Yosuke’s family has guarded a strange pact for generations: once every sixty years, a mermaid leaves them an egg to protect until it is ready to be returned to the sea. In exchange, the coastal town receives calm waters and abundant fishing. When commercial development reaches the village and Yosuke’s father turns the sacred egg into a tourist attraction, the family’s old promise collides with modern economic ambition and threatens the relationship between the town and the sea that sustains it.