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 Lagoona: 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 High. The ghouls--except for Toralei!--think it's totally fintastic being fishified, but it's not all fun and games. There's a dangerous, scary Kraken lurking at the bottom of the sea. Will Lagoona and the ghouls sink or swim?