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.
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.
Sirena, Lyla and Nixie follow Zac to school, but the school principal confronts them and takes Sirena's Moon Ring.
When a waitress spills a drink on Lyla, she rushes to the cool room -- just before transforming into a mermaid!
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.
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.
Zac's struggle with his merman secret intensifies when his girlfriend, Evie, throws him a pool party.
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.
While listening to Sirena sing, Lyla has an idea: If Sirena sings the "Enchantment Song" to Zac, he'll fall under her spell.
Against Nixie and Sirena's advice, Lyla helps Zac get to the Moon Pool. But when the entrance suddenly closes, they're trapped inside.
Fearing they'll never get the pod to return to Mako, a despondent Nixie finds herself at odds with Lyla and Sirena.
Sirena is collecting shells on the reef when she sees merman Zac. Luckily, she torpedoes away before he recognizes her.
The full moon is once again upon Mako, and the mermaids have to stop Zac from getting the all-powerful Trident.
Now that he knows Sirena, Lyla and Nixie are mermaids, Zac begins to wonder about his school principal, Rita, and why she helps them.
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.
While showing off his brand-new smartphone, Cam takes video footage of Zac swimming as a merman.
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.
The mermaids keep a close watch on Zac at the Halloween party, but he evades them with Cam's help.
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 Trident is active and dangerous, and the mermaids must take it from Zac. Knowing this, Zac searches for the perfect hiding place.
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.
When Rita's cat, Poseidon, steps through a puddle of mermaid powders, he is transformed into a person.
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.
While Lyla tries to teach Zac more about the ocean and mermaid history, Nixie refuses to trust him.
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.
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.
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 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.
When Zac has a powerful vision about his connection to Mako Island, he and the mermaids visit the island to investigate.
Erik's curiosity about the girls and Mako Island becomes a problem when he decides to investigate further.
When Evie's dad decides to take tourists on dive charters to Mako Island, the gang must find a way to change his mind.
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.
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 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.
When a young mermaid follows Ondina back to Mako Island after a mermaid council meeting, it could spell trouble with the pod.
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.
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 Mimmi loses her moon ring in the Ocean Café's cooler room, it creates havoc for David and his brother's business.
On the night of the full moon, Zac and Erik swim to Mako Island and unlock the merman chamber, revealing a startling secret.
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.
Ondina sneaks Erik into the grotto to duplicate a pearl he gave her, but she accidentally turns him into a copy of Poseidon.
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 moon 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.
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.
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.
The chamber's true purpose is revealed: to drain the magic from all mermaids. Zac risks his life to shut it down and restore the mermaids' magic.
Ondina is tasked with teaching the younger mermaids. A mermaid from China joins the pod. During a full moon ceremony, a terrifying beast emerges.
When marine park workers are sent to investigate the reported sea monster, the mermaids must create a credible explanation to stop further inquiries.
Weilan casts a knowledge transference spell on Zac to help him pass a biology test. But more than just science facts transfer from her body to his!
When Evie tries using her new moon ring to magically finish her chores, things quickly get out of control. Joe makes an upsetting announcement.
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.
Zac begs Weilan to teach him an Eastern spell few have mastered, which he hopes to use against the dragon. Joe encounters a water ball.
On the eve of the full moon, Ondina and Mimmi defy Veridia's orders. Evie becomes suspicious when she spots Zac and Weilan meeting in secret again.
After overhearing Ondina blaming her for Evie's loss, Weilan runs off. Meanwhile, Mimmi and Ondina combine their powers to try to restore Evie.
Zac realizes a shell he found has magical powers, and he accidentally sends the mermaids back in time five years, where they meet his younger self.
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.
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.
Weilan mistakenly makes tea for Rita using a magic growth accelerator. Can the mermaids cure Rita before Mrs. Trumble takes over her principal job?
A priceless artifact recovered by a deep-sea treasure hunter provides a clue as to how the mermaids can defeat the water dragon.
The vision reappears to Mimmi and Zac, this time speaking to them. Weilan learns a new detail about the dragon legend that suggests Nerissa is alive.