WHILE MERMAIDS HIDE ON ATLANTIS...

...THE MAINLAND IS DESTROYED...

Cordelia, Wade, and friends return to the mainland to find a nuclear junkyard. With the human population drastically reduced, most blame the mermaids and selachii for deserting them and hiding the sanctuary of Atlantis.

When a new prophecy says they must unite with a third, a human, Wade and Cordelia open their land to the survivors. But sharing Atlantis comes with a price. Prejudices are rife and many are out for revenge. And the third in their trio is anything but cooperative.

Distracted by the politics on the island, Cordelia misses the creeping return of Wade's old flame, Stephanie. Seeking vengeance, Stephanie transforms into a sea witch and evokes the hound of the ocean, a terrible beast from the ancient depths that kills without prejudice.

With Atlantis threatened, Cordelia's powers misfiring, and a betrayer among their ranks, mermaids and selachii are once again at risk of extinction. Only if they can remain united with the humans and vanquish the hound and Stephanie will they have a chance to reclaim their island.