Young mermaids of the Mako Island pod, Sirena, Lyla and Nixie have the task of keeping trespassers off the magical island. On a full moon, they fail to stop a land boy, Zac, from falling into the sacred Moon Pool, and the power of the moon transforms him into a merman! As punishment the mermaids are banished from their pod as it flees Mako.

To rejoin the pod, Sirena, Lyla and Nixie must turn Zac from part-time merman to full-time land boy. With the power of Sirena’s Moon Ring, the mermaids give themselves legs and walk on land to begin their task. But when Zac spectacularly uses his new-found powers for good, the mermaids realise their plan may be simple, but not easy.

Attempting to capture Zac in his natural environment, Sirena, Lyla and Nixie follow him to school. When their plan fails, the school principal, Rita Santos, confronts them and confiscates Sirena’s Moon Ring. In trying to get the Moon Ring back, the mermaids uncover Rita’s secret - she too is a mermaid! Begrudgingly, Rita agrees to help the mermaids while they’re on land.

While Nixie and Sirena discover shopping, Lyla looks for Zac at The Ocean Café. When a waitress accidentally spills a drink on Lyla, she rushes to the cool room - just before secretly transforming into a mermaid! Trapped and unable to regain her legs, Lyla’s tail is on the line.

Relentless 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 inexperienced with their powers, the Moon Ring creates a blizzard in Rita’s house! Lyla and Sirena fall ill with Snow Rash and it’s up to Nixie to find a mermaid cure before a land doctor takes them to hospital for tests.

Joe, who delivers seafood to the Ocean Café, has a new boat and takes his brother David out for a spin. His fish-finder locates a dolphin, but it’s not like any dolphin he’s seen before. Out having a swim, Nixie is in trouble as Joe decides to chase the ‘dolphin’. While Lyla and Sirena awkwardly try to befriend Zac on land, Nixie gets trapped under a jetty and desperately needs their help!

Zac’s struggle with his merman secret intensifies when his girlfriend, Evie, throws him a pool party. At the party, Cam pushes Zac into the pool and the mermaids see him in his transformed state. When they help him keep his secret from the rest of the guests, he finds new allies, and the mermaids are one step closer to getting back to their pod.

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. Then they can return to their pod. Despite their best efforts to keep him indoors out of the moonlight, the power of the full moon is too strong and Zac is drawn to the Moon Pool, where he gains even more power!

While listening to Sirena sing, Lyla has an idea. If Sirena sings the Enchantment Song to Zac, he’ll fall under her spell and they’ll have the chance to take his powers. It’s a risky plan, but Lyla talks Sirena into it. Except the song misfires, and the Café’s waiter David falls under Nixie’s spell. Lyla and Sirena must find a way to reverse it.

Against Nixie and Sirena’s advice, Lyla helps Zac get to the Moon Pool. But when the Moon Pool entrance suddenly closes Lyla and Zac are trapped inside! Zac discovers a new chamber and discovers an ancient prize - a Trident. Before he can obtain it, Sirena and Nixie arrive to save them. Now Zac’s sights are set on the mysterious Trident. The mermaids don’t know what will happen if he gets it.

Fearing they’ll never get the pod to return to Mako, a despondent Nixie finds herself at odds with Lyla and Sirena. Seeking solitude, she meets Ben, a young boy whose family is leaving their home to live on a yacht. Things couldn’t be worse for Ben. But as Nixie shows him the magic of mermaids, Ben looks forward to life at sea and Nixie regains her hope.

Sirena is collecting shells on the reef when she sees merman Zac. Luckily, she torpedoes away before he recognises her. But it’s not enough - Zac saw her tail and now believes there’s another merman in the Mako waters. Without giving away their secret, the mermaids must put Zac off their scent and keep him away from the island.

The full moon is once again upon Mako and the mermaids have to stop Zac from getting the all-powerful Trident. But the full moon draws Zac to Mako where he fights off the mermaids and seizes the Trident. In their struggle, Zac sees the mermaids’ true identity, tails and all! But the Trident vanishes from his grasp. Feeling betrayed, Zac poses a new threat that the mermaids never expected.

Now that he knows Sirena, Lyla and Nixie are mermaids, Zac begins to wonder about his school principal, Rita, and why she helps the mermaids. Zac quickly uncovers the truth - Rita is also a mermaid. He uses this insight to get out of doing homework, but Sirena, Lyla and Nixie won’t stand to see Rita pushed around. The mermaids convince Zac to leave Rita out of their problems.

When David invites Sirena to sing with him at the Ocean Café, their connection grows. Seeing Sirena’s attachment to a land boy, Lyla and Nixie fear that Sirena might not want to return to the sea. To stop David and Sirena growing closer, Lyla and Nixie sabotage Sirena’s audition, only to realise what a terrible mistake they’ve made.

While showing off his brand new smart phone, Cam takes video footage of Zac swimming as a merman. During an argument with Nixie in the café, Cam accidentally loses his phone. The mermaids must work with Zac and Cam to retrieve the phone before anyone sees the footage of Zac and discovers his secret.

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! But Evie isn’t about to hand it over, and Zac knows the ring holds great power. Over lunch with his parents, Zac and the mermaids scuffle for possession of the Moon Ring, and accidentally cast a spell on Rita.

A full moon. A Halloween party at the café. The perfect chance for Zac to seize the Trident. The mermaids keep a close watch on Zac at the party, but with Cam’s help, he evades them. The mermaids aren’t sure how Zac did it, but they’re certain he has unlocked the Trident from the chamber. He denies it to the mermaids, but Zac’s quietly thrilled. Finally, the Trident is his!

The Trident is every bit as powerful as Zac imagined. Only, he can’t get it to work! Zac wonders if the Trident needs to be activated by moonlight and breaks into Rita’s secret Grotto. While the mermaids try to take the Trident from him, Sirena’s Moon Ring triggers a glow from the Trident. Zac escapes with the now activated Trident ... and the ability to wield its mighty power.

The Trident is active and dangerous, and the mermaids must take it from Zac. Knowing this, Zac searches for the perfect hiding place and decides on an old warehouse. When the mermaids discover it’s hiding place and try to take the Trident, the ancient weapon counteracts their powers. Zac can’t wait to see what else it can do!

Sirena’s sister Aquata returns to Mako with wonderful news - Sirena can return to the pod! Unfortunately, the invitation isn’t extended to Lyla and Nixie. When Zac finds the Moon Pool he tries to kick Lyla and Nixie out. In a showdown between Zac and the mermaids, Lyla manages to seize the Trident from Zac, and hides it in a cave deep in the ocean. Sirena decides to stay and defend the island.

When Rita’s Cat, Poseidon steps through a puddle of mermaid powders, he is transformed into a person. A girl - identical to Evie! A boy Cat, in a real girl’s body, Cat-Evie’s behaviour raises many eyebrows. Zac and the mermaids must work together to reverse the spell before Cat-Evie and the real Evie cross paths.

Zac’s continued search for the Trident has Lyla worried. Although she hid it well, Zac manages to find the Trident. But Lyla is right behind him, and as the two battle for possession, a flash of energy knocks Lyla unconscious. Zac rushes her to Rita’s place for help, and uses the Trident to save her. Rattled and guilty, Zac realises just how dangerous a weapon it is.

For the first time, there are no secrets or differences between Zac and the mermaids. While Lyla tries to teach him more about the ocean and mermaid/merman history, Nixie refuses to trust him. Suddenly, Nixie finds she has more in common with Cam than with Sirena and Lyla.

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 the Moon Pool entrance suddenly closes, Evie is trapped underwater with her oxygen running out! Zac and the other mermaids come to rescue Evie, and their true mermaid identities are revealed to her. Meanwhile, Cam escapes with the Trident.