Union soldiers escape in a hot air balloon from a Confederate prison camp during the American Civil War. They crash-land in the ocean. They are washed ashore on an unknown island where gigantic animals live. The animals are the result of experiments by Captain Nemo. He is an unknown benefactor to the castaways as they struggle to survive on the island. A volcano threatens to erupt. The stranded group escapes from the island on a pirate ship. The volcano destroys the island. The animated "monsters" that the castaways encounter are a giant crab, a giant flightless bird, giant bees, and a giant cephalopod.