"Brenden Vetch found the Od School of Magic beneath a cobbler's shoe on a busy street in the ancient city of Kelior."
He came to be their gardener -- Od herself had sent him. Od had founded the school many centuries ago, and given a labyrinth, a small one, and left behind a map, except that no one who entered the labyrinth found the same path, so it wasn't very useful. The school soldiers on, training wizards for the royal service and curbing those whose magics are more dangerous.
Meanwhile, the wizard Tyramin has set up in the Twilight Quarter and is giving shows. The quarter warden, Ardeth, is assigned the task of discovering whether it's really wizardry or just tricks. The Princess Sulys is making her own life about the corners of court, since no one pays much attention to her, including studying little magic tricks, nothing serious enough for the school, with her foreign grandmother. And a new crop of students has enrolled in the city.
And there is the little question of what, if anything, Od is doing and where she is. She does vanish for decades on end and not reappear for long when she does. (However did she know they needed a gardener?)
And it all weaves together into a tale of wonders in McKillip's gorgeously lyrical prose.