This episode is all about Sam.
We start out with Sam running from cops and goes into a bar. There he meets a girl and tells her that he doesn't remember who he is. She decides to help him. He gets flashes and finds a hotel and goes there. There are fake IDs and he has another flash and finds out he's Sam. Then decides to go visit Bobby and drives the Impala. Someone is trying to kill him and it turns out to be himself. Turns out he's in a coma from the wall being torn down by Castiel. He did it so they would stop fighting him. Sam has to face and kill himself in order to put his soul back together. One is Sam without his soul and he shoots him. Then we find out the bartender that's been with him is actually one he shot when a demon of some sort had her at knife point and said he would kill her and soulless Sam just shoots her to get to that demon. He then has to face Sam that remembers hell. He ends up stabbing him and gets all his soul back.
Meanwhile, Dean and Bobby decide to go after Castiel and Crowley since they have blood from the Dr to open up a gate to Purgatory. Castiel has the blood and Crowley and Raphael come and take the blood. They perform the ritual with the blood and it doesn't work. Dean & Bobby are watching nearby. Castiel comes and he had given them fake blood and he has all the souls in him and is all powerful. He kills Raphael and lets Crowley go (he has other plans for him). Dean tries to convince him to release the souls but he won't. Sam comes and stabs Castiel but he has he can't be killed since he's no longer an angel but the new God. He tells them to either bow down to him or face destruction.
And that is how the season ends.