The fifth season picked up right where the fourth season left off. Sam killed Lilith, who just happened to be the final seal to release Lucifer. Things look bad for Sam and Dean, but then a bright light happens and they are whisked away from the danger.
They go see their buddy the prophet Chuck. He tells them that Castiel was killed by the other angels for disobedience. They learn that they must retrieve Michael’s sword to defeat Lucifer. Some demons come and fight Sam and Dean, but it ends well.
They head to the place where they believe the sword to be, but Zach and his angels are there. They inform them that Dean is the sword. Dean thinks they mean he is a weapon, but in truth he is the vessel. Angels must have permission to take someone’s body (same with Lucifer).
Dean refuses and Zach begins torturing them. A bright light shows up and Castiel kills two of the angels. Zach asks how he came back, and Castiel says something like “how do you think?”
The episode ends with Lucifer convincing his vessel to allow him to take control. Dean informing Sam that he cannot forgive him and that they will never be like they were.
The season seems to be going towards a three way battle, Angels vs. Lucifer, with Sam, Dean, and Castiel searching for God who could stop everything. Apparently it was God who saved Sam and Dean when Lucifer returned.