While exploring a new cavern, Cave Carson and crew watch as a humanoid alien falls and dies. Using materials left by the alien, the threesome use the Mighty Mole to descend and ultimately come upon an alien city lit by a sphere in the sky. The aliens tell Cave that a faction, led by Xanon, want to work their way to the surface to flee a deadly gas they’ve been exposed to, not caring that it could mean the death of all humanity. Bulldozer is captured.
With the help of Arhun’s cave beasts, Cave and the Underworlders attack Xanon in the city. Bulldozer is freed as the city is destroyed. Having escaped, Xanon orders his men to unleash the power of the artificial sun.
Xanon has the artificial sun doggedly follow Cave and crew as they try to evade it with the Mighty Mole. Meanwhile Xanon captures Alrud and his men and prepares to use the elevators to ascend to the surface and spread their disease. Cave uses the Mole’s thermo-ray to make the sun go supernova, destroying what’s left of the underground civilization and all its inhabitants, including Xanon and his men. Shielded by the Mighty Mole, Cave returns to the surface.