Disaster seems imminent when scientists discover that the planet Mongo is about to crash into Earth. Luckily, heroic young Flash Gordon is on hand to lead an investigative mission into outer space and onto the speedily approaching planet. There, he and his best girl, Dale, who is along for the ride, learn that Ming, the devious ruler of Mongo, has purposely put the planet on a collision course with Earth, and only Flash can stop him.

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!

本片剧情接续《小美人鱼》电影，主要是由多伦多和雪梨两地的迪士尼动画部门所共同制作。

爱丽儿和埃里克王子在陆地成亲后有了一个女儿，名字叫做美乐蒂。这个小女孩并不知道自己有一半的美人鱼血统，她一心向往美人鱼的海底世界，希望能够变成美人鱼徜徉在大海之中，跟她妈妈当年正好相反！海巫婆乌苏拉的妹妹莫吉娜一心要为她的姐姐复仇，于是就利用美乐蒂，说可以帮忙把她变成美人鱼。乐不可支的美乐蒂就这样跟着莫吉娜前往了海底。丢失了爱女的爱丽儿和埃里克异常焦急。为了寻找爱女下落，爱丽儿决定重返大海。

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.