I am a little behind in recapping (ok, a lot), so I am going to give just the basics...well, as much as I am actually capable of doing that. I was really dreading watching this episode. All the hype told me that there was going to be a third Winchester brother introduced; apparently John let one slip though in the years of hard hunting that followed his wife's death. I couldn't see how the dynamic of the Winchesters could get any better, so I watched this episode with a great deal of reluctance.
The show started with a blonde woman running though a house, obviously I terror. She locks herself in the bedroom and blocks the door...only to be dragged under the bed and that is the end of her.
Our boys get a phone call. It is from Adam Milligan looking for John...his dad. Kid checks out, his existence holds up to Sam's research and coincides with pages of John's diary that had been ripped out. S & D meet him in a diner and he passes all the "evil detection" tests that Dean lays out for him: silver cutlery, holy water. Adam met John when he was 12 and for a several years, John made it down to see him on his birthday. He is 19 now and called for John because his mom (the blonde) disappeared. At this point he thinks our boys are old coworkers of Dad's and they were all mechanics (like Sam told his teacher in the flashback episode). John had met the mom when he was investigating 17 grave robberies from that town in 1990 and then last month bodies started disappearing and there have also been deaths.
At the house, Dean discovers the claw marks that Mom left in the floor as she was dragged under the bed. They lead to a vent that the Dean goes into after he loses rock, paper, scissors. There are entrails and blood in the vent. Back in the motel, the boys come clean to Adam about being his brother and being hunters and after a discussion on whether to teach him some thing s to defend himself (Sam for, Dean against), Adam completely accepts the truth.
There was a bartender that recently died mysteriously and it turns out that he had been a cop back in 1990 and he had worked with John on the case and had taken all the credit (that was ok, John wanted anonymity). He was confident that the cause of the grave robbings had been taken care of way back then. Sam and Adam, meanwhile, were in the motel room when the power went out and they vacated as soon as they could to avoid the big vent in the room and what would come out of it. At the truck there is a manhole under it and Sam is grabbed...but Dean shows up and saves him by shooting under the truck, wounding whatever was under there.
Dean goes back to the crypt where the original tombs were disrupted and ends up in a big, creepy cavern underneath that looks like it was a church previously. Sam and Adam (I really liked this kid, despite my reservations, good actor) are back at his mom's house and they are salting every exit...except the front door that MOM walks in. Sam says "Shoot it! It is not your mother!" to which Adam relies "I know" just when Dean finds Adam's corpse at the crypt. So Adam and Mom are ghouls (which have been encountered before) and they are the spawn of the one that John killed 19 years ago and this has all been revenge. Dean gets out of the cavern through a (former) stained glass window and shows up to kick some butt...since Sammy is tied down and cut up for the ghouls to feed on. Adam was actually John's son...its just that ghouls take on the appearance of their last kill, as well as the memories.
The end is Dean telling Sam that he finally understands why Sam and Dad butted heads so often...they are practically the same person. Dean always tried to be just like Dad, but never was. Sam says that he will take that as a compliment, but Dean tells him to take it whichever way he wants.