At school, after Haruka's and Rin's unsuccessful duel, a scarlet-haired girl observes the group with a friend and remarks of them being old acquaintances before Makoto and Nagisa are reprimanded by a teacher for trespassing at the swimming club. As Makoto and Nagisa visit Haruka that evening, they discover the scarlet-haired girl turns out to be Rin's sister, Gou Matsuoka, attempting to pay Haruka a visit. They then learn from her that Rin is now attending Samezuka Academy upon his return from Australia.
After the match, Haruka and the others are once again reprimanded for trespassing on Samezuka Academy grounds. Makoto and Nagisa run into Gou and they all agree that Rin's attitude had dramatically changed in the past few years, simultaneously learning that Gou had been the one who sent him to the Iwatobi Swimming Club while Nagisa suggests that they form a school swimming club.
Gou proposes that the swim club join an indoor gym so they can practice swimming until summer, but due to its expense, they seek sponsorship from the school. After Nagisa's numerous unsuccessful attempts to get Rei to join the swim club, Gou informs the club that she had successfully scheduled a joint swimming practice with the Samezuka team at their indoor pool, but they would still require a fourth swimmer.
The Swim Club (with their fourth member, Rei) officially starts training in the refurbished pool using an old training regiment of Gou's had unearthed. The following day, the guys try to come up with designs for a club uniform but Gou instead gives Rei's lessons higher priority and makes a revamped swimming regiment during which he must master swimming in one week before their first prefectural tournament. During the weekend, Rei attributes his inability to not having proper gear, hence prompting a visit by the team to a sports store to buy swimming essentials. Gou runs into Seijuro there who was surprised to see her. Later, she overhears Rin's and Haruka's conversation outside the store, getting even more worried over her brother's drastic personality change.
Believing that the added pressure will help the Swim Club in their training, Gou continues to make her countdown calendar for the upcoming prefectural tournament. Meanwhile, as the guys decide on which events they will each participate in at the tournament, Gou discovers a decades old training regiment of the past Iwatobi Swim Club, and excitedly proposes that they use it and venture to an island for a summer training camp. On the day of the trip, Sasabe takes the Swim Club to the island where they coincidentally discover the Samezuka Swim Club, along with Rin, training at a special swimming facility on the island. As they prepare for dinner, Gou heads to a nearby convenience store and briefly runs into Aiichiro and Rin who shows some concern for Makoto after Gou explains their training, before returning to his usual stoic self.
The next morning, the group (who was on the other island during the night) swims back to the coast where their tens are, as well as Gou and Miho, who didn't know what happened last night. When they finally arrive at the coast, Gou and Miho watch them as they sleep and wonder what they're doing. Gou tries to wake them up, but doesn't succeed.
At the day of the tournament, Miho notices that Gou isn't very happy and tells her not to dwell on it, but Gou replies that that's not it and that she wanted to see her brother and Haruka swim together, not against each other. After the unsuccessful day at the swimming tournament, Miho and Goro left Makoto, Nagisa, Rei and Gou alone. Gou then decides to tell the group that she signed all of them up for the medley relay on the second day of the tournament. The group protests a little for Gou not telling them about it sooner, but Rei immediately agrees on swimming in the relay, making Makoto and Nagisa agree as well. They go to Haruka's house with a goal to make him agree on swimming in the relay with them as well, but nobody's there. They break in the house and start searching for Haruka in the hope that he's somewhere in the house. After going in every room and not finding him there, they decide to call him, but he doesn't answer. Rei, Nagisa and Gou leave him a voice message, but soon discover by Makoto that he doesn't bring his phone with him often.
The group attends a summer festival, running into Gou and Chigusa. Makoto compliments Chigusa and Gou for their looks, but Chigusa misunderstood him, thinking that he complimented their yukatas instead of their looks, making Makoto feel a bit uneasy.
After the team finished on one of their practices, Nagisa, Makoto, Rei and Haruka together with Gou, Goro and Miho had a hot pot in Goro's house even though it's summer. There, Gou discovered the old clubs photos, which of course includes Rin in it.