Yosuke Yashiro's family has a strange, unique tradition—once every 60 years, they receive an egg from a mermaid where it sits protected in a shrine. After the egg matures, the family dutifully returns it to the sea, and the process is repeated. In exchange for this favor, the merpeople bless their coastal town with bountiful catches of fish and calm seas.

However, commercial developers encroach on the sleepy seaside hamlet and Yosuke's father is lured away from tradition toward modern prosperity, turning the mermaid's egg into a tourist trap. What will happen to the promise their family made to the mermaids all those years ago?

Included one-shot: Volume 1: Summer of Tension