H2O's story revolves around Takuma Hirose, a blind young male junior-high school student, though the cause for his blindness is undetermined. After his mother died an unexpected death, it left a deep emotional scar on him, which caused him to become very lonely and reserved. Due to this, Takuma and his uncle move from the city out into a rural area and Takuma is enrolled into a new junior-high school. At his new school, he meets several new girls, though three of whom he gets to know the most out of anyone else. They are, the firm and obstinate Hayami Kohinata, the kind and obliging Hinata Kagura, and the cheerful and mysterious Otoha, who is a spirit of Hinata's dead sister. As Takuma interacts with these girls, his medical condition gradually begins to heal. When the summer festival came, Takuma realized that he needs Hayami and said to her that he wants her to stay by his side. Rumors spread and the village elder knew about their relationship so he revealed to Takuma that the reason of his mom's death was because of the Kohinata family. Takuma became confused and out of his mind, which lead him to hurt Hayami and lost his sight. Takuma then realized that the whatever he can see is nothing but an illusion. In the final episode, Takumi gained mental disorder due to emotional depression and considers Hayami as his mom. He then saves Hayami from being killed by the village elder and decides to move in Tokyo with Hayami. While they were living in Tokyo, Hayami makes Takuma realize that he have to accept the death of his real mom and Hayami dies while saving a boy from being hit by the train. Many years later, Takuma's blindness had been healed and he makes a windmill where he and Hayami will always stay, then Otoha approached him, for the first time in many years. She tells him that the spirits allowed Hayami to be alive again. She runs away and Hayami approaches Takuma.