Spoilers below, read with caution!
I don't know if I've ever given any show a 10 but in my book this episode gets one. It kept me totally immersed in the story from beginning to end and of course left me hanging for next weeks episode to tie up all the loose ends and there are some doozies. Roman has kidnapped the brilliant kid who has the tablet and is forcing him to translate the tablet or else Roman will kill the kids mom. Meanwhile Roman Enterprises is putting drugged corn syrup in all their food products which is causing the general population to become totally complacent for future manipulation and processing by Roman Enterprises.
Sam and Dean are attempting to determine how they will get the items they need to destroy the Leviathans so they summon Crowley as they will require his blood as well. They also find out from Crowley where the alpha vampire is who is the third person they need blood from. Crowley tells them he will only give his blood when they have everything else they need to destroy the Leviathans.
So they hit the road to find this vampire and stumble into his house finding a seemingly innocent victim of the vampires. They rescue her and she tells them the vampire king has moved to his safe house. So they "rescue" her and hit the road again to find the vampire king with her assisting in finding him. Bobby continues to assist them but as push comes to shove they need to prevent Bobby from getting further involved.
Locating the vampire kings haven and attempting to break into the vampires house they fund they've been outed by the "victim" and are captured by the vampire king. He is all ready to kill them when they explain to him he is simply another victim in the Leviathans plans. The vampire doesn't initially believe them and when Edgar, Roman's henchman, arrives to meet with the vampire he has Sam and Dean locked away. While negotiating with Edgar the vampire king asks what the plans are for the Leviathans and the vampires. Edgar replies the vamps are scum just like the humans and everything else and a fight breaks out between them. Meanwhile Sam and Dean have escaped and enter the room as the fight is starting. Dean attacks Edgar only to be stopped by Edgar but Sam comes up from behind beheading the Leviathan. The vampire king then realizes what is going to happen if the Leviathans aren't stopped and gives Sam and Dean his blood to help with defeating the Leviathans. Back at the motel Bobby has entered the body of a maid and opened the safe containing his whiskey flask so that he can take control of himself. Sam and Dean arrive back at the motel to find Bobby gone as Bobby has headed off to deal with Roman.
This episode ends with Roman summoning Crowley and telling him they have much to discuss.
This episode is so great because it carries the Leviathan story towards its ultimate end. I enjoyed the prediaments that Sam and Dean got into and Bobby's continued hatred of Roman. I think this is one of the best acted episodes for our two heros and felt the story was extremely well written and really started fitting all the pieces together for next weeks denouement.