Yosuke Yashiro's family has a strange, unique tradition -- once every sixty years they receive an egg from a mermaid where it sits protected in a shrine. Once the egg matures the family dutifully returns it to the sea, where the whole process is repeated. In exchange for this favor, the mer-people bless their coastal town with bountiful catches of fish and calm seas. But as commercial developers encroach on the sleepy seaside hamlet and Yosuke's father is lured away from tradition towards modern prosperity and turns the mermaid's egg into a tourist trap, what will happen to the promise their family made to the mermaids all those years ago? Summer of Tension