The film centers around Union soldiers escaping in a gas balloon from a Confederate prison camp during the American Civil War. They end up crashing in the ocean, only to find themselves washed up on an unknown island where gigantic animals abound. It would later be revealed that the animals were the result of experiments by the presumed-dead Captain Nemo. He has been an unknown benefactor to the castaways as they struggled to survive on the island. The island's volcano threatens to erupt. After a skirmish with pirates, the stranded group manages to escape from the island on the pirates' ship as the volcano destroys the island.