When Rin's childhood friend Sousuke Yamazaki transfers to Samezuka Academy, their reunion sparks memories of their past and their shared dream of swimming together. Joined by Captain Mikoshiba's brother, Momotarou Mikoshiba, and the dedicated Aiichirou Nitori, the Samezuka relay team is born, strengthening their bonds of friendship. Yet, as competitions and rigorous training unfold, Sousuke's mysterious return starts to impact his performance in the water. Concerned by Sousuke's struggle, Rin is determined to unveil the secrets weighing on his friend and hindering his swimming prowess.