Sousuke Yamazaki, Rin Matsuoka's childhood buddy, has recently transferred to Samezuka Academy. When they meet again, they reminisce about their childhood days and reconfirm their desire to swim together. The Samezuka relay team is established, and their relationship grows, including Rin, Sousuke, Captain Mikoshiba's younger brother Momotarou Mikoshiba, and hardworking Aiichirou Nitori. However, as the competitions, races, and training continue, the cause behind Sousuke's unexpected homecoming begins to bother him and negatively impact his swimming. When Rin notices Sousuke's transformation, she is determined to find out what is keeping him back.