Mugi, a transfer student from Tokyo now living in a southern island of Japan, can stand the new sunburn and the bullying he receives at school. It's the ocean, the water that he doesn't know how to swim in, and the brother-in-law who's making his older sister so happy, that he can't handle. Ever since his parents passed and this stranger became 'family,' Mugi's felt like there's nowhere he belongs, like maybe he should just... drown. Feeling out of place in his own home, he turns for refuge to his one friend, local girl Matori, whose grandmother tells them about a mysterious legend surrounding mermaids in a secret cove who will grant one wish to whoever they meet. So when Mugi's brother-in-law gets swept off to sea trying to save a carless swimmer in rough waters, it's up to Mugi to try to make things rightsink or swim.