Born Under A Bad Sign

Born Under a Bad Sign starts with Dean frantically looking for Sam. He had just taken off on Dean and has been missing for a week. Sam calls Dean and tells him that he doesn't remember anything and he's covered in blood.
Dean tries to help Sam figure out where he's been doing for the last week. All the evidence shows a wild sign of Sam that Dean has never seen: smoking menthol cigarettes, drinking excessively, being loud, rowdy and just plain mean. They also find a bloody knife in the car that Sam apparently stole.
They go to a gas station to ask if anyone saw him. After the clerk freaks out and threatens to call the cops, Dean asks the guy which direction Sam went when he finally left the gas station. They head in the same direction. Sam tells Dean to pull on a dirt road and at the end of the road is a house with some high tech surveillance cameras. The house had been broke into and when Sam and Dean go into the house, they find the body of a man. The man turns out to be a hunter, Steve Wandell. Dean spots a video camera in the room that the body is in so they view the footage and it is clearly Sam attacking the man.
Dean is trying to cover their tracks while Sam is still freaking out so Dean trashes the computer so that no can view the same footage and implicate Sam. They head back to the motel room and Sam tells Dean that he needs to kill him. That Dean promised their dad that if Sam turned evil Dean would stop Sam. Sam hands Dean a gun but Dean can't do it. Sam then smashes the gun against Dean's head and leaves.
Dean tracks Sam by the GPS in his cell phone. Sam is in Duluth, Minnesota visiting an old friend, Jo Harvelle. Jo notices a unusual burn on Sam's right forearm. Sam is acting strange and sets Jo on edge. Jo asks Sam to leave and that's when Sam gets angry and attacks Jo. He ties her up and then taunts her by telling her the story of how her father died. He tells her "My daddy shot your daddy in the head."
Dean shows up and Sam gets even madder. He gags Jo and tells Dean that he needs to shoot Sam or Sam will kill Jo. Dean refuses to shoot Sam again but Dean throws holy water on Sam and he steams like a boiling cauldron. Sam is possessed. Dean follows Sam and they stalk each other, each trying to stop the other. The demon in Sam tells Dean that it wants to kill all the hunters and that with Sam as its vehicle, they will just let him right through the door. The demons shoots Dean and gets away. Jo finds Dean and he is alive but the bullet entered his left shoulder. Jo doctors Dean up but is upset when Dean tells her that she can't go with him to track Sam.
The demon shows up on Bobby's doorstep. Bobby lets Sam in and offers him a beer but he put holy water in his beer. Bobby tells the demon " don't try to con a con man" and knocks him out.
Sam/the demon wakes up ties up in a chair and under the devil's trap. Dean is there and they try to exorise the demon but the demon has a trick up its sleeve. The demon is the demon that they exorised out of Meg in the first season. The demon has placed a binding link, the burn on Sam's right forearm. It breaks the devil's trap and attacks Dean and Bobby. The demon is beating the crap out of Dean when Bobby takes a hot poker and burns the mark on Sam's arm, breaking the hold the demon has on Dean.
They are right back at square one. Bobby asks them if the heard of the hunter, Steve Wandell and tells the boys that they need to keep a careful eye because Wandell's buddies are after whoever killed him. Bobby also gives them charms that will fend off demon possession.
Sam tells Dean that he was awake for some of his possession. Sam yells at Dean for not shooting him. Dean tells Sam that when their dad told him to shoot Sam it was only if Dean could save Sam. They end the episode with Dean telling Sam: "You full on had a girl inside you for like a whole week. Its pretty naughty."