A boy, orphaned and raised in the Frozen North by reindeer herders, boards his birth parents' sled and sets its special compass to "Home," where he uncovers a plot that threatens his mermaid godmothers and other friends of his family.

Mermaids Beattie, Mimi, and Zelda's summer on land with temporary legs is going swimmingly-until they receive a strange message ordering them to return home at once. The beloved queen of the Hidden Lagoon has been fishnapped, and some seriously bad mermaids have taken over instead! Now, the girls are the only ones who can stop them. If they're going to save the day, they'll need a clam car, some piranha-print nail polish, and possibly a talking seahorse...