Clownfish are disappearing from the bay, so the girls spring into action to find out why. A mysterious diver could be the culprit.

When the stone circle causes Cleo to forget that she can turn into a mermaid, Emma and Rikki scramble to keep her from transforming in public.

Lewis promises to tell his Valentine's Day sweetheart, Abby, a big secret; Cleo is jealous and scared that he will give away the girls' mermaid identities.

The girls and Bobby the dolphin untangle a mystery when Bobby's twin, Robby, is captured by someone in a mysterious speedboat.

When her father goes to sea on a mission, Cleo's worries begin to affect her dreams. Her nightmares could be trying to tell her something.

Tired of the racket Miriam and Zane are making at their beach party, Bernie leaves his cave giving the Vandals a chance to move right in.

To protect the White Mermaid, the girls guide explorer Stanley Finch to the underwater stone circle, whose magic makes Stanley think he's a pirate.

When a spell forces Rikki to remain a mermaid until the next full moon, she has to find a way to lie low, and underwater, for three full days.

Lewis is thrilled when the university hosts an underwater robot competition, but the girls fret over the safety of their marine friends.

Emma's younger brother catches a glimpse of her mermaid tail and thinks he's seen a sea monster, and soon the whole town is in an uproar.

The mermaids turn to a cranky giant octopus for help when the Vandals invade the bay with a horde of stinging jellyfish.

Having played one too many practical jokes, Teddy can't get anyone to believe his warning that an underwater volcano is about to erupt

Lewis joins a famous explorer in a high-tech submarine to hunt for a sunken treasure ship, but a mishap strands them at the bottom of the sea.