As Dr. Abner Perry (Peter Cushing) and David Innes pilot their innovative Iron Mole drilling machine through the rugged Welsh mountains, a series of catastrophic events begin to unfold. Their test run is suddenly disrupted when the intense heat from the magma crust surrounding their device causes them to lose consciousness, relinquishing control to the menacing forces dwelling beneath the Earth's surface. Upon regaining consciousness, Perry and Innes discover they are plunging further into the Earth's core, where the temperature begins to drop, allowing them to stabilize themselves.
After emerging onto a bizarre terrain filled with flora and fauna reminiscent of prehistoric times, their initial awe is swiftly overshadowed as they become captives of the Sagoths, ape-like beings intent on enslaving every human tribe in this extraordinary realm. These Sagoths are subjugated to the telepathic Mahars, sinister flying reptiles equipped with mind control abilities.
David's growing empathy for the imprisoned Princess Dia (Caroline Munro) galvanizes him to incite a revolt within the oppressive magma mines. Captured alongside Ra, another rebellious slave, both men are soon faced with execution in a battle arena. A giant dinosaur is unleashed to attack them, but as the spectators roar, it is Perry's voice that echoes through the crowd, guiding David to expose the creature's weak spot. Together, David and Ra not only defeat this monstrous threat but also overcome a Mahar, earning the respect and admiration of the oppressed multi-tribal masses.
Motivated by their newfound courage, Innes, Ra, and Dia unite to inspire the subjugated tribes, encouraging them to collaborate in their fight for freedom. Meanwhile, Perry dedicates his efforts to teaching the humans how to construct and skillfully utilize bows and arrows. As the human slaves band together against their oppressors, they ultimately succeed in overthrowing the Mahars and subduing their minions.
With their mission complete, Perry and Innes prepare to return to the world above, escorting Princess Dia back with them. Nevertheless, despite her deep affections for David, Dia is convinced she cannot adapt to life above ground, just as he struggles to fit into her world. A heartbroken Innes ultimately resigns to accept her decision, opting to return to the UK with only Perry.
In an extraordinary twist of fate, a minor calculation error results in the Iron Mole breaking through the ground beneath the White House lawn, much to the astonishment of two Secret Service agents stationed nearby. These typically composed law enforcement officials, attired in their neat black uniforms, are left dumbfounded, staring in disbelief as the metallic giant erupts from the earth like a colossal mechanical creature.
