Is it "Somewhere to Elsewhere (http://archiveofourown.org/works/259331/chapters/405194)"?

Defeating the Leviathan and Raphael's armies has come at a great cost to Dean: losing Castiel, just after realizing his true feelings for the angel. When Castiel miraculously returns – human and unaware of his past – Dean must help him recover his memory on a complex and dangerous Quest. With his own mind finally in one piece, Sam finds himself falling for the kind-hearted doctor who aided Castiel and gave them all a real home. And all the while, dark forces are watching, waiting for Castiel to regain everything, plotting to rip it all from his grasp.