Max, "Inky," and Luke are the youngest members of a secret underwater society dedicated to protecting the books and artifacts of their home from evil invaders. They call themselves the S.Q.U.I.D. Squad, and they never turn down a dangerous adventure. So when an army of rogue zombies finds their hidden aquatic outpost, it is up to the S.Q.U.I.D. Squad to create the powerful conduit that will save their community.

In their desperate search for the mysterious Heart of the Sea to power the Conduit, the brave, young explorers discover new allies, the turtles, to help in their quest. There's just one problem: zombies hate turtles. A full-on zombie-turtle war breaks out, threatening to wipe out the turtles and expose the secret society's mission to the vengeful, book-hating pillagers. Can the S.Q.U.I.D. Squad restore peace before all is lost?