Takumi Nanase, a naive young man, is friends with Tetis, a dolphin who avoids other human beings. While waiting for Tetis, Takumi hears someone singing melodiously. Beckoned by the voice of this beautiful girl Ruka, Tetis appears in the ocean, surprising both Takumi and Ruka. Brought together by this fateful meeting, the two young people begin a friendship involving difficulties and challenges.