The novel centers on a young, independent, unnamed, wealthy traveler (the narrator), who visits a friend, a mining engineer. They explore a natural chasm in a mine which has been exposed by an exploratory shaft. The narrator reaches the bottom of the chasm safely, but the rope breaks and his friend is killed. The narrator finds his way into a subterranean world occupied by beings who seem to resemble angels. He befriends the first being he meets, who guides him around a city that is reminiscent of ancient Egyptian architecture. The explorer meets his host's wife, two sons and daughter who learn to speak English by way of a makeshift dictionary during which the narrator unconsciously teaches them the language. His guide comes towards him, and he and his daughter, Zee, explain who they are and how they function.
The hero discovers that these beings, who call themselves Vril-ya, have great telepathic and other parapsychological abilities, such as being able to transmit information, get rid of pain, and put others to sleep. The narrator is offended by the idea that the Vril-ya are better adapted to learn about him than he is to learn about them. Nevertheless, the guide (who turns out to be a magistrate) and his son Taee behave kindly towards him.
The narrator soon discovers that the Vril-ya are descendants of an antediluvian civilization called the Ana, who live in networks of subterranean caverns linked by tunnels. Originally surface dwellers, they had fled underground thousands of years previously to escape a massive flood and gained greater power by facing and dominating the harsh conditions of the Earth. The place where the narrator descended housed 12,000 families, one of the largest groups. Their society was a technologically supported Utopia, chief among their tools being an "all-permeating fluid" called "Vril", a latent source of energy that the spiritually elevated hosts are able to master through training of their will, to a degree that depended on their hereditary constitution. This mastery gave them access to an extraordinary force that could be controlled at will. It is this fluid that the Vril-ya employed to communicate with the narrator. The powers of the Vril included the ability to heal, change, and destroy beings and things; the destructive powers in particular were powerful, allowing a few young Vril-ya children to destroy entire cities if necessary.
Men (called An, pronounced "Arn") and women (called Gy-ei, pronounced "Jy-ei") have equal rights. The women are as strong as, if not stronger than the men. They marry for just three years, after which they are free to remarry or to remain single.
Their religion posits the existence of a superior being but does not dwell on his nature. The Vril-ya believe in the permanence of life, which according to them is not destroyed but merely changes form.
The narrator adopts the attire of his hosts and begins also to adopt their customs. Zee falls in love with him and tells her father, who orders Taee to kill him with his staff. Eventually both Taee and Zee conspire against such a command, and Zee leads the narrator through the same chasm which he first descended. Returning to the surface, he warns that in time the Vril-ya will run out of habitable space underground and will claim the surface of the Earth, destroying mankind in the process, if necessary.
Output: What warning does the narrator give Zee once they reach the surface?