Ondina and Mimmi, two young mermaids from the Mako pod, are determined to break Zac's connection to Mako Island and reclaim Mako for the pod. They run away and join Sirena on land, where they soon learn that Zac's connection to Mako is stronger than they thought.

Their job gets harder when they accidentally turn Zac's girlfriend Evie into a mermaid. Then there's Erik, the handsome new boy in town who reveals he is a merman and develops a romance with Ondina.

But their biggest challenge is the ancient merman chamber that Zac discovers on Mako. Zac must learn to control the mysterious force that draws him to the chamber, while the mermaids must work with Zac to unlock its secrets – before it falls into the wrong hands. Along the way, friendships will be tested and stunning truths will be revealed.