Briana, daughter of the Archimage had vanished.
Fig Newton summoned a demon to help find her.
Too bad it was a succubus, a seductive devourer of men.
Some time had passed since Briana and Fig had saved the Earth from sinister aliens (as told in the Archimage's Fourth Daughter). Together, they struggled to put together a "magic show" that would provide them enough money on which to get by.
Fig was infatuated by Briana, but, as she explained, she loved him like a brother. Briana was homesick, a fish out of water on the Earth and longing for home on a distant world, one for which the crafts of thuaumaturgy, alchemy, magic, sorcery, and wizardry were common.
Briana vanishes, and Fig's pursuit to find her takes him to the Gates of Hell in central Asia, the catacombs of Paris, cold, drab modern Moscow, and a place to which no mortal had ever travelled before.