|Sam goes up to the bar and acting kinda funny, orders a banana daquiri and introduces himself as "Gary" to Crystal, a hot cougar at the bar. Well, just as they are leaving the bar together, we see that this isn't actually Sam at all but rather a scrawny kid in Sam's body...hmmmm.....A day and a half earlier in Massachusets, Sam and Dean are visiting their old favorite babysitter. Apparently their dad would leave the boys with her while he would go off to fight ghosts. Well, it turns out she called them to come over to her house to do more than just reminisce: they have a ghost in their house, and though it started off relatively harmlessly, a few days ago, it attacked their tweenage daughter Katie by carving "murdered child" into her stomach. So the boys send the family on vacation and get down to business solving the mystery.
At the local diner, we get a quick cameo of the dude from the first scene that is masquerading as Sam in a bar. He serves them their food in a ridiculous American flag outfit and manages to creepily stare at them the whole time they are at the diner. Moving on to the mystery of the poltergeist...Sam does an internet search and finds out that the home used to be owned by Isaiah Pickett and he hung a woman for witchcraft there named Maggie Briggs.
That night, outside the house, Sam comes back from doing some futher digging (the investigating kind, not digging graves) and he gets shot with a tranquilizer dart by some invisible force and passes out. When he comes to, he is dressed in that ridiculous American flag outfit from the diner, and a police officer calls him Gary and tells him that his family has been worried about him. Confused, he gets in the cop car and they deliver him "home" where his "mother" and "father" are grateful to see him but accuse him of being drunk. He starts to piece things together a little bit when he looks at his own reflection in the window and sees someone else.
Have I mentioned the kid who exchanged bodies with Sam is in his mid-teens, scrawny and not super manly looking? So you can bet he is enjoying having Sam's hot body. When Dean gets home, they plan to get on the road and Fake Sam manages to grab their cell phones and secretly throw them in the garbage. Then Fake Sam (clearly not yet 16 in real life) asks to drive, and reverses smack dab into a dumpster. And from there, they take off.
The Real Sam calls all of Dean's many cell phones to tell him what's been going on, but as we know, fake Sam has thrown them out. So Real Sam decides to find out all he can about Gary by going through his room. After finding all the classic signs of a nerd he decides the kid is a "frustrated virgin"...who does witchcraft. At breakfast, he continues to probe for information and later Gary's sister confirms that there is a big, leatherbound book out there somewhere. He sees that Gary's parents are obsessed with the plan (doing good at school, getting into a good college, etc.)
Fake Sam tells Dean that Maggie Briggs is buried in Isaiah Pickett's basement. He never actually hung her, she was carrying his unborn child and he killed her and buried her in the basement (hence the "murdered child" scratches on Katie). Fake Sam then asks Dean to turn up the music, which Dean finds strange.
Dean and fake Sam find where Maggie is buried and Dean starts digging. Fake Sam has a loaded gun pointed at his head and his finger on the trigger and Dean doesn't notice. Just when he is about to shoot, he gets knocked down by some invisible force. Dean goes to help him and gets knocked down himself. Maggie appears and fake Sam burns the body just in time and the ghost goes up in flames.
So real Sam is in school (as Gary of course) and he finds the witchcraft book in his locker. He sneaks out of school with the book to try to reverse the spell when he is stopped by Gary's friends. Apparently skipping class is way out of character for Gary. They try to convince him to stay and that they can help him through whatever is going on, but he won't listen. Just then, one of them shoots him with a tranquilizer dart.
Fake Sam and Dean are celebrating at a bar and when "Sam" orders a bacon cheeseburger, Dean starts to realize something is up. He crashed the car, he has different taste in music, he thought today was an "awesome" day when really it was just another day in the life, etc. But in the end he lets it go. Gary (as Sam) gets up to talk to that cougar lady and here is where we sync up with the first scene.
Real Sam is tied up with Gary's friends holding him hostage. One of them, Trevor, calls real Gary to ask where he is. Gary apparently didn't know that Sam was in his body as well. And they talk about how Gary needs to hurry up and kill Dean already. Then the cougar comes in the room dressed as a Dominatrix, and the phone call gets cut short.
The kids are Devil worshippers and they tell Sam that the demons have a bounty out on Dean's head. They are summoning all the witches and demons everywhere to try to kill him, and these three high schoolers want to be the ones to do it. Sam tries to talk them out of it, and Norah seems to be listening to him. But Trevor decides he is going to summon a demon to help in killing Dean. The demon he summons comes to life in Norah.
So, Trevor explains to Norah that he has summoned her because he as Dean Winchester for her. She is even more excited when she realizes that Sam is in Gary's body, meaning Sam's body is out there somewhere for her to steal as well. She plans to let Trevor go unharmed, but Trevor gets greedy and starts talking about wanting his reward. He asks for a million bucks and a girl to fall in love with him, and of course, the demon inside Norah kills him instantly.
Gary (as Sam) tries again to shoot Dean but Dean catches him and demands to know who he really is. He then checks his voicemail and hears all those messages from the real Sam. Gary starts freaking out and tells Dean that Sam is in his friend's basement when the demon in Norah's body shows up and knocks Dean out. She unties Gary and asks him what he wants from her. He says he wants to be a real witch, which she can help him with only if he goes to meet the devil. Of course, this is where Gary starts to chicken out a little.
Dean gets up and he and the demon fight. The demon seems to be kicking Dean's butt when Gary starts speaking in Latin to exorcise the demon. Dean joins in and before long the demon is sucked up into the sky in a cyclone shaped cloud of dust.
The next scene opens with they boys performing the ceremony to get each in his own rightful body and when it works they threaten Gary to keep him from doing it again. When they drive him home, Sam gives him some big brother-type advice about rebelling a little, not letting your parents control your life and that Norah likes him.
Back in the car, the two brothers are back together at last.