Een uitgebluste reddingsduiker krijgt nog één klus voor zijn kiezen: zijn vrienden redden van een monsterlijke megalodon, waarvan men lang dacht dat hij was uitgestorven.

Researchers on the undersea lab Aquatica have genetically altered the brains of captive sharks to develop a cure for Alzheimer's disease. But there's an unexpected side effect: the sharks got smarter, faster, and more dangerous. After a big storm damages their remote research facility, they must fight for their lives.

On a volcanic island near the Kingdom of Hetvia rules Count Dakkar, a benevolent leader and scientist who has eliminated class distinction among the island's inhabitants. Dakkar, his sister Sonia and her fiance, engineer Nicolai Roget have designed a submarine which Roget pilots on its initial voyage just before the island is overrun by Baron Falon, despotic ruler of Hetvia. Falon sets out after Roget in a second submarine and the two craft, diving to the ocean's floor, discover a strange land populated by dragons, giant squid and an eerie undiscovered humanoid race.