When an unwelcome figure from the girls' past returns to the Gold Coast, the mermaids are faced with some unpleasant realities. Caught up in the middle, Zane Bennett is just trying to protect the friends he hopes to one day regain - and the choices he makes set them all on a course they never expected. A story about a merman. Set after final episode of Season 3.

Danger is coming, more than anybody has imagined. Evie realises she is the only one left who is not a 'real' mermaid. Erik is looking for a real home. Zac wants to forget everything that has happened, but the pull of the moon sets him on a dark path. Meanwhile, the merman chamber on Mako holds a secret that should never be uncovered. Set post-2X13.