With the restoration of the Eastern merpeople pod and Nerissa, years have passed and things seem quiet for the merpeople and humans who have had experiences with Mako Island. That is until the full moon begins to restore Evie McLaren's mermaid powers and she takes Zac to Mako to heed a desperate call from Veridia, the Head of the Mako Pod's Mermaid Council. It appears that several merpeople have gone rogue to form a cult dedicated to summoning Dagon's Trident.
It turns out that Erik has resurfaced and is looking to find an apprentice who will help the mermen take revenge on humans and mermaids for their ostracization - and that apprentice so happens to be Charlotte Watsford. To combat the Cult of Dagon, thirteen swords of power must be wielded to destroy the monsters of Dagon. Zac and Evie cannot battle the Cult alone, as they will need help from merpeople past and present to be able to save the Earth from a prehistoric menace to all.
