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.
Castiel thought that losing his grace was the worst thing that could ever happen to him, forcing him to be mortal until he could once again become divine.
He was dead wrong.
There are worse things, things unimaginable to even him. As he runs from his actions and what he has done, he realizes that angry creatures are on the prowl and a man with green eyes might be his only hope.