ask for money to pay for their own suits, and their parents all agree on the condition that it's in exchange for not getting them birthday, christmas, or graduation gifts for the rest of high school. The twins parents, having to buy two suits, also insist they'll have to get a job and pay them back.
After being given the go-ahead, the group then gets ready to measure the 35 different parts of the body that the diving supply company will require for their made-to-measure suits. The reason that the measurements need to be accurate is that if there are any spots in the suit that are too spacious air will get trapped, and if the suit is too tight it won't fit properly. A properly fitted dry suit is needed for proper bouyancy.
While Ai and Futaba begin taking measurements and choosing colours for their suits, Hikari notices that Makoto is missing from the room and goes to look for him. She finds him outside, loking at all the old, damaged equipment. He explains that he's the kind of person who is terrible at getting rid of things even if he knows it's something he can't use again because he feels like all the memories are still there in the objects. Hikari says nothing, but understands. Mato appears, and reminds Makoto that they're not throwing the equipment away, they're going to get them repaired because they'll be useful for future students to use. In return for using school funds to repair them, Mato asks Hikari and Makoto to make sure the equipment is well-maintained even though they won't be using it anymore. They agree.