Gilly loves to flap her flippers in Lighthouse Bay...until a mysterious monster starts menacing the market! It swipes Gilly's flippers, snatches fish, and nabs nets. So Gilly hatches a plan: she is going to catch the sneaky, rotten monster! With the help of her goldfish, Finnegan, and the townspeople, she builds a trap.

But when things go awry, Gilly discovers she and the monster may have been after the same thing all along--a friend.