Tokiko "Toki" Aoyama and her father move from Tokyo to stay with her grandmother in the sleepy coastal town of Sunimori. However, under its serene veneer, Sunimori is fabled to be near an ocean full of mermaids and mermen. One of Toki's earliest memories is of a merman saving her from drowning in the ocean. Although she clearly remembers the experience, Toki meets Yousuke Narumi, a boy skeptical of these beings' existence.

As Toki settles into her new surroundings and makes new friends at school, she learns about an annual Sunanomori tradition where a summer festival is held in gratitude for the merfolk and Wadatsumi, the spirit of the sea. With the many mysteries surrounding the local folklore filling her mind, perhaps only during the festival will the answers be revealed to her, much like the tide leaving behind seashells in a town of sand.