Haruka Nanase has a deep love for water and a strong drive to swim. Back in elementary school, he teamed up with his three friends—Rin Matsuoka, Nagisa Hazuki, and Makoto Tachibana—to win a relay race. After that victory, the four went their separate ways. Years later, they reunite in high school, but Rin is not interested in rekindling their old bond. He attends a different school and only wants to prove he is a better swimmer than Haruka. Following a tense meeting, Haruka, Nagisa, and Makoto create the Iwatobi High School Swim Club, but they need one more member to enter the upcoming tournament. Nagisa recruits Rei Ryuugazaki, a former track athlete, to fill the spot. As the competition draws near, the four grow closer through intense training, aiming to win the event and settle the rivalry between Haruka and Rin once and for all.