The Mermaid Festival is an annual festival that takes place in the coastal town of Suminoe and on the night of the festival, Takumi meets a girl with a beautiful singing voice. From this chance encounter, Takumi's daily life slowly begins to change. The 7th project by developer, feng.

The ‘ningyo matsuri’ (mermaid festival) is held every year in the village of Suminoe-chou, which faces the Pacific Ocean. Takumi helps out with his family’s popular cafe Mermaid with his osananajimi Mizuka and his mother Isana.

On the night of the festival, Takumi came upon a girl with a beautiful voice. She was surprised at the unexpected visitor and accidentally slipped into the sea. He saved her and since he couldn’t let her be in her drenched state, he invited the mysterious girl Shione to his house. When Isana decided that Shione would stay in their house and Matsuri, who he had met at the festival, transferred into his school, his days gradually began to change.