Maybe your character was caught masturbating or relieving herself in a forest. Maybe she stumbled into his cave hideout. Maybe she already lost a fight or was knocked unconscious and is now bound and ready for taking. Maybe she's just plopped out of the sky from another world and her powers don't work. Maybe she has no money and nowhere to stay. Maybe she's just a slut or prostitute looking for some cash or a good time. Whatever works.