Lyla and Nixie have left Sirena to join in a search for a new home. Out at sea, Sirena is unable to convince the mermaid council that Zac is not a threat to them. Two new mermaids, Ondina and Mimmi, head to the mainland with Sirena to try to permanently remove Zac's merman abilities. The two try to warn Zac about his impending encounter with his seventh full moon since becoming a merman but Zac insists he doesn't need their help. Undeterred, that night, Ondina and Mimmi lure Zac to the Moon Pool and attempt to use a powerful spell to remove his powers. Evie, fearing the mermaids may harm Zac, comes to his rescue, only to be transformed into a mermaid herself.
With Evie having a hard time adapting to her new life as a mermaid, Mimmi decides to use a shapeshifting spell from within Rita's grotto to turn Evie back to normal. The spell, however, doesn't work and, as a result, Evie is covered in a pink slime which keeps her stuck in her mermaid form. Zac hides Evie in the café's refrigeration room and then runs off to stop the spellcasting while Cam and Sirena keep Carly and the new waiter, Erik, from entering the room and seeing Evie with a tail.
Ondina does not trust Zac, convinced that the next full moon will make him a threat to mermaids. When Zac suggests using Sirena's moon ring to simulate a full moon to find out, he has a vision in which he glimpses a place on Mako Island never seen before. He and the mermaids head to the island to investigate. They discover a mysterious chamber on the island from Zac's vision. When Zac experiments with a stone carving in the chamber, Ondina suddenly disappears. Meanwhile, at the café, Cam is being overprotective of Evie, much to her annoyance. Erik overhears the two talking about Mako and heads to the island to explore it. Back at the chamber, Zac is able to reverse what he did causing Ondina to reappear in the forest where she is found by Erik, who seems to have an interest in her.
Erik's interest in Ondina continues to grow. He delivers a seafood order to Rita's house and uses this opportunity to talk to Ondina. While out for a walk with Erik, Ondina is accidentally sprayed and turns into a mermaid in front of him. Distraught and despondent over having her secret blown, she tearfully admits what had happened to Zac and Evie, who inform the others. Mimmi convinces Ondina to try to tell Erik to keep quiet about what he saw. When she does, he not only promises to keep her secret, but reveals that he is a merman and that his kind live primarily on land and travel frequently to keep their secret. The others are worried given the history of mermaids and mermen, but Ondina expresses trust and possible interest in Erik.
Trouble arises for the merpeople group when Evie's father, Doug, decides to take tourists on dive charters to Mako Island. Meanwhile, Erik and Zac bond with each other and quickly become friends. While out for a swim, the two narrowly avoids being run over by Doug's boat. While meeting at the café to discuss the situation, Ondina becomes upset at Erik when he doesn't seem to understand how important Mako is to her. Later, an unseen merperson destroys the divers' air compressor, putting Doug's business in jeopardy. Zac immediately accuses Ondina of damaging the air compressor but Ondina claims innocence. Zac eventually deduces that the true culprit is Erik. He confronts Erik and orders him to pay Evie's father back. Mimmi uses her ability to talk to whales to have them gather for tourists to enjoy. Afterwards, Zac apologises to Ondina and the two make peace, with Ondina finally accepting Zac as a friend.
Erik becomes curious as to why Zac and Cam are no longer friends. While Cam is on life saving duty, Erik shows up and asks him about his estrangement with Zac. Meanwhile, Sirena begins giving Evie lessons on how to use her powers. But when Sirena mentions that Rita was once her teacher, Ondina arrogantly claims Rita's teaching methods are outdated. The two play an ancient mermaid game to see who was taught their powers better. Back at the beach, Erik is accidentally splashed and is forced to rush into the ocean. Cam goes after him and is shocked to learn that Erik is a merman. When Cam paddles back to shore without Erik, his boss thinks he's lost Erik and a search and rescue operation is launched. To get Cam out of trouble, Erik stages a fake rescue, making Cam a hero. Erik now has a new friend in Cam and Zac is not happy about it.
It is the night of the full moon and the mermaids are worried about what effects the moon will have on Zac and Evie. Zac's plan to avoid the moon is dealt a blow when his father takes him and Cam on a camping trip to Mako Island. Erik decides to come on the trip as well so he can learn more the island. The three mermaids stay over at Evie's house that night to keep watch over her. During the full moon, Zac is drawn to the chamber he previously discovered with Erik following him. The moon also causes Mimmi to have a vision of Zac on Mako Island. She and Ondina heads to the island, leaving Sirena and Carly to look after Evie on their own. Evie ultimately catches a glimpse of the moon and falls under a moon spell, making her hyper and longing to be with Zac. She escapes to Mako with Sirena in pursuit. The two find Zac, Erik, Ondina, and Mimmi at the currently active chamber. Zac nearly attacks Evie, but fortunately snaps out of his own moon spell when the chamber deactivates. The next morning, Erik offers to help Zac figure out the chamber's secret, but he is reluctant to accept.
Mimmi's curiosity about the human world leads her to spend a day at Zac and Evie's school. A newly enrolled Erik is obsessed with finding out more about the merman chamber on Mako Island. He asks Mimmi for more information on the chamber, insisting that he wants to help her get her home back, but Mimmi does not trust him. Things soon get out of hand when Mimmi runs afoul of the pompous science teacher who happens to detests Rita and attempts to make both of them look bad during a lab experiment. In the process, Mimmi and Rita are splashed and Zac and Evie are forced to drag them to safety. Afterwards, Evie gives Mimmi a computer so she can continue to learn more about the human world without leaving the grotto.
Ondina meets with the mermaid council and convinces them to give her, Mimmi, and Sirena till the next full moon to remove Zac's powers. A young mermaid, named Neppy, follows Ondina back to the mainland, inspired by Ondina's dedication and wanting to help her save Mako. The girls disapprove of her presence, but she convinces them to give her some food before sending her back to the pod. While Ondina is away, Neppy wanders off and, after seeing Erik swimming, sets him adrift in a boat with Carly. She then witnesses Evie helping Erik with her powers. Later, Ondina tells Neppy the truth about Erik and Evie and admits she kept this from the mermaid council. As Neppy begins to question Ondina's dedication to Mako, she loses her temper and hurts Neppy's feelings. Neppy runs away and is forced into hiding by a group of boys. Feeling guilty, Ondina asks Zac and Evie for their help to search for her. Zac finds Neppy and, after scaring the boys away, convinces her that he is not an enemy and takes her back to Rita's grotto. Ondina apologises to Neppy and takes her back to the pod.
Evie's father accidentally drops his watch, which was a gift from his deceased wife, into the marina. When Evie dives in to try to find it, she is spotted as a mermaid by David. Amazed by what he saw, David begins telling everyone about his mermaid, putting Zac's, Erik's, and the girl's secret at risk. However, no one believes David and he is mocked in public. Sirena believes that the time has come to tell him the truth, but the other mermaids forbid it. Undeterred, David attempts to prove mermaids exist with underwater cameras. To get David off their trails, Ondina stages a hoax by having Cam pose as a mermaid in front of the cameras, publicly humiliating David, much to Sirena's chagrin. As a result, David gives up his search and decides that it is better for the mermaid to be left alone. Sirena implies to him that she will one day tell him the truth.
Rita meets with the new handsome director of the local marine park to discuss a new marine science elective and Rita catches his eye. Meanwhile Sirena and Mimmi create a body lotion that moisturises the applier's skin and makes them look younger. When Ondina tries some on and likes it, she adds too much jellyfish extract to the lotion when she tries to make more. As a result, she begins to act childish and spoiled. Rita tries some on too and also acts childish during a class tour at the marine park. The situation worsens when Rita goes for a swim, putting her secret in jeopardy. Zac, Evie, and Sirena must get Rita under control while Mimmi keeps an eye on Ondina. The lotion eventually wears off and Rita comes back to her senses, revealing that she helped a recently rescued and struggling dolphin.
When David's brother, Joe, gives him a crayfish order for the café, David is unhappy with how small they are. To help David out, Mimmi and Sirena use their magic to enlarge the crayfish, only for Mimmi to lose her moon ring in the refrigeration room. When it does not turn up, they try searching for it in Rita's fish order, but it is not found. Mimmi then realises its in the box of crayfish that Joe took back after he accuses David of stealing his larger crayfish. Mimmi sneaks into Joe's warehouse to look for it but is caught by Joe, who interrogates her and threatens to call the police. Meanwhile, while working to install a sprinkler system with Evie and her dad, Zac suddenly has a vision where he sees through Mimmi's eyes. He rushes to her aid and gets her moon ring back. Afterwards, when Mimmi reveals she also had visions about Zac, Rita silently suspects something about Zac and Mimmi's apparent connection.
With the full moon approaching, the mermaids plan to take Evie into the Moon Pool that night and teach her to channel the moon's influence, so she doesn't lose control again. Mimmi talks to Rita about the visions she's been sharing with Zac and thinks it is an ability she inherited from her mother, Nerissa, but Rita has no answers for her. Meanwhile, Cam convinces Zac to work together with Erik to unlock the merman chamber's secrets. To prevent interference from the mermaids, they have Cam cover for them. However, the girls see through Cam's deception and realise what Zac and Erik are up to. Sirena and Evie head to the Moon Pool while Ondina and Mimmi prepare to go stop Zac. They and Rita are approached by Veridia, the head of the mermaid council. She is determined to stop Zac at all costs and has them lead her to the merman chamber. Rita, worried that Veridia might harm Zac, follows them. The group confronts Zac and Erik in the chamber. As Veridia prepares to attack Zac, Rita intervenes, revealing that Zac's true mother is Nerissa and that he had always been a merman. Upon hearing this, Mimmi steps in front of Veridia, refusing to let her hurt her newfound brother. Ondina chooses to side with her best friend against Veridia. An angry Veridia brands Mimmi and Ondina as traitors and banishes them from the pod. Meanwhile, in the Moon Pool, Sirena successfully teaches Evie to control the moon's influence. Back at Rita's grotto, the news of Zac's true identity proves too much for him to bear. Feeling utterly overwhelmed, Zac goes home and talks to his parents, who appear to break down and tearfully tell him the truth.
Zac struggles to come to terms with the revelation that he is a real merman who was given up for adoption by Mimmi's mother. He isolates himself from everyone else and refuses to accept Mimmi as his sister. When a couple of jerks show up and ridicule Zac by calling him a quitter, Zac becomes very angry and begins recklessly using his powers against them. Mimmi and Evie manage to calm him down before he goes too far by reminding him that, unlike Mimmi, he has two parents who love him. Afterwards, Zac talks to his parents who tell him about how they found him and adopted him as a baby. The next day, Zac finally accepts Mimmi as his sister.
When Cam comes into contact with water from the Moon Pool, he absorbs the water into his body and gains the ability to control water. The water also enhances Cam's swimming and earns him a shot in the school swim team trials. Zac, Evie, and Erik are worried about what effects the Moon Pool water will have on Cam and tries to convince him not to swim but Cam does not listen. Their worries are proven correct when the water soon begins to have unpredictable effects. The mermaids must figure out how to remove the Moon Pool water from Cam's body before he swims in the trials.
Erik finds a pearl and gives it to Ondina as a gift. When Erik says he wishes he could find more to make a bracelet for her, Ondina sneaks him into Rita's grotto and uses a duplication spell to create more pearls. Poseidon is in the room, and Erik takes an immediate liking to him. Things soon get out of hand when Ondina accidentally spills some magic powder on Erik, turning him into a second Poseidon. Ondina makes things worse when she tries to reverse the spell and inadvertently creates two more Poseidons for a total of four cats. When three of the cats wander off, the girls must find them, figure out which cat is Erik, and reverse the spell before Erik permanently becomes a cat.
Ondina and Erik are spending increasingly more time together causing Mimmi to feel neglected. Meanwhile, Cam studies a photo of the stone pedestal from the merman chamber and discovers that the stone carving matches a satellite photo of Mako Island. He also suspects that the symbols on the carving correspond to the outcrops on the reef surrounding the island. He shares his thoughts with Erik and the two decide to investigate the outcrops on the reef. The two find a symbol of a trident on the outcrop but when Erik touches it, he is teleported into a realm of endless water with no way out. Cam rushes back to land and asks Zac and the three mermaids for their help to rescue Erik. While out on the island, Ondina and Mimmi get into a heated argument about their different opinions of Erik causing a rift between them. The group soon realize that they must risk activating the chamber to rescue Erik. Mimmi is able to figure out the order of the pedestal's code and Erik is set free when Zac activates the chamber. Afterwards, Ondina and Mimmi reconcile with Ondina promising never to let Erik come between their friendship.
Sirena sees a video of a mermaid enchanting a kayaker with an enchantment song and it has unexpected consequences. Sirena falls under an enchantment that causes her to care for nothing more than being with David and she begins following him obsessively. Mimmi manages to obtain the video and watches it. She realizes Sirena is under the spell of a northern mermaid siren song that affects men and southern mermaids. Mimmi, being a northern mermaid, is immune to the effects of the song. That night, Sirena enchants David with the song and takes him to the Moon Pool to make him her man. Fortunately, Ondina and Mimmi are able to break the spell by singing a counter enchantment song. Sirena comes back to her senses and Ondina uses her moon ring to send David back to his boat with no memory of what happened to him that night.
Zac plans a surprise party for Evie's birthday without telling her. He acts like he has forgotten it's her birthday which sends her into a depression. To cheer Evie up, Mimmi lets her try on her moon ring. When Mimmi is out of the room, the ring accidentally activates and shrinks Evie. While shrunken, Evie falls into a fish bowl that Zac intends to give to her as a gift. The girls take the fish bowl to the party unaware that Evie is inside. At the party, Evie tries unsuccessfully to get someone's attention. When Evie fails to show up and Zac learns how he unintentionally made her feel, he becomes depressed and believes she is now refusing to talk to him. As Mimmi tries to comfort him, he tells her how special Evie is to him. Evie overhears this and is greatly touched. She also manages to finally get Zac's attention. The mermaids restores Evie to normal size and she expresses her gratitude to Zac for his beautiful words. The gang then returns to the party to enjoy themselves.
In need of money, Mimmi decides to take on a job at the marine park. She meets a young man named Chris who seems overly strict and rubs her the wrong way at first. She learns, however, that he has a passion for dolphins and dreams of working with them in the dolphin training program. Chris's attempts to gain approval for the program fall on deaf ears so Mimmi decides to help him out. She has him close his eyes while she gets in the water and successfully helps him to connect with a dolphin. He and Mimmi then share a kiss. When Chris shows what he can do to the marine park's director, he is accepted into the dolphin training program. Although Mimmi is disappointed to learn that Chris will be away in America for 3 months, she is happy that she helped him achieve his dream and Chris expresses his gratitude to her before leaving. Meanwhile Ondina gets hired at the café as a back-up waitress, but her poor work ethic ultimately gets her fired.
The next full moon is approaching and Zac will have an opportunity to learn about what the merman chamber does. Ondina is worried about what might happen if Zac activates it but Mimmi refuses to go against her brother's wishes and intends to let him activate the chamber. At the Moon Pool, Ondina runs into Veridia who has come up with a plan to destroy the chamber by having Ondina use her power to turn the chamber's power against itself when Zac activates it. Ondina agrees under the condition that she, Mimmi, and Sirena be accepted back into the pod and Veridia agrees. Later, however, Zac changes his mind about activating the chamber when Rita informs him that he and Mimmi are descended from the merman who built the chamber and refused to activate it after falling in love with a mermaid. Ondina meets with Zac and manipulates him into continuing with his original plan to learn the chamber's purpose. Zac agrees under the condition that he goes alone. That night, Zac sneaks away to the merman chamber with Ondina following him. When Mimmi learns that Zac had lied through a vision, she and the others rushes to the chamber. When Zac enters the code into the chamber, it seemingly recreates the trident. As Zac reaches for it, Ondina stops him and prepares to carry out Veridia's orders. The others intervene, warning Ondina that destroying the chamber may destroy the island itself. Zac reaches for the trident again but it turns out to be a hologram that signifies the trident is necessary to start the chamber. With the trident already destroyed, the chamber's threat is seemingly over. The next day, Veridia lifts the girl's banishment and informs them that the pod will return by the next full moon.
With her banishment lifted, Ondina is eager to return to the pod. Her excitement is short-lived, however, when Erik brings up the possibility that returning to the pod will mean she may never be able to see him again. Ondina becomes depressed, not wanting to have to choose between Erik or the pod. Erik decides to throw her a party at the café to cheer her up. When a pendant of Rita's goes missing, Mimmi accuses Erik of stealing it to make money for the party. Erik does not deny it, but is hurt by the accusation. When the pendant turns up and Mimmi learns that Erik had actually gotten the money using his own secret stash of treasure he collected out at sea, she apologizes to him. Meanwhile, Cam and Carly have trouble admitting their true feelings for each other. When the party ends, Cam musters up the courage and admits his true feelings for Carly. They share a kiss and become a couple. Afterwards, Ondina concludes that if Mimmi could not trust Erik, neither will Veridia or the pod. She then makes the difficult decision to leave the pod and stay on land to be with Erik.
Mimmi is heartbroken by Ondina's decision to leave the pod and refuses to speak to her. While working at the marine park, a little girl steals a toy dolphin and Mimmi pursues her. Ondina comes to the park to talk to Mimmi about her decision to stay the land. However, both of them ends up getting splashed by the dolphins. The girls transform and are forced to hide, but not without the little girl seeing them. The girl draws in a huge crowd to show everyone the mermaids. Sirena is able to get them out of trouble by telling the crowd that Mimmi and Ondina are actresses in mermaid costumes for an attraction. She then gets Mimmi and Ondina to reconcile. Meanwhile Cam and Carly have their first date fishing near Mako Island. Their boat's anchor gets stuck forcing them to swim to the island and await rescue. Much to Cam's humiliation, any attempt he makes to impress Carly, she does better. Zac arrives and helps Cam by freeing the anchor and letting him take the credit to impress her. Back at Rita's grotto, the girls decide that they must find a way to get the pod to see that humans and mermen can be trusted so they can live in both worlds.
David goes out for some training and leaves Carly in charge of the cafe. When Erik is late for work again, Carly becomes fed up with his frequent tardiness and fires him. Meanwhile, Evie catches a cold and it causes her to lose control of her powers. Whenever she sneezes she lights things on fire. Carly witnesses one of the sneezes and becomes suspicious that Evie is hiding something from her. The mermaids try to come up with a cure, but it merely changes the effect of the sneeze from fire to ice. Eventually, all three mermaids also catch the cold. At the café, Evie sneezes and freezes Cam in front of Carly. Zac and the girls then flee to Evie's house. When Carly later shows up, demanding answers, she finally discovers Zac's and the girls' secret. Carly is upset at first, but she soon forgives Evie and agrees to keep the group's secret.
Erik learns from Cam that while the trident was broken, the trident's stone that powers it wasn't. Erik concludes that it is the stone rather than the trident that can activate the merman chamber. Sirena reveals to Zac, Ondina, and Mimmi that she has kept the stone hidden at Rita's grotto the entire time. Although the stone has seemingly lost its power, it becomes active again when Zac touches it due to his stronger connection to Mako Island. Rita realises that the chamber's purpose is to drain the magic from the moon pool and all mermaids by amplifying the effect of the stone. Zac takes the stone and goes to hide it in a safe place. When Erik realizes Zac has the stone, he confronts him over. Erik believes there has to be more to the chamber's purpose besides destroying mermaids and becomes angry when Zac refuses to activate the chamber. He demands custody of the stone but Zac refuses to hand it over. Realizing the stone is not safe with him, Zac turns to Cam and asks him to hide it. Erik tries to bribe Cam into giving him the stone but Cam refuses to lose his friendship with Zac twice and, instead, gives it back to the mermaids. Erik later finds the stone in Rita's grotto and is confronted by Mimmi and Zac. Erik uses the stone to drain Zac's powers. With the stone now in his possession and Zac's powers inside it, Erik will be able to activate the chamber.
With the full moon approaching, Ondina tries to warn Erik not to activate the merman chamber, but Erik refuses to listen even to her. A now powerless Zac has seemingly given up despite encouragement from Mimmi, Evie, and Cam. Zac later musters up the courage and follows Erik to the chamber to try and stop him. The girls also race to the chamber to stop Erik. Zac fights with Erik for the stone but he shakes him off and finally activates the chamber. The girls attempt to destroy the stone with their moon rings, but it merely absorbs their power. The chamber drains the magic from the moon pool and slowly begins to kill the mermaids except Evie. Horrified by what he's done, Erik tries to deactivate the chamber but is unsuccessful. Evie arrives and helps an injured Zac to his feet. He successfully deactivates the chamber and saves the mermaids but seemingly dies in the process. Desperate to save her brother, Mimmi grabs the stone and places it over Zac's heart. The stone restores Zac's power and revives him. The stone then crumbles, ending the chamber's threat to mermaids for good. The next day, Ondina breaks up with Erik and he leaves the Gold Coast. Sirena reveals she's a mermaid to David. Veridia meets the group at Rita's grotto and praises Zac for his heroic actions. She makes him and Evie honorary members of the pod and gives Evie her own moon ring. The group joyfully swims out to greet the pod as they return to Mako at last.