Ever since she rescued him from drowning, Kou has had feelings for his classmate Ayame, who he refers to as a mermaid. But much to his dismay, Ayame is in love with someone else: Kou's older brother, the advisor of their school's swimming club.

Kou, who is willing to die for Ayame's love, believes that she is foolish for chasing after someone who will never reciprocate her feelings. Even so, Ayame continues to desire the affection of Kou's brother, hoping that he will eventually notice her. Although she was the one who initially saved him, can Kou return the favor to Ayame one day?