The narrative begins at Castle Trump, where the young baron is engrossed in a rare fifteenth-century manuscript written by the philosopher Don Fum. This ancient text posits that the interior of the earth is hollow and inhabited. Inspired by this notion and urged on by his restless foster-brother, the dog Bulger, the baron resolves to set out in search of the subterranean portals. His preparations are blessed by his parents, and he travels across Russia to the northern wastes of the Urals.
After parting with government guides, the baron engages a surly teamster named Ivan to traverse the rugged mountain foothills. Ivan proves treacherous and attempts to murder the young explorer, but Bulger saves his master with quick cunning. Left without transport, the baron presses on afoot and with the aid of mountain cattle discovers the legendary Giants' World. Descending into this vast abyss, he encounters a narrow opening known as Polyphemus' Funnel, which he successfully navigates by covering himself in black lead to slip through the tight space.
Emerging onto a magnificent marble highway, the travelers enter the land of the Mikkamenkies, a transparent folk whose translucent bodies reveal their beating hearts. Here the baron is received by Queen Galaxa and solves a royal succession crisis involving the true identity of Princess Crystallina. The narrative continues as the pair journey further underground, encountering the Formifolk—blind and deaf ant-people who communicate through delicate pressure-languages and read raised silver script with their feet.
Continuing their subterranean voyage in a shell boat, they brave dark, monster-infested rivers before reaching the frigid realm of the Koltykwerps. Ruled by King Gelidus and his daughter Schneeboule, this icy domain features a magnificent palace lit by a massive rock-crystal lens that focuses distant surface sunlight. The baron encounters frozen prehistoric creatures and witnesses the curious revival of a chimpanzee named Fuffcoojah.
The tale crescendos as the travelers make a breakneck, ninety-mile descent down an icy slope on a massive block of moving ice. This thrilling escape lands them safely in a new and mysterious region of the under-world, where further wonders await discovery.
