Uriel and Castiel forcibly recruit Dean to torture the captive Alastair. They need to find how, who or what is killing angels as one of another 66 Seals in which both Dean and his former captor play mind games against each other to break one another down. Sam is forced to call upon Ruby to help him find Dean before he emotionally cracks, while the rouge angel, Anna Milton, also returns to ask Castiel where he stands on his position as an angel of the Lord - does he obey all heavenly rules without question? Written by
Did You Know?
The title of this episode comes from the expression, "How many angels can dance on the head of a pin?" Some early theologians discussed this question, though some believe it may have been merely a debating exercise, rather than a true academic inquiry for a concrete answer. However, other theologians used this expression mockingly, as an example of questions they deemed useless because it had no practical use. Therefore it became a metaphor for an exercise in futility. Since Dean was forced to torture Alistair without anything useful coming from the act, the metaphor fits the episode - Dean's descent back into torturing was, in fact, an exercise in futility. See more
The righteous man who begins it is the only one who can finish it.
References Little Shop of Horrors