After one "merely" good episode, Supernatural returns to greatness with one to please both new and especially long-time viewers. In #1.9 the ghost of Sam's mother told him, "I'm sorry". What did she mean? Well, we've waited for three years...
As the name implies, this is a great orientation episode for newer viewers who haven't been following the show and a great returning point for those who gave up because of Season 1's crappy Monster-of-the-Week episodes but have returned after hearing how great Season 4 is. And for those who've seen every episode, this offers plenty of treats.
Of the guest actors, the relatively inexperienced Amy Gumenick and Matt Cohen are just right for their roles, but obviously the star here is Mitch Pileggi, once again proving his name is not Walter Skinner. After The X Files - no hyphen! - he's done impressive roles in such different shows as West Wing, Nip/Tuck, C.S.I., Boston Legal, The Reaper, Stargate: Atlantis, Grey's Anatomy and especially in Criminal Minds. That last one and this *are* among his greatest performances.
Since the basic concept on which this episode is built upon has been done to death, the writers have wisely concentrated on characters and offer great material for great actors. The amount of revelations and twists is quite impressive. It's also so tightly scripted that there is room for only a blink-and-you'll-miss-them appearance from Sam and Ruby. Even Castiel gets only a couple scenes, and he's the driving force of this episode! Tonewise, while this episode may have plenty of laughs, it's the drama that grips you. The final act really is powerful stuff.
The one flaw here is that this is mostly just tying up loose ends and filling the holes in the back-story from the first three seasons. It may be that Kripke really knew from the beginning the stuff revealed here, but the fact that he has admitted the Angels *not* having been in his original Mytharc idea makes one wonder if all the stuff from seasons 1 to 3 is just Retconned here so that it *appears* the Angels' first appearance as late as three fifths into the Mytharc makes sense. Contrast with Babylon 5, where the Shadows and the Vorlons were in from the first season on. Also, this episode really does not advance the Big Mytharc that much and compared to the best of the best that is yet to come, it would be exaggerating to give this more than 8/10, however enjoyable it may be.