Boom Broom doesn't think his life could get any worse. Ever since his mother was swept away by a twister, his family has gone crazy. They refuse to leave the house and Halvor, the Viking descendant who rents the room over their garage, will only cook fish for Boom and his sister Mertyle to eat.

When Boom finds a baby mermaid who seems to grant unlimited wishes, he thinks his luck has turned around. That is, until his sister is hit by the curse of the merfolk and starts to turn green. Now Boom and his best friend Winger must find a way to return the foul-tempered merbaby to its mother and save Mertyle before it is too late.