The angeli come up with a new game plan that horrifies Sam, Dean and Castiel; an old friend returns. Watch Supernatural s05e18 Point of No Return. The investigation and drama continues
At the end of Supernatural season 4, Sam turned his back on his brother, left him behind and went off to do what he thought needed to be done in order to stop Lilith. In "99 Problems," it was Dean's turn to do the same thing.