Baron Munchausen is informed by an astrologer cousin of his about the Selenites, an extraterrestrial race which is native to the Earth's Moon ("Selene" in Greek). The Selenites have reportedly discovered the secret to immortality. The Baron organizes an expedition to the Moon, hoping to convince them to share the secret with him. He and his companions arrive just as another alien race invades the Moon, and they are caught up in a war.