Plot advancement, background enlightenment and Crowley after the break heh.
Sam goes on a hunt solo because Dean says he is too busy chasing Abbadon. Dean is actually terrified of how the mark or cain is making him feel and goes on a drink binge. Sam's hunt leads him to meeting an ex nun who actually met his grand-father and by her inaction was the result of Abbadon infiltrating and killing the entire Men of Letter organization.
Dean calls Crowley who sensing Dean's hesitation sets up an encounter with a 'hunter' who is really a demon in Crowleys employ, it works an Dean is back on course. Sam kills a demon in Abbadon's employ, finds out Abbadon is stealing innocent peoples souls and turning them in to a mindless loyal army to her.
Regarding the past? The Winchesters grandfather and friend showed up in a small town persuing a hunt. They killed two demons, the Abbadon demon and one of her other minions knocked grandfather out, Abbadon took over his friend and that's how she got in to the Men of Letter. The surviving nun? She kept her mouth shut out of fear and shortly after left the church entirely. She's the old lady who tells Sam about the demon in town stealing souls.
Really good episode!