Ondina doesn't want to choose between Erik and the pod when the pod returns. To cheer her up, Erik decides to throw a party in the café.

On the night of the full moon, Zac and Erik swim to Mako Island and unlock the merman chamber, revealing a startling secret.

When Mimmi goes to the first day of the new school year, things get out of hand fast when she runs afoul of the science teacher.

Ondina adds too much jellyfish extract to Mimmi's body lotion, and the results put Principal Santos in dangerous waters at the marine park.

When the pod decides Zac is a threat and must give up his merman powers, two new mermaids, Mimmi and Ondina, are sent to make sure he obeys.

With the full moon approaching, Zac's plan to avoid Mako Island is dealt a blow when his father insists they go on their annual camping trip.

When David sees Evie in the water as a mermaid, it puts Zac and the girls' secret at risk unless they can convince David he's wrong.

With Evie now a mermaid, Mimmi and Ondina try a dangerous spell to change her back to a human form, but the results are less than ideal.

Erik's curiosity about the girls and Mako Island becomes a problem when he decides to investigate further.

When a young mermaid follows Ondina back to Mako Island after a mermaid council meeting, it could spell trouble with the pod.

When Mimmi loses her moon ring in the Ocean Café's cooler room, it creates havoc for David and his brother's business.

When Evie's dad decides to take tourists on dive charters to Mako Island, the gang must find a way to change his mind.

When Zac has a powerful vision about his connection to Mako Island, he and the mermaids visit the island to investigate.

Sirena gives Evie mermaid lessons, which rubs Ondina the wrong way, while Erik tries to get to the bottom of Zac and Cam's falling out.

Ondina sneaks Erik into the grotto to duplicate a pearl he gave her, but she accidentally turns him into a copy of Poseidon.

Zac struggles to come to terms with the news that he's a real merman, he's adopted and Mimmi is his sister.

After Cam absorbs moon pool water, he discovers he has power over water, which gets him a place in the trials for the school swim team.

When Erik dives down and touches one of the rock outcrops on Mako reef, he's transported into a sphere of endless water with no way out.

When Sirena sees a video that shows a kayaker being lured by a mermaid's song, she starts to follow David obsessively, and he falls under her spell.

A surprise birthday party is in the works for Evie, who accidentally shrinks herself and falls into an aquarium after trying on Mimmi's mood ring.

Mimmi takes a job at the marine park, where she meets Chris, a young man who is desperate to get into a dolphin training program.

On the night of the full moon, Zac and Ondina sneak away to Mako, and Ondina reveals her plan to destroy the chamber.

At the marine park, Ondina and Mimmi get into trouble when a dolphin splashes them and a little girl sees them transform into mermaids.

When Evie comes down with a fever that causes the temperature in the room to soar, Carly suspects that her friend is keeping a secret from her.

When Erik learns that the trident stone is the missing element needed to activate the merman chamber, Zac gives it to the mermaids for safekeeping.