From Antisense Scanslations:

Lotte has always been fascinated by the surface world, but mermaids are forbidden to ever have contact with humans. One day, she finds a handsome young man adrift in a shipwreck and takes him to shore.

More obsessed with the surface than ever, Lotte jumps at the chance to become human when a demon named J offers her a magic potion. But there's more to J's potion - and his intentions - than meets the eye...