I'm really disappointed, actually. That wan't very good at all. That was just... ugh. I have a lot of complaints. But let's start first with the plot. God, I'm so annoyed. Grr.
Dean is working a case with Rudy, another hunter, but then Dean tells Rudy to get lost and that he'll handle it himself. Rudy calls Sam and tells him where Dean is, since he can tell something is terribly wrong with him. Dean finds a vampire's nest, and allows Rudy to get stabbed and killed before he take out the vampires, killing them ruthlessly. He goes back to his hotel room and has a meltdown, destroying the whole room. Then, he leaves a note that says "she's all yours" with the keys to the impala for Sam to find, and he takes off.
Sam leaves Cas with Rowena, telling him to please find a way to enact the spell that will remove the Mark of Cain. He tracks Dean to the hotel room, finds the destruction and the note, and then... he gets a call from Dean. Dean has summoned Death, and Death has offered to remove him from Earth to protect everybody from the Mark of Cain. He can't kill Dean, and he can't take the Mark off of Dean unless Dean passes it to somebody else. Since Dean is unwilling to do that, he'll have to continue, immortal, with the Mark for the rest of eternity. Turns out, if the Mark is not tethered to a person, it releases something called the Darkness, which existed before the world was created. God created a Mark to tether the Darkness, and he gave it to Lucifer. Lucifer then gave it to Cain, who gave it to Dean. Dean calls Sam and tells him where he is. It's time to say goodbye.