Dr. Abner Perry, an elderly Victorian inventor, and his youthful sponsor, David Innes, test their “Iron Mole” digging machine by burrowing under a Welsh hill. When something goes wrong with the angle of descent, the machine starts drilling straight down. Both men fight to change course until they faint from the intense heat of the Earth’s inner mantle. Nearing the Earth’s core, they awaken to find they are drilling through ice instead of molten iron. Abner turns the contraption upward, but they hit an underground lake that causes the Iron Mole to lose power as they bore into a giant cave. There, the men find themselves in a subterranean world full of prehistoric plants and a pink sky. In the cave, a creature with a dinosaur's body and bird's head pursues the men. They escape, only to be taken prisoner by hominids who shackle them to a group of other enslaved humans. Marching through the jungle, David speaks to Dia, a woman dressed in animal skins, and notices Hoojah, another male slave, looking at him. Dia warns that Hoojah is not to be trusted. Later, while resting, Abner explains that the “sky” is actually magma twenty miles above. Dia says their captors are the Sagoths, soldiers of the Mahars, who rule the underground Kingdom of Pellucidar. The Sagoths force the prisoners to their feet, but before they can get up, Hoojah grabs Dia’s wrist. When David defends her, Dia and the rest of the prisoners stare at him with hostility. The prisoners are marched to the city of Phutra, where the entrance is a tunnel protected by a wall of flowing lava that the Sagoths control with a lever. When David realizes that Dia is missing, an elderly slave named Ghak says that Hoojah ran off with her, adding that David insulted Dia by not claiming her as his mate after fighting Hoojah. Ghak explains that Dia is a princess who is hiding from Jubal, a man with the strength of many men. The captives are marched into a misty chamber where a dozen Pterodactyl-like Mahars telepathically command the Sagoths to separate the prisoners into groups. While David is sent into a tunnel to chisel stones, Abner becomes a scribe to copy the Mahars’ books. Later, Abner tells David that the Mahars’ power comes from lava; however lava threatens the city, so the slaves are forced to build canals to control the lava’s flow. The next day, David is whipped by a Sagoth, but he kills his tormentor with his hammer and escapes from the city. When David steals a piece of meat from a fire, Ra, another human, attacks him. During their struggle, a man-eating plant grabs Ra. However, David rescues Ra by using Ra’s knife to cut the plant’s tendrils. When David asks why humans do not fight the Mahars, Ra leads him into a tunnel that opens above a huge antechamber. Below, four women are immobilized by the Mahars’ telepathic control. One of the Mahars swoops down and feeds on a woman's flesh. As David and Ra search for Abner, they are quickly recaptured and Ra is chained to a stone pillar. As other prisoners watch, David is forced to fight a dinosaur with a wooden spear and he defeats the beast by stabbing its ear. One of the Mahars attacks David, but Ra breaks free of his chains and uses the shackles to strangle the monster. The prisoners rise up, kill their Sagoth guards and try to escape, only to find a wall of falling lava blocking them. Abner leads David though a small passageway to the Mahars' hatchery, a large, egg-like container that is suspended over a pool of molten lava. Abner explains the Mahars use this machine to control the fire curtain. Outside, David and Abner see Hoojah chasing Dia, who breaks off the pursuit when a fire-breathing toad appears. David attempts to fight the creature off with a rock, but Abner saves them by shooting the beast with a bow and arrow. Dia fears Hoojah will return and kill David, but David relieves her anxiety with a kiss. Soon, Hoojah returns with Jubal, an enormous man carrying a mace. Abner raises his bow, but David defeats Jubal himself with a spear. After Hoojah runs off, Ghak and Ra appear to tell David that Dia is now his woman, but David declines to accept his mate until they successfully conquer the Mahars. Much later, after uniting the warring human tribes of Pellucidar and training them in archery, David explains his battle plan to a war council. David bids Dia farewell and heads off to battle. However, Dia spots and follows Hoojah, who is captured by the Sagoths. Dia warns Abner that Hoojah may tell the Mahars about the impending battle. Since it is too late to call off the attack, Abner leads Dia and Ra further into the tunnels, where they are stopped by Sagoths. Ra escapes and makes his way into the hatchery, but he is severely wounded before he kills the guards. Although barely able to move, Ra turns a capstan that shuts off the lava flow to the fire curtain, then redirects the lava to destroy the hatchery. With the fire curtain down, David’s troops fight their way into the main chamber but find it empty. Meanwhile, Abner and Dia are brought into the feeding chamber where the Mahars use their mind control to freeze them in place. David arrives with his troops, but a wooden gate blocks them. Abner’s archers enter the chamber from above, open fire, and ovecome the Mahars. David breaks through the gate and kills the last of the Mahars with a spear. Meanwhile, the lava causes the tunnels to explode. David leads the human slaves out of the caves, but attempts to turn back when he realizes Ra is not with them. However, Abner stops David just as the caves and the city of Phutra blow up. Later, a victory celebration is held and Abner fixes the Iron Mole. Before leaving, David announces that, according to custom, he is claiming Dia as his wife, but Dia explains she is now queen and must stay with her people. David sadly boards the machine and he and Abner drill to the Earth’s surface. However, instead of coming up in Wales, they break through the lawn of the White House in Washington, D.C.