The Mermaid Festival is an annual festival that takes place in the coastal town of Suminoe, not far from Tokyo. The excitement for the festival is especially felt in bustling activity in the cafe, “Mermaid”, which is also home for Takumi Nanashi. Takumi’s days consist of helping out at the cafe alongside his childhood friend, Mizuka Moriya (even if she’s not actually officially employed by the cafe) and his mother, Isana Nanashi.

On the night of the Mermaid Festival, Takumi meets a girl with a beautiful singing voice. Surprised by the unexpected visitor, he also ends up rescuing her when she accidentally falls into the sea. Unable to leave the mysterious girl standing there dripping wet, Takumi invites her over to his home to dry off.

After calming down, she introduces herself as Shione Katagai. It is then that Isana decides that Shione should stay with them at their home. Soon after, Matsuri Motosuwa, who Takumi meets at the festival, ends up transferring to his school and soon Takumi’s daily life slowly begins to change.