Nirai Canai, the underwater kingdom that lies on the far side of the ocean. Mare, the mermaid princess of the kingdom, is angry at the humans who pollute the sea. However, her father, the king, doesn't even try to deal with it. When Mare reaches the end of her patience, in a decaying cave, she finds out the existence of a great "Queen of the Sea" who once ruled over all. Mare's admiration for the Queen grows, but when she learns that the "divine arrow" with which the Queen is sealed in has been stolen by humans, she decides to go to land. Where she lands is a small island called Mitaka Island, inhabited by humans.

(Source: Kyoto Animation, translated)