Boom Broom thinks his life can't get any worse. Ever since his mother was swept away by a twister, his father has been locked in the attic, leaving twelve-year old Boom to care for his little sister Mertyle. Low on money, Boom brings home a feisty, seaweed-covered fish from the reject seafood bucket by the docks. But when the seaweed parts, Boom and Mertyle find that their supper is actually a foul-tempered merbaby.

Boom wants to use the creature to get rich, but Mertyle won't hear of it. She loves the mud-scented baby and wants to keep it for herself. But when strange things begin to happen, Boom is afraid they might be cursed! With the help of their Viking housekeeper, Halvor, and his secret society, Boom and his sister set off to find the home of the merfolk, in search of a cure. To Catch a Mermaid is a whimsical fantasy adventure reminiscent of Eva Ibbotson and Roald Dahl.