During his time in junior high school, Haruka Nanase, voiced by Nobunaga Shimazaki, decides to join the swim team. However, his tenure is cut short after a significant falling out with his former rival from elementary school, Rin Matsuoka, who has recently returned from studying in Australia. This conflict creates a ripple effect, leading Ikuya Kirishima to feel a deep sense of resentment toward Haruka. Eventually, the swim team disbands entirely as Asahi Shiina moves away and Ikuya departs to study abroad.
As the story transitions to high school, Haruka begins preparing for various tournaments alongside a new group of teammates, including Makoto Tachibana, Nagisa Hazuki, and Rei Ryugazaki. During this time, Rin has reconciled with the group and is also focusing on his own competitive goals. Despite being in his final year of high school and even receiving interest from a university during the prefectural competitions, Haruka remains undecided and lacks a clear plan for his life after graduation. Meanwhile, Makoto finds a new sense of purpose and inspiration to become a professional swim coach after he helps Hayato, the younger brother of Kisumi Shigino, master the art of swimming.
The pressure reaches a breaking point during the regional tournaments. Overwhelmed by the scrutiny of university scouts watching his every move, Haruka suffers a crisis of confidence and abruptly stops swimming in the middle of his individual event. When Rin confronts him about his lack of a dream or ambition, Haruka reacts with an uncharacteristic burst of anger, leaving his friends deeply concerned for his well-being. This tension culminates in a heated argument between Haruka and Makoto. During this clash, Makoto reveals his own ambition: he intends to move to Tokyo to attend university and pursue his dream. This revelation leaves Haruka feeling even more adrift and lost.
Seeking to help his friend, Makoto prompts Rin to take Haruka back to Australia. This journey serves as a turning point, inspiring Haruka to pursue swimming at the collegiate level. Upon returning to Japan, he makes amends with his teammates. With their bonds strengthened and a renewed sense of hope, Haruka and his team fight their way to a 6th place overall finish at the national championships.
In a brief post-credits sequence during the nationals, Sosuke Yamazaki accidentally gets lost, only to stumble upon the individual medley events. There, he witnesses Ikuya, representing Shionezaki High School, dominate the competition and overtake all other swimmers. Following the tournament, Ikuya is seen discussing his physical condition with his older brother, Natsuya Kirishima.
The narrative then shifts to focus on Sosuke Yamazaki, who is Rin's childhood best friend. Sosuke transfers into Rin's class at Samezuka Academy, bringing with him a hidden burden. While the two had promised as children to pursue their swimming careers together, Sosuke has suffered a severe shoulder injury caused by overuse. Despite the pain, he continues to push himself as they prepare for upcoming competitions. While Rin remains intensely focused on the sport, Sosuke struggles to find a meaningful reason to keep going, which places a significant strain on their friendship.
At the regional tournament, the truth finally comes to light as Rin discovers the extent of Sosuke's injury. Having come to cherish the deep bonds he formed with his teammates, Sosuke is determined to compete in the team medley one last time. The team manages to secure second place, successfully qualifying for the nationals. Recognizing the severity of his condition, Sosuke decides to step away from the competition to seek necessary medical treatment. Even though he is forced to drop out of the national championships, he remains supportive of his peers, helping Aiichiro Nitori and Momotaro Mikoshiba with their preparations.
In a final post-credits scene, Sosuke warns Rin about the threat posed by Ikuya, noting that Ikuya's freestyle form and speed are remarkably similar to those of Haruka. Simultaneously, Ikuya chooses to skip the nationals' awards ceremony to contact Natsuya. As the chapter closes, Gou becomes determined to recruit fresh talent for the upcoming school year, while Aiichiro Nitori, Momotaro Mikoshiba, Nagisa Hazuki, and Rei Ryugazaki reaffirm their rivalry and organize a joint practice session. After Asahi Shiina says his goodbyes to his team at Kazami High School and receives a message from Kisumi, the story concludes with Haruka and Makoto arriving in Tokyo to begin their next chapter.
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