Vor Mako taucht eine unbekannte Meerjungfrau auf und befreit eine junge Meerjungfrau aus einer Notlage. Es ist Weilan, die aus Shanghai angereist ist und eine Weile bei Rita wohnen wird. In der darauf folgenden Vollmondnacht halten die Meerjungfrauen ihre Vollmond-Zeremonie ab. Doch plötzlich werden sie von einem furchterregenden Wasserdrachen bedroht. Zum Glück wird er von Joes Fischerboot abgelenkt und die Meerjungfrauen können fliehen. (Text: KiKA)

Cam discovers he has a power, which hes takes advantage of by using it to get him a place in the trials for the school swim team.

Ondina sneaks Erik into the grotto to create a duplicate a pearl that he gave her, but she makes the mistake of turning 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.

Sirena sees a video of a kayaker being lured by a mermaid song and becomes obsessive about David.

Surprise birthday party for Evie comes with some surprising consequences.

Mimmi gets a job at the Marine Park. There she meets Chris who works a popcorn cart and desperately wants to get in the dolphin training program. Since everyone else has a job, Ondina decides to get one too but it doesn't go so well.

Ondina is given the task of destroying the Merman Chamber on Mako Island during the full moon by Veridia. Rita tells Zac about the merman who built the chamber and why he decided not to activate it.

Ondina is faced with a choice: staying on land with Erik or returning to the pod. Erik rents out the Café to throw a party for Ondina but Mimmi is suspicious of how he is affording it. Cam pursues Carly romantically.

Mimmi is upset that Ondina has decided to stay on land. Cam takes Carly fishing near Mako Island for their first date and run into some trouble. At the Marine Park, a bratty girl sees Ondina and Mimmi transform.

Evie catches her father's cold which throws her mermaid powers into a frenzy. Things get worse when Sirena, Mimmi, and Ondina catch it. With David gone, Carly is left in charge of the busy Café. When Erik shows up late again, she fires him.

Erik learns that the stone from the trident is all he needs to start up the Merman Chamber. If activated, the chamber will drain all mermaids' powers. Cam is given the stone for safekeeping and has a tough decision to make.

Erik is determined to activate the Merman Chamber even though Ondina begs him not to. Zac feels powerless without his magic and seems to give up. The mermaids decide to try to destroy the chamber. The chamber's true purpose is revealed.