Everywhere Gray and Barkley swim, fish are whispering of a growing threat, of a shark named Finnivus who will eat anything or anyone, and won't stop until he has seized control of the entire ocean. Finnivus has an armada of vicious battle sharks to help him expand his empire, and Gray realizes that it's up to him to save the Big Blue. But first he must train with Takiza, the wise and mysterious fighting fish who has promised to teach his young apprentice...

Gray turns out not to be the last megalodon in the ocean after all, and he explores his family's heritage, but when the conniving mako Velenka escapes from her underwater prison, peace in the Big Blue is in danger.