After work he has drinks with his neighbor Sid, who complains good-naturedly that Dean never talks about his past. Dean, after some prodding, tells him that for most of his life he "lived on the road" and "did crap jobs that nobody else wanted." He further elaborates, saying that he worked in pest control with a partner. A cute waitress Brigitta strokes Dean's arm and gives him her number, which he tears up. On leaving the bar, Dean hears a scream and goes to investigate a nearby building site. He sees blood smears and what looks like claw marks on the wall, but he finds nothing but pigeons.
The next day on the way to work, he sees the claw marks on telephone poles near his house and then on his garage door. In his garage he is startled by a neighbor's Yorkie, but then more ominously finds some sulfur on the ground. He tells Lisa to take Ben to the movies, and starts unpacking weapons from the trunk of the Impala. From a strong box containing his leather jacket, he takes John’s Journal when he is attacked by Azazel. As he passes out, Azazel is attacked from behind by none other than Sam, who drives through Azazel to thrust a syringe in Dean's chest.
Dean regains consciousness in an old house. On seeing Sam, he assumes he is dead, but Sam assures him he is alive. Sam demonstrates he is human by cutting himself with a Silver blade and drinking holy salt water. He tells Dean he doesn't know how he got out of Lucifer's Cage - he just woke up in Stull Cemetery. Sam says he tried to contact Castiel in hopes of getting an explanation, but he didn't respond to any of his prayers. To Dean's shock, Sam has been back for nearly a year. Sam explains he wanted Dean to live a normal life, and knew if he reappeared Dean would leave Lisa and Ben.
Sam reveals he's been hunting with cousins from their mother Mary's side of the family and introduces him to Gwen Campbell, Christian Campbell, and Mark Campbell. The biggest surprise is that their grandfather Samuel Campbell has been resurrected.
They reveal they have been hunting Djinn who can disguise themselves as normal people, and who are pursuing Sam and Dean. The Djinn seem to want revenge for the Djinn that Sam and Dean killed a couple of years ago in 2.20 What Is and What Should Never Be. They attacked Sam, dosing him their hallucinatory poison, before they came after Dean. Sam was saved by Samuel, who has an antidote - the same antidote that Sam injected into Dean, breaking him out of his hallucination of Azazel. Johnny Campbell has gone to watch over Lisa and Ben, but he is killed by the Djinn. Luckily Lisa and Ben are safe, and so Dean takes them to Bobby's place.
Dean is further distressed to find that Bobby also knew Sam has been back for a year, but Bobby argues as Sam had, that he wanted Dean to have a chance at a normal life. Dean tells them how much he suffered trying to deal with his grief, and looking for a way to save Sam. Dean is also distressed at the danger he has now put Lisa and Ben in, and apologizes to Lisa. She assures him the last year has been "the best year of her life" and that he is a great father to Ben.
Dean, Sam and the Campbells return to Dean's home where the Djinn are still lurking. Samuel tells Dean that over the past few months they have been dealing with a lot of unusual supernatural activity, and argues that hunting is in Dean's blood and he needs to come back to it. Dean suggests they need to leave so the Djinn will attack.
After they leave, Dean asks Sam about his time in Hell, but Sam doesn't want to talk about it. Just then they see the neighbours Sid and his wife being attacked. Dean rushes over despite Sam's protestations, and is attacked himself. As Dean's getting another, more powerful dose of the Djinn's poison, Brigitta tells him, "That's for our father." He starts hallucinating that Lisa and Ben are attacked by Azazel in a parallel to what happened on the night Mary was killed - with Ben being made to drink the demon's blood, and Lisa being burned on the ceiling. Sam takes on the other Djinn with help from Samuel and then rescues Dean. Samuel, with help from the other Campbells, overpowers Brigitta, and they place a bag over her and take her away.
Afterwards, Sam argues that Dean needs to return to hunting, and that he needs Dean and his compassion, with him. But Dean refuses, saying he needs to keep Lisa and Ben safe. He offers the Impala for Sam to take, but Sam declines, saying that he already has his car set up the way he likes it, before driving off.
Info from Superwiki