Misato Mizuki, first year high school student. Her parents are on business trips abroad, and she lives with her sister-in-law, Shiori, who runs a coffee shop called "Seaside" on Kaigandori. Her brother is dead, so her sister-in-law is a widow. One day, her dog Baron informed me that I picked up a girl on the beach. The girl, who called herself Marina, seemed to have known me before. What's more, her true identity is a mermaid!?I was attracted to Marina with so many mysteries, but her classmate Nagisa Takagi interrupted... A strange love story.