Evie tries to adjust to her new situation with Zac's help. Sirena pressures Ondina and Mimmi to change Evie back. Carly and Erik become suspicious when Cam blocks the entrance to the cool room where Sirena is hiding Evie.
Sirena uses her moon ring to simulate a full moon to test it's effect on Zac. He has visions involving his connection to Mako Island. He and the girls go there to investigate. After overhearing about the island, Erik goes to explore it.
Zac tries to convince Evie that there are good things about having a tail. Erik takes a seafood delivery to Rita's house and questions Ondina about Mako Island. Secrets are revealed.
Lifeguard Cam is left in charge of the beach and his boss' son. Sirena gives Evie lessons on using her mermaid powers much to the disdain of Ondina. Erik questions Cam about his lost friendship with Zac while dealing with bothersome Ned.
It's the first day of the new school year and Mimmi wants to know all about it. She follows Rita there and sits in on a science class where she makes quite an impression the teacher. Erik shows Cam the photos he took in the Merman Chamber.
Ondina meets with the mermaid council to discuss Zac and Mako Island but she doesn't tell them about Erik and Evie. A young mermaid named Neptina follows her back and volunteers to help. Neppy finds Erik and follows him causing mischief.
When David sees Evie in the water as a mermaid, it puts Zac and the girls' secret at risk when he starts telling anyone who will listen about seeing a mermaid.
Sirena and Mimmi make a lotion that helps keep you young. Ondina uses it all so she makes a slapdash batch to replace it. Rita uses some before going on a class trip. Rita begins acting childish as a result and could end up in hot water.
Joe gives David small crayfish for the Café so Mimmi uses magic to make them larger but loses her Moon Ring in the cool room. Zac and Evie help Doug install a sprinkler system. Zac has a vision of Mimmi and goes to her rescue.
On the night of the full moon, Sirena takes Evie to the Moon Pool to help her control the moon spell. Veridia, Ondina, and Mimmi go to the Merman Chamber where Zac and Erik are trying to unlock it. Secrets are revealed.
