Yosuke Yashiro's family upholds a peculiar, singular tradition: once every 60 years, they receive an egg from a mermaid, which is kept safe within a shrine. Once the egg has matured, the family dutifully returns it to the sea, and the cycle begins anew. In return for this service, the merfolk bless their coastal town with abundant fish catches and calm waters. However, commercial developers encroach upon the quiet seaside village, and Yosuke's father is drawn away from tradition in pursuit of modern prosperity, turning the mermaid's egg into a tourist attraction. What will become of the promise their family made to the mermaids so many years ago? Included one-shot: Volume 1: Summer of Tension