Also known (to me) as The Astonishing Dorothy and the Wizard in Oz.
Anyway, long story short, Dorothy falls through a crack in the Earth caused by a massive earthquake in Northern California (undoubtedly inspired by the devastating quake that hit San Francisco in 1906). Coincidentally (Dorothy is big on coincidence), the Wonderful Wizard of Oz (aka Oscar Zoroaster Phadrig Isaac Norman Henkel Emmannuel Ambroise Diggs) floats down into the Earth too. He got lost in a balloon again, presumably because after decades of farting around in the Emerald City, he forgot how they worked.
Dorothy and Wizard subsequently have many subterranean adventures before Deux Ex Ozma whisks them to Oz, where the Wizard takes up permanent residence. Ozma throws the Wizard a parade, presumably totally cool with the whole "the Wizard kidnapped me as an infant and usurped my throne" thing. That's very big of her.
Note that at least the problem with the Deux Ex Machina of Dorothy suddenly remembering that Ozma can always bring her to Oz on Saturday afternoons is fixed in Eric Shanower's scripts for the Marvel comics. He actually foreshadows Ozma's intervention, first by setting up - at the end of "Ozma of Oz" - that Ozma will watch for Dorothy on Saturday afternoons and bring her to Oz if a signal is made, and then by reiterating this fact to the Wizard early at the beginning of this story. This scenario sets up a ticking clock for the heroes, who know they merely have to survive long enough for Ozma to notice they're about to die.
Shanower still leaves in the massive inconsistency of the Wizard's baby-napping, however. In the "Land of Oz" comic, it's still very clear that the Wizard took the baby Ozma to Mombi, but in the "Dorothy and the Wizard in Oz" comic, Ozma tells the Wizard that she was born into Mombi's care. I guess it just wouldn't be Oz without the occasional massive plot hole.
Also, "invisible bears" is the most terrifying concept I've ever heard of.
Author: L. Frank Baum • Year: 1908 • Info:Tor Online