In this hit sequel to "H20: Just Add Water," secrets will be unlocked from an ancient merman chamber on Mako Island. Before the secrets fall into the wrong hands, young mermaids Ondina and Mimmi, along with merman Zac, must learn to control the force.
Zac struggles to come to terms with the news that he's a real merman, that he's adopted and Mimmi is his sister. Mimmi refuses to give up on her newfound brother, but Zac rejects her. A confrontation with some local boys causes Zac to use magic. It looks like he's about to reveal his secret until Mimmi talks him down. Zac admits to Mimmi he doesn't know who he is, but the painful confession is healing. Zac is finally able to accept his new identity.
After Cam absorbs moon pool water, he's thrilled to discover that he has power over water, which gets him a place in the trials for the school swimming team. Zac and Evie try to talk Cam out of competing, but he refuses to listen. When Ondina attempts to remove Cam's power, she causes water to be uncontrollably attracted to him. Now the race is on for the mermaids to find a solution before Cam's aquatic magnetism is revealed to the world.
Ondina sneaks Erik into the grotto to duplicate a pearl he gave her with magic she has learned from Mimmi. But Ondina accidentally turns Erik into a copy of Poseidon and when she attempts to reverse the spell, she turns the two cats into four. Three cats escape from the grotto and the mermaids must find them and change them back, otherwise Erik may be stuck as a cat forever.
Cam realizes the shape of the pedestal in the merman chamber is identical to Mako Island, and that five decorative inlays on the pedestal correspond to five rock outcrops on Mako reef. When Erik dives to one of the outcrops and touches it, he vanishes, transported into a sphere of endless water with no way out. Zac and the mermaids help search for Erik. In the process of freeing him, Zac discovers the key to activating the merman chamber.
When Sirena sees an online video that shows a kayaker being lured by a Northern mermaid's song, she becomes enchanted. She starts to follow David obsessively, even disrupting him at school during an exam. By the time Mimmi works out what has happened, David has already fallen under Sirena's spell and she is luring him into the moon pool. The mermaids are just in time to prevent disaster. When the siren spell is broken, David has no memory of what happened.
A surprise birthday party is in the works for Evie, but Zac is so tight-lipped about it that she thinks he's forgotten. Mimmi tries to cheer Evie up by letting her try on her moon ring. Evie accidentally shrinks herself and falls into an aquarium, which Zac is giving her as a present. The mermaids bring the aquarium to the party without realising that Evie is inside. Evie struggles to get Zac or the mermaids to notice her, while hiding from everyone else.
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. Meanwhile, Ondina takes a job at the cafe where she has to work with Erik - and deal with customers. Chris and Mimmi's relationship develops and Mimmi helps Chris learn to work with dolphins. Chris gets the gig and has to go away, but in a sweet farewell, Mimmi gets her first kiss.
On the night of the full moon, Zac and Ondina sneak away to Mako. Veridia has made a deal with Ondina to destroy the chamber. Zac is in the process of activating the chamber when Ondina reveals her plan and Mimmi, Sirena, Evie and Erik arrive. A hologram appears revealing that one more element is needed to complete the activation - the trident, which was lost. The chamber cannot be activated. Ondina tells Veridia, who agrees that the pod will return to Mako next full moon.
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. But hiring the café is expensive and when Rita's precious pendant disappears, Mimmi falsely accuses Erik of stealing. The party is a success - Cam and Carly even share their first kiss. Mimmi's suspicion makes Ondina realise that the pod will never accept Erik. She decides to stay on land rather than return to the pod.
Ondina and Mimmi are at odds over Ondina's decision to stay on land. At the marine park, they get into trouble when a dolphin splashes them and a little girl sees them transform into mermaids. Sirena arrives just in time to create a distraction. The adventure reminds the mermaids just how important they are to each other.
