The King is a bit... eccentric - a sort of Calligula type, but in his employ is the most powerful wizard in the land. He is continuously ordering the wizard to perform humiliating, monotonous and pointless tasks.
What is to stop the more sensible wizard from getting sick of this, and just refusing to carry out the King's orders, or quitting his post, or doing something to the King?
The story is set in a kind of medieval-with-magic type of world, and the wizard's power is such that he could easily hold off the King's guards, certainly for long enough to escape if the King turned on him.
Just to explain the magic is like high-level D&D (not the same but this is just to give an idea). The wizard is not a slave of the King, he's a high ranking member of staff, a bit like Merlin, who is loyal but not to the point of putting up with this continuous abuse. The idea is that a King who didn't have a powerful wizard working for him would be highly vulnerable.