Sweetness and Serpio Waymender strike across the desert, aiming for the haunt of a man named Devastation Harx, the leader of a mail-order religious group that has ensnared the persecuted, handicapped Serpio. Serpio can "see" Sweetness' secret companion, the spirit of her dead twin that she calls Little Pretty One. But when they find Devastation Harx on his huge airship, Sweetness learns that she has been betrayed -- Little Pretty One is something quite different than she had known, and Devastation Harx covets her twin for the power she can give him over the "angels" (AI's) which control the Martian climate, and, indeed, reality itself.

So Sweetness finds herself again on the run, trying to find Devastation Harx and reclaim her twin and save the world. At the same time her redoubtable Grandmother Taal has decided that family is more important than the rules of train society, and she has left the train to look for Sweetness. Add in a group of anarchic performance artists and comedians, a dream artist, a man who can cross the various alternate realities of Mars, and many more wonderful characters and landscapes, and you have the enchanting melange that makes up Ares Express.

McDonald plays archly with the idea that this is a "story", as Sweetness keeps telling herself, and occasionally uses this as license to provide neatly plotted but highly coincidental encounters and rescues for our heroes. And he has great fun with the idea of using some notion of quantum computing to maintain the "reality" of this glorious "manformed" Mars -- at the same time this is quite fun but somewhat subversive of the SF basis of the story -- so that it occupies a sometimes uneasy perch between all-out Fantasy and nominally plausible Science Fiction. But if, at times, this stretches the reader's willing suspension of disbelief to the point of severe strain, at other times it works to add delight, as with the beautiful trip, late in the book, through a series of alternate Marses, including of course those of Bradbury and Burroughs and others of the great SF chroniclers of the Red Planet. This may not be the most serious or the most significant SF novel of the past year, but it just might be the most fun. I loved it wholeheartedly.