Called "When the Levee Breaks," the episode was a detour into Sam's tortured soul, mangled even further by his druglike dependence on Ruby's demon blood. At first it seemed that Ruby's blood was just making Sam stronger - after all, he's had demon blood in him since infancy - but now we know for certain that the blood is actually an addiction. Without it, Sam loses focus and power. He can't think of anything except getting another suck on Ruby's knifed-open arm.
With the apocalypse bearing down upon the world, Dean and Bobby decide Sam has to detox. He can't be demoning out when Lilith opens the 66th seal and Lucifer comes to town. So they stick Sam in Bobby's basement, inside a giant tank emblazoned with about twenty million anti-demon symbols, where Sam goes into the ugliest withdrawal you ever saw. He hallucinates that Alasdair is torturing him, has a depressing conversation with his younger self, and imagines that an evil version of Dean is calling him a monster (not entirely untrue, since real life Dean is worried that Sam is monstering out).
But perhaps most troubling, he has a vision of the boys' mother telling him that he's doing the right thing by guzzling demon blood. She tells him that sometimes Hunters have to do what seems like the wrong thing in order to make things right, and that he's the only person who can stop the apocalypse. If he can use demon blood to make himself strong enough to defeat Lilith. She points out that Dean is scared of his power, and basically tells him to go against Dean's wishes and seek out Lilith for a showdown. Unlike the other apparitions that have appeared in Sam's withdrawal, she doesn't seem to be a hallucination. For one thing, she doesn't say what he expects. And for another, we know she's crossed over from the other side before to help him. So we're left with the uneasy idea that maybe she really does think Sam should fight Dean on the demon blood question. She also says that though the demon blood is "evil," Sam can learn to control it. (Hmm, hints about Sam's future?)
And she's not the only one who starts to question the wisdom of keeping Sam locked up during End Times. Bobby keeps asking Dean if it really is a good idea to get rid of their best weapon in the fight against Lilith. The more Bobby ponders, the more he begins to think that letting Sam get stronger on demon blood might be the kind of dark bargain they need to strike to win the war against Hell. If Sam does get strong enough to fight Lilith, however, Bobby admits that he'll be changed forever. "A monster," says Dean. And that's the crux of the problem. As an angry, weepy Dean puts it to Bobby: He would rather lose the war than win it by turning his brother into a monster.
That's when Dean goes to Castiel for help. The angry-faced angel is in full dick mode, though, and completely plays Dean for a chump. First he secretly gets rebel angel Anna arrested and sent back to Heaven. Then he tells Dean that the angels think Dean is the only one who can stop the war, and arm-twists Dean into "choosing" to become an obedient servant of the Lord. Which, as Bobby puts it, makes him the angels' bitch. And we already know that being the angels' bitch is only slightly better than being the demons' bitch. So Dean has sold his soul to the angels.
Which was a really bad idea, because Castiel is double-crossing him. He slips into Bobby's basement and unlocks Sam's bonds as well as the demon-proof door (I guess it's only demon-proof, not angel-proof - Bobby should really work on his supernatural security paradigm). What the heaven is Castiel up to?
Sam pretty much immediately runs to Ruby, slicing open her arm and sucking down a giant dose of demon blood. She claims she's been off tracking Lilith's movements so they can pick her off before she opens the final seal. Turns out that only Lilith can open the 66th seal, because she's got special status as the first soul that Lucifer turned to hellishness. Take her out, and stop the apocalypse.
Unfortunately Dean figures out where Sam is hiding out with Ruby and bursts into their honeymoon suite with anti-demon knife drawn. After a scuffle, Ruby hightails it out and it's pure Dean vs. Sam action. Of course Sam overwhelms Dean with his rejuvenated powers. From the floor, his face squashed into shattered glass, Dean issues an ultimatum: "If you walk out that door, don't you come back!" Serious Winchester tragedy. No, Sam, don't do it! Dean, don't say that!
He walks out. Next week: Showdown.