Imagine your mind and body at war with each other on a regular basis. That's the plight of the gifted, those rare individuals have the falling sickness. They're capable of molding faerie energy, but the results leave them besieged by seizures.
Erin is one of the gifted, and after rescuing a fey lord from his sidhe tormentor, he offers to teach her to mold energy. If she learns her lessons well, she will transform into a faerie and never have a seizure again, but a decade of abuse has made her suspicious and hot-tempered. If she doesn't learn to control those impulses, she may transform into the very thing she hates.