In front of the lone tunnel leading out from the small village of Tamasato, Yuuto draws a picture of a mermaid living in the deep ocean where light does not reach. One day, he looked up from his drawing and there was a girl in front of him who looked just like the one on his canvas. She called herself a ‘sinner’ who came to this town to repent for her sins. Yuuto and the townspeople welcomed the girl, Ai, to their town.

Come spring, the entrance ceremony was underway. The new students took pictures and videos in front of the school with their parents to commemorate this special day. However, it was a depressing event for Yuuto, who lives alone after the death of his father and the abandonment by his mother and older sister. While he had gotten used to it, this time he suddenly felt very lonely due to the recent passing of grandmother, the sole close relative he had left.

Upon finding out about Yuuto’s loneliness, Ai confided that she was abandoned by her parents. She searched for a reason for living, whereas he ran away from reality through his imaginative drawings. Even though they were polar opposites, their backgrounds also made them quite similar. Gradually, they became closer to each other, while harbouring secrets of their own.