Vivian is still trying to figure out how she fits into this new world of myth and magic. Having uncovered her parent's secrets, now she's not sure she wants to have anything to do with the crazy life they've been hiding from her. Unfortunately, the folks at RISE have a different idea. There's a new bad guy in town. Poseidon, the king of the ocean, is looking for the key and, since she's the one who lost it, it's up to her to find it. If the power-hungry water king gets his hands on the key he'll wield its power and flood the earth. With the help of her friends, and a few land loving mermaids, Vivian will need to travel from the city streets of Tokyo to the deepest depths of the ocean. Can she stay ahead of Poseidon, help the mermaid resistance, and, finally, bring the key to safety?