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.

The vision reappears to Mimi and Zac, this time speaking to them. Weilan learns a new detail about the dragon legend that suggests Nerissa is alive.

Cam betrays Nixie after planning to become a merman and rule Mako.

Mimmi and Zac are startled by a mysterious yet strangely familiar vision. Cam's dual role as Carly's boss and boyfriend becomes difficult to manage.

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

While listening to Sirena sing, Lyla has an idea. If Sirena sings The Enchantment Song to Zac, he will fall under her spell. Everything goes south when David hears the song and falls in love with Nixie.

Nixie agrees to help Cam unlock the Trident, but Evie wants to come along.

Ondina is tasked with teaching the younger mermaids. A mermaid from China joins the pod. During a full moon ceremony, a terrifying beast emerges.

Lyla and Sirena attempt to use the Moon Ring to stop the rain, but instead make it snow indoors.

Zac's struggle with his merman secret intensifies when his girlfriend, Evie decides to throw him a pool party.

Zac leads Lyla to the Trident; they fight for control of it and Lyla is knocked unconscious.

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.

Weilan comes up with the idea of using a Chinese puzzle box to trap the sea dragon. Cam panics when Carly suggests they take dance lessons.

Evie's moon ring lesson with Ondina takes an unexpected turn. At the marine park, Mimmi discovers the reason for a rescued dolphin's distress.

While Ondina teaches the little mermaids to walk on land, her fastest-learning student wanders off and soon runs into Chris and Karl on the beach.

When Mimmi and Chris go on a date at the beach, Ondina goes to great lengths to protect her. Cam tries to lure more upscale customers to the cafe.

Nixie and Sirena won't help Lyla try to "zap" Zac's powers, so Lyla decides to do it alone. This misfires when a waitress spills a drink on Lyla and Lyla rushes to hide in the refrigeration room before she transforms into a mermaid! When she can't dry her tail, Nixie and Sirena must save her before she is discovered!

The girls fear their pod will never return. A despondent Nixie gives "advice" to a young boy named Ben. Zac researches tridents online and Cam snoops. Rita tells Lyla and Sirena about the trident and a pod of mermen that rose up eons ago.

Rita and the girls worry about the upcoming full moon's effect on Zac. Nixie thinks they should let Zac find the trident so they can take it from him and hide it. Lyla thinks they should come clean with him so he'll in hopes that he'll understand and stay away from it. While Lyla warns Zac to keep away from Mako Island during the full moon, Evie sees them holding hands and breaks up with him. Cam discovers that the girls know Zac's secret but feels uneasy about them helping him. Under the light of the full moon, Zac goes after the trident with the girls close behind.

Cam gets a new phone and films Zac swimming.

After finding a Moon Ring on the sea floor, Zac gives it to Evie. Now, the mermaids must get it back.

Lyla teaches Zac about mer history and the ocean but Nixie doesn't trust him.

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.