Maaya Nanako transfers to and joins the swimming club. She annoys the girls and manipulates the boys. Makio forms the Bust Building Alliance with her. Kaname and Amuro visit Mirei’s home and meet her parents and Makio decides to investigate Momoko’s and Ikamasa’s relationship. The team prepares for the next big tournament and get really united and fired up when provoked by a rival school, resulting in them blazing through the tournaments preliminaries.
As the Ninagawa house is almost rebuilt, a new student arrives at . Maaya Nanako, fashion model, two times middle school backstroke champion and former neighbour of Kaname, has returned to . Despite her being all friendly, Kaname mainly remembers her as a bossy bully. Amuro, telling her she is living with Kaname, invites Maaya for a swim in the ocean, which Maaya reluctantly accepts. Seeing Amuro in her swimsuit, she throws herself at Kaname, claiming Amuro is laughing at her small chest, annoying both Kaname and Amuro.
At the club’s pool Maaya shows off her swimming skills. She starts to annoy Momoko with her arrogance and keeps annoying Amuro by clinging to Kaname. When Kaname, annoyed as well, shakes himself loose of her grip and Momoko explains they do not have the funding for a pool of Maaya´s tastes, she starts manipulating the male members of the swimming club into disliking Kaname and wanting to find jobs to generate more funds. When Momoko is about to get really mad, Amuro tells Maaya to behave and that everyone is swimming at the club because it is fun, shutting her up. But when she tries to get Maaya to swim with her, Maaya kicks her into the pool.
Not wanting to change clothes with the other girls (because her breasts are smaller than almost all of them), Maaya manipulates the boys into transforming part of the storage room (which is their dressing room) into a private dressing room for her. When Maaya arrives for training the next day, her private room is nearly finished …and being tested by Amuro, Mirei and Makio. Annoyed by this and the boys trying to watch them through the curtains, Maaya angrily slaps the wall and it, weakened by the redecoration by the boys, collapses, destroying the storage room.
Arriving at school Maaya is annoyed to find Kaname arriving together with Amuro, blaming Amuro’s breasts for it and calling Kaname a breast freak. Later, in the library Makio tells Maaya she would love to have a model’s figure like her, but with big breasts. Realizing Maaya has the same wish, despite comments that suggest the opposite, Makio decides to form the Bust Building Alliance with Maaya and they begin massaging each other’s breasts during every break. After a few days they think their method works, but soon they realize their breasts were just temporarily swollen due to all the rubbing.
When Mirei is ill, Amuro is asked to deliver her homework, since they are both in the same class. Fearing she will get lost, she drags Kaname with her. They are welcomed by Mirei’s seemingly normal parents and in Mirei’s room Amuro quickly starts to go through her drawers, just as she did in Kaname’s room. After dressing up as a bunny girl, she discovers a bra that matches the panties in Kaname’s room, which she of course tells Kaname and Mirei. When Amuro is about to discover Mirei’s kokeshi collection, Mirei jumps up to stop her and she loses her consciousness when hitting her head. Kaname decides to warn her parents, finding them in the kitchen, nearly naked. He falls ill due to the shock.
Lately there are rumours in the swimming club that Momoko and Ikamasa are dating. Makio and Amuro decide to investigate and ask their fellow club members if they noticed anything. When Sanae claims to have seen them enter a love hotel after school, they decide to follow them after the training, dragging along Kaname. It turns out Momoko and Ikamasa followed a narrow alleyway next to the love hotel Sanae thought they entered. Makio, Amuro and Kaname follow them and end up in a sporting goods store, where Momoko and Ikamasa are picking up new swimsuits they ordered there for the next meet. Asked by Makio if they are dating, both deny, but Ikamasa also tells them in middle school they used to shave each other, earning a beating from Momoko.
As Momoko is having the club members practice starting (Maaya refuses, claiming starting is her speciality), Manabu Domon, Takura High swimming club’s advisor, is peeping at (and filming) them, wishing he could be Umishou’s advisor as well (because of all the girls, which Takura High lacks). Knowing that is impossible as long as Momoko is around, he arranges she gets called to the staff room. Soon after she left the pool, Manabu shows up, claiming he was just passing by when he ran into Momoko, who asked him to train the swimming club today. Failing to make Maaya do as he pleases and then using a dubious training method on Mirei, the other Umishou members grow suspicious. Momoko, realising she has been had, returns and kicks Manabu unconscious. After Kaname dragged him out of the pool, Amuro decides to give him CPR …using a bicycle pump. As Manabu runs away they all hope in vain that he has learned his lesson.
When being driven home after a photo shoot Maaya spots Amuro and decides to sneak up on her and scare her. She orders her father to stop the car and quickly follows a fast moving Amuro by foot. Right after her attempt to scare Amuro she herself is scared by a monkey Amuro just met. Asked what she is doing, Amuro explains she went for a walk and got lost. After laughing at her, Maaya decides to return to her father’s car, without any intention to help Amuro. When it appears she has gotten lost as well, Amuro offers to try to find the way back home together, which Maaya refuses. Hours later, when she finally seems to have shaken off Amuro, Maaya spots a camp fire and decides to ask for help. The person at the camp fire turns out to be Amuro, who had run into the monkey again and together they made a camp. Maaya reluctantly stays and the next morning they are awoken by Momoko. It turns out their little camp site was behind Momoko’s house.
Since everyone will have midterm exams the next day, Momoko decides to finish training early. She explains that every year the club with the largest number of failures gets the Idiot Trophy and that its current holder is the swimming club. Wanting to not earn it this year, she promises, that anyone who fails even one subject will be crucified on the fence surrounding the pool. Ikamasa decides to hold a study party at Kaname’s house and Momoko agrees, saying the worst should attend it. After an hour without any real progress they decide to take a short break and go to a convenience store. On the bridge they meet a stranger asking for directions and Amuro helps him fluently in English, explaining to the others her father has a lot of foreign friends. After the midterms Momoko asks who failed anything. Makio and Kaname are both safe this year, but Amuro has gotten the Idiot trophy, since she failed everything, including English, since she cannot read or write it. Of course the other boys failed at least one subject as well, so Momoko ties them and Amuro to the fence.
During training a week before the prefectural meet Amuro dives into the pool and suddenly she loses all strength and has to be pulled out. The others realize one of her X’s is missing: during her dive it was blown away by the wind. Thinking the missing X is for some reason related to her weakened state, the club members start to hunt down the X, leading to all kinds of crazy situations. After some failed attempts they see the X has landed on Maaya’s head, but when Kaname tells her she angrily throws it away, because of her disliking Amuro. In the end they are still searching, not seeing it landed on Ono-chan’s head.
One of Amuro’s X’s is still missing. As various members try to see if substitutes have any effect, Kaname remembers Masashi telling him the X’s are a memento of Amuro’s grandmother and he thinks her current condition is due to the shock of losing one. Since substitutes failed, they decide to hunt down the real X once more. By now it has landed on the head of a huge Buddha statue. They try to reach it by having the whole team sit and stand on top of each other, but they appear to be one person short. Just when this human ladder starts to collapse, Amuro jumps up, climbs up the ladder and grabs the X. It turns out she was weakened because she had not eaten anything during lunch and while the others were forming the ladder she ate some offerings. Later, during dinner, she tells the curse is fake and her grandmother was a notorious liar.
The swimming club has travelled to the gym where Maaya normally trains, since it has a 50m pool, like the pool of the prefectural meet. Maaya shows up with her personal coach, Iori, who most of the girls know from a weekly column in a swimming magazine. Of course Maaya does not like the others giving him any attention. When everyone starts training, Kaname quickly notices the difference in skill between Iori, who is exclusively training Maaya, and Manabu Domon. Seeing Amuro return to Kaname after a swim, Iori tells Maaya to train by herself for a bit and moves over to Amuro asking her if she wants some advise. Of course, this enrages Maaya, who tries to get Amuro away from Iori, but accidentally kicks Kaname towards the pool. Iori manages to catch Kaname before he falls into the pool and scolds Maaya for acting spoiled. Now Maaya aims her anger at Iori, which forces him to confess he did not want to get close to Amuro, but to Kaname.
After training on the day before the prefectural meet Momoko tells everyone to be at the meeting place at 8:30. Sanae tells Makio it will be Momoko, Ikamasa and her last tournament if they lose, since they are in their final year. Ikamasa tells the other boys to show up for a quick meeting in the girls’ dressing room at 9 pm that evening. Kaname arrives last, finding the other four playing mah-jong. Ikamasa explains it has been his dream to play with his fellow club mates inside the club room and that that evening could be his last chance for it. When the others ask Kaname who he loves, Amuro or Mirei, he accidentally mentions having Mirei’s panties. He manages to get away with it by claiming to be joking and asks Ikamasa about him and Momoko, receiving the same answer as before: there is nothing going on between them. When looking around if the girls left anything worth seeing (like a garter belt, Kaname correctly guessing as Mirei’s), Isao Tsutsumi discovers the girls’ new swimsuits. The next morning when arriving to pick up the swimsuits, Momoko finds the boys (including Kaname, who is forced into one for losing at mah-jong) asleep and wearing the swimsuits.
Arriving at the prefectural meet the next day, the boys are tired, since Momoko forced them to clean up the dressing room and wash the swimsuits. At the meet the Umishou team runs into the affiliated High School’s swimming team, led by Momoko’s direct rival, Karena Nanjou. When the girls change into their swimsuits, Maaya (who had already annoyed Momoko by arriving late) gets into an argument with Karena for standing too close to her and having large breasts. When Karena claims to have the biggest breasts in the room, Amuro starts feeling her up, followed by feeling up Mirei and claims Mirei’s breasts are definitely bigger. Karena retorts her team will definitely beat Umishou at swimming, firing up the Umishou girls, who decide, as a team, to beat Shinomiya. The boys, claiming they do not need unity, since swimming is an individual sport, are surprised about the unity in the girls’ team.
The preliminaries start and each of the Umishou girls, fired up as they are, manages to reach the finals. The first event of the finals is individual medley, Mirei’s race. Shinomiya’s Tomoko Naruko, who is Mirei’s opponent in that race, realizes Mirei did not get much sleep, though that is not because of the race, but because of the lost garter belt. Mirei falls asleep at the start, but manages to start perfectly, despite normally being a bad starter, and wins the race.
The girls try to give a short preview for the next volume, but Ikamasa wants to give some fan service is. Asked by Amuro to give some service as well, Mirei rejects, saying that is her usual role.