Amuro Ninagawa and her father arrive at Umineko City, where she meets Kaname Okiura. She joins the high school swimming club and has her first tournament. During a storm her houseraft is wrecked and they decide to live with Kaname’s family until it is rebuilt. Kaname learns Amuro’s grandmother went missing at sea after a swim and that fellow club member Mirei Shizuoka has a secret.
Amuro Ninagawa and Masashi, her father, arrive from Okinawa by houseraft at Umineko City, where she meets Kaname Okiura, Momoko Orizuka and Maki “Makio” Ikuta on the beach. Greeting them, she wonders if she has met Kaname before in Okinawa. Seeing Makio wearing a swimsuit she yells she likes to swim and swims away, stunning the others with her skill.
At the start of Amuro’s first day at Umineko High School she immediately goes to Kaname after spotting him. Momoko shows up and, interested in Amuro’s swimming style the day before, challenges her to a swimming race.
The Ninagawa family houseraft is anchored at Onoshima Island, an island connected to Umineko City by a bridge. Kaname and Momoko spy on them and after pulling Kaname into the water, Amuro learns he cannot swim, which surprises her.
Amuro joins the swimming club and meets its members. Masa "Ikamasa" Ikariya decides to hold a shaving party, which of course leads to chaos. While hiding from Amuro, Kaname notices Mirei Shizuoka’s secret perverted side for the first time. (Though he thinks he misheard her)
Walking home together (Kaname lives on Onoshima Island as well) Amuro asks Kaname if he wants to be able to swim, claiming his expression tells her that. As Kaname explains he does not mind only supporting his club members, Amuro falls into the ocean and finds a seal, Ono-chan, who starts living near Onoshima.
Arriving at school with Kaname, Amuro greats Mirei, asking where she lives, while Kaname recounts what he knows about her. Training proceeds as usual, boys ogling the girls (especially Mirei and Amuro) and Momoko trying to get them to leave. Kaname, still thinking he misheard Mirei the day before, gets some proof from Mirei that he most likely did not.
The swimming club participates in a yearly three-way school competition, held at Takura High School. Momoko wins (200 m freestyle) and Ikamasa shows off and finishes last (100 m butterfly). Makio, using a kickboard (special permission), fails to impress too, while Mirei is admired and cheered on even by guys from other teams. Amuro (50 m freestyle) stuns everyone with her style and speed, but gets disqualified, though, for not surfacing within the first 15 meters, which leads to Momoko getting angry with Kaname for forgetting to teach Amuro that rule. Still, Amuro had fun.
Ikamasa’s family restaurant has financial problems and begs Momoko for help. The girls decide to help as waitresses dressed in their swimsuits, as proposed by Mirei, who of course is embarrassed by all the stares, but loves seeing the customers get aroused. The plan works, though the second day the Ikariyas manage to mess up, enraging Momoko by trying to sell swimsuit pictures of the girls.
On a hot Sunday Kaname plans to stay inside playing games, but instead Amuro takes him to the beach to show off her new bikini. While Kaname wonders if this could be called a date, he does not realise Amuro has taken him into the water up to his waist. When Amuro makes him notice this, he starts wondering if Amuro’s presence suppressed his fear.
The swimming club has a two days training camp to improve teamwork. The girls are spending the night in a classroom, while the boys stay in the night duty room. During the training Momoko, thirsty from violently drilling the club members, drinks a couple of glasses of a sports drink made by Amuro and starts to behave weird. It turns out, when making the drink, Amuro by accident used sake instead of water.
At the end of the first day of the training camp the girls are taking a bath (except for Mirei), while the boys are playing mah-jong. After the girls go to sleep, Mirei slips out to take a bath. Suddenly the boys enter the bathroom as well. Mirei hides and then tries to escape, knocking down Kaname, who does not recognise her because of the fog. In her hurry she forgets her panties, which a bit later Kaname finds.
The swimming club celebrates the end of the training camp with a barbeque at Amuro’s houseraft. When it starts to rain most of the girls take shelter in Amuro’s house and the others hide in a nearby cave. In the quickly worsening weather the slowly disintegrating houseraft floats away with the girls still on it.
Until they have repaired their houseraft, Amuro and Masashi will stay at Kaname’s home. Since Amuro is not used to a normal house or life, this of course causes problems like taking a shower using a garden hose or invading Kaname’s privacy. Trying to stop Amuro from going through his belongings, Kaname trips, knocks over a shelf and falls on top of her. It also causes the panties, which Kaname hid on top of his lamp, to fall down as well.
At 6 am Kaname is roughly awoken by Amuro, who wants to go for a walk. Masashi tags along, looking for things for repairing the houseraft. Kaname wonders how Amuro can be so carefree after losing her house and belongings and she answers the most precious thing in the world to her is already in the sea and Masashi explains a year ago her grandmother got lost at sea after a swim. He also tells he is originally from Umineko City and ended up on Okinawa after meeting Amuro’s mother. Kaname realised that although he met Amuro a month ago and they are almost constantly together, he barely knows or understands her.
Okiura films the girls doing a preview for the second volume, announcing a new club member.