When Lagoona Blue, her ghoulfriends, and Gill Webber try to save Toralei Stripe after she accidentally falls in the Monster High pool, they are all sucked through a vortex that turn them into fish. They have been brought there by Posea Reef, the daughter of Poseidon, who has a mission for Lagonna: dive deep and face her biggest fear. Until she succeeds, Lagoona and her friends are trapped in a magical underwater world and cannot return to Monster...