In Asia, archaeologist Dr. Roger Bentley leads a team, including Dr. Paul Stewart, Dr. Jud Bellamin and Etienne Lafarge, on a dig for Sumerian artifacts. They discover a 5,000-year-old stone tablet bearing cuneiform writing, which describes the dedication of a temple to the goddess Ishtar. During a sudden earth tremor, the tablet is broken, but the next day a worker uncovers an oil lamp whose inscription tells the tale of Sharu, who constructed an ark to rescue his people from a massive flood. The ark came to rest on top of the mountain Kuitara, and Sharu and his people built a civilization there. Ignoring his colleagues’ misgivings, Roger arranges for an excavation on top of the mountain, and they set out on the treacherous winter climb. Led by guide Nazar, they struggle up the mountain, narrowly surviving an avalanche, and after finally reaching the summit, they encounter the ruins of a Sumerian temple. Just then, however, Paul falls through a crack in the ground, disappearing into a deep shaft. The others go after him and, after a long descent, find Paul dead. They prepare to climb out, but when Nazar taps a loose rope tether, a rock slide commences, killing the guide and trapping the other three inside the mountain. Noting that the caves they are in have been previously excavated, Roger leads them on a search for another way out, while Etienne grows progressively weak and nervous. Finally, they reach a huge, dimly lit cavern, and discover a piece of a statue of Ishtar, which indicates to Roger that they have stumbled into Sharu’s ancient, buried city. The men lie down to rest, but are quickly attacked by mole-like creatures, who drag them through the soil into a cave prison. There, skeletons of some unfortunate mole people are shackled to the wall, and the scientists examine the skeletons' large skulls and long, clawlike fingers. Soon, the men are beckoned to a temple filled with the underground civilization’s other inhabitants, Sumerians whose 3,000-year-old ancestors were trapped underground after an earthquake, and who have remained faithful to their ancient customs. After centuries of living inside the mountain, the Sumerians have evolved into albinos who cannot tolerate direct light. The archaeologists are brought before the head priest, Elinu, and the king, Sharu, who believes the strangers to be evil spirits and sentences them to death. The guards attack, but after Roger shines his flashlight on them, they fall in agony, and Sharu declares the strangers to be holy messengers bearing light, or what they call “the fire of Ishtar.” Roger and Jud flee into the tunnels and stumble upon the mole creatures’ hellish lair, where they are whipped and starved by the Sumerians. A panicked Etienne attracts the attention of a mole person, who kills him and then runs off. Elinu then finds Roger and Jud in the tunnels and, although he remains skeptical of their divinity, invites them to a feast. There, Roger and Jud learn the history of the Sumerians, who, because they can grow only enough mushrooms to support 150 of their people, must kill the rest. Roger stops the guards from cruelly whipping a non-albino, Adel, whose relatively dark skin has condemned her to slavery. The next day, Roger and Jud tour the working facilities and marvel at the society’s inventiveness, while at the same time Elinu rouses his priests to steal the flashlight and stage a coup against the weak king. Later, the Americans again stop a guard from beating a mole person, unwittingly causing the death of one of the guards. Over the next few days, while Roger searches for a way out of the cave, and begins to fall in love with gentle Adel, the mole people grow more rebellious. Sharu asks Roger for the flashlight to use as a weapon against the creatures, but Roger refuses, and instead Sharu orders three of the creatures whipped to death. Roger and Jud drain the flashlight’s battery in order to put a stop to the beating, after which one of the mole people tries to communicate its gratitude. The creatures slow down their food production, and in response, Sharu orders three women to be sacrificed to the light of Ishtar, which is actually a crack in the surface through which sunlight pours in, instantly burning the albinos’ flesh. Soon after, Elinu discovers Etienne’s body, proving that Roger and Jud are also mortal, and orders them killed. To restrain them, the men are fed poisonous mushrooms and then pushed into the lighted area, which, unknown to the Sumerians, poses no threat to them. Meanwhile, Adel notifies the mole people, who attack the Sumerians and gain control, and then help Adel into the lighted area. Roger, Jud and Adel easily climb the shaft to the surface of the mountain. As they leave the area, however, an earth tremor occurs, killing Adel and, once again, completely sealing the entrance to the city below.