A chance discovery leads American mining engineer Ben Harris and his acquaintance Harold to a lost city under the sea while searching for their friend Jill. The city is inhabited by a group of smugglers led by the cruel and tyrannical Captain, who hid there in 1803 and, due to a strange mixture of oxygen, have not aged in over a century. However, the volcano that powers the city has become unstable. The Captain now imprisons them until Ben can come up with a means of stabilizing it.