When Cam takes the Trident to Mako behind Nixie's back, the mermaids realize that he intends to turn himself into a merman so he can rule Mako Island.

Behind Sirena and Lyla's backs, Nixie helps Cam get into the Moon Pool to unlock the Trident. But Evie insists on coming along for the dive.

While Lyla tries to teach Zac more about the ocean and mermaid history, Nixie refuses to trust him.

Zac manages to find the Trident, but Lyla is right behind him. As the two battle for possession, a flash of energy knocks Lyla unconscious.

When Rita's cat, Poseidon, steps through a puddle of mermaid powders, he is transformed into a person.

Sirena's sister, Aquata, returns to Mako with great news: Sirena can return to the pod. Unfortunately, the invitation isn't extended to Lyla and Nixie.

The Trident is active and dangerous, and the mermaids must take it from Zac. Knowing this, Zac searches for the perfect hiding place.

The Trident is every bit as powerful as Zac imagined -- but he can't get it to work. He wonders if the Trident needs to be activated by moonlight.

The mermaids keep a close watch on Zac at the Halloween party, but he evades them with Cam's help.

Zac finds a Moon Ring on the ocean floor, and when he gives the ring to Evie, the mermaids know it's no ordinary ring -- they have to get it back.

While showing off his brand-new smartphone, Cam takes video footage of Zac swimming as a merman.

When David invites Sirena to sing with him at the café, their connection grows. Lyla and Nixie fear that she might not want to return to the sea.

Now that he knows Sirena, Lyla and Nixie are mermaids, Zac begins to wonder about his school principal, Rita, and why she helps them.

The full moon is once again upon Mako, and the mermaids have to stop Zac from getting the all-powerful Trident.

Sirena is collecting shells on the reef when she sees merman Zac. Luckily, she torpedoes away before he recognizes her.

Fearing they'll never get the pod to return to Mako, a despondent Nixie finds herself at odds with Lyla and Sirena.

Against Nixie and Sirena's advice, Lyla helps Zac get to the Moon Pool. But when the entrance suddenly closes, they're trapped inside.

While listening to Sirena sing, Lyla has an idea: If Sirena sings the "Enchantment Song" to Zac, he'll fall under her spell.

It's a full moon, and the mermaids think that if they keep Zac out of the moonlight, he'll lose his powers and return to his land-dwelling self.

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

Joe has a new boat and takes it out for a spin. His fish finder locates a dolphin, but it's not like any dolphin he's seen before.

Rain keeps the mermaids indoors and unable to look for Zac. Lyla and Sirena try to use the Moon Ring to stop the rain, but they create a blizzard.

When a waitress spills a drink on Lyla, she rushes to the cool room -- just before transforming into a mermaid!

Sirena, Lyla and Nixie follow Zac to school, but the school principal confronts them and takes Sirena's Moon Ring.

Sirena, Lyla and Nixie must turn Zac back into a land boy. Using Sirena's Moon Ring, they give themselves legs and walk on land to begin their task.

During a full moon, a land boy named Zac falls into the Moon Pool, and the power of the moon transforms him into a merman.