After using a hot air balloon to escape from a Confederate prison camp, Capt. Harding (Michael Craig), Cpl. Neb Nugent (Dan Jackson) and Sgt. Pencroft (Percy Herbert) crash into the ocean and are later washed up on the beach of a strange island. The men soon run afoul of the local wildlife, which include giant crabs and prehistoric creatures. To make matters worse, the island's central volcano begins to erupt, threatening to take the whole island down with it.