Rin Matsuoka"s longtime companion, Sousuke Yamazaki, has just enrolled at Samezuka Academy. When they meet again, the pair recall their youth and renew a promise to swim side by side. Together with Rin, Sousuke, Captain Mikoshiba"s younger sibling Momotarou Mikoshiba, and diligent Aiichirou Nitori, the Samezuka relay squad takes shape, strengthening their bond. Yet as competitions, races, and drills progress, Sousuke"s abrupt return begins to weigh on him and impair his performance. Observing this shift, Rin urges an investigation into the cause that holds him back.
