S9, E10: Dean's best chance of saving Sam lies with Crowley. Devastated after Kevin's death, Dean vows to find the angel that killed him, not only to free Sam, but to seek revenge. Castiel arrives at the bunker and suggests they try to separate Sam's brain from the angel's so they can talk to Sam and have him expel Gadreel. However, there's one small problem: the only person who knows how to do that is Crowley.