The angels have fallen. Castiel lost his grace, and is now a human. He experiences hunger, cold, need, but still pondered at the thought of love. Dean takes him under his wing and helps him understand his new way of life; every aspect. Even though his grace is lost, the affair between a man and an angel is forbidden. Not to mention, an angel and a Winchester. In addition to the internal conflicts between the two, they also have to worry about the bounty put on Castiel's head by the other fallen angels.

Forbidden things are like a beautiful hypnotizing magnet. Once you get pulled in, it's hard to pull back. But maybe I liked being pulled in, especially if it was by her.