There was once a prosperous kingdom under the sea, where mermaids lived happily, until the fairies raised them to the surface. That was when the mermaid Hlaine was captured, kept in a tank within a museum, forced to perform tricks for the fairies' entertainment. As centuries passed, the mermaids raised to the surface grew legs and learnt to live among fairies. Mae, a young girl on an overnight school trip to the museum meets Hlaine and becomes enamoured with mermaid culture. Might this be Hlaine's chance to return home?