Flash Gordon, Dale Arden e Dr. Alexis Zarkov visitam o planeta Mongo para frustrar os planos malignos do Imperador Ming, o Impiedoso, que colocou seu planeta em rota de colisão com a Terra.

A group of 21st-century colonists inhabit an underwater city called Pacifica. Originally intended as a purely scientific installation, the U. S. government wants to stash all its gold reserves from Fort Knox there, along with a fantastic new radioactive element. The brother of Pacifica's returning former commander plans to steal the gold and on top of that, the city faces destruction by an asteroid from outer space!

Ariel e seu príncipe encantado Eric, vivem felizes, após a sereia filha do Rei dos Mares, ter abdicado de sua condição para tornar-se mulher. Eles já têm até uma bela menina chamada Melody. Mas, a terrível Morgana arquiteta um plano diabólico para vingar-se de Ariel e Eric, obrigando-os a retornar aos mares para derrotá-la, contando com a ajuda do caranguejo Sebastião e do peixe Linguado, amigos do primeiro filme.

Survivors of a sinking ship are rescued by Captain Nemo and his submarine crew. They are taken to an underwater city where they may spend the rest of 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.

An engineer, a psychologist and several other disparate types take part in an experiment to see if people can live for extended periods of time in a city built under the ocean.