During the Civil War, reports of a sea monster attacking ships alarm the Navy. An investigation finds the monster is a submarine, the "Nautilus," commanded by vengeance-seeking Capt. Nemo (Allen Holubar), who was once an Indian prince. Years earlier, adventurer Charles Denver (William Welsh) caused the suicide of Nemo's wife and the disappearance of his daughter. Nemo tracks Denver to a distant island where, unknown to him, a wild native girl matching his daughter's description is also hiding.