Ted quickly gives us some backstory and we soon learn that Ralph has been kidnapped by Tyrannus, a Hulk villain who lives at the center of the Earth, and who has a grudge against the Mole Man.
They are provided with access to Ralph's experimental tunnelling machine, which, rather than being a clone of the Thunderbirds' Mole, instead vaporises a hole through the rock with a giant laser. They arrive soon enough in a giant underground cavern, described by Iceman self-consciously as "straight ouf of a Jules Verne yarn". He's right - it is as incongrous in the science-based world of the X-Men as magic gems that make you an unstoppable avatar of a demon - or a pre-Ice Age redoubt in the Antarctic.
And it's also pretty weird, on the face of it, to brainwash your enemies by finding the river Lethe (which, in Greek legend, induced forgetfulness in all who drank from it), vaporising it into mists, while making sure you aren't affected yourself by using an oxygen mask. Which the Mole Man does to the X-Men. But no, that's not even the half of it. The Mole Man and Tyrannus are having a stand-off with giant androids. Moley has built himself a 30-foot tall one made of solid diamond, so Tyrannus is focing Ralph Roberts to make him one out of, well, cobalt.
The X-Men win in the end, natch, but they've spent nearly 2 whole days, got very confused about what genre of story they are in, and still haven't made any progress in finding the Professor.