|While on the road, Sam receives an email from Becky, one of his old friends from College. She tells him that her brother Zack has been arrested for killing his girlfriend. She is sure he is innocent as she was with him at the time. Worried for his friend, Sam convinces Dean that they need to go to St. Louis to help. They tell Becky that Dean is a detective and start investigating. When they look at the security tape the police have as evidence, they notice something strange about Zack when he appears on the.tape. They suspect they may be be looking for a doppleganger as the killer.Meanwhile, a man arrives home to find his wife tied up and bleeding. She begs him not to hurt her anymore. Puzzled, he looks around the apartment and comes face to face with someone who looks identical to him.
At 5:30 the next morning, Dean and Sam are looking in the back alleys for clues to the whereabouts of Zack's double. They hear about the second attack and realize that rather than a doppleganger, they are probably looking for a shapeshifter. They follow a trail left by the creature into the sewers and get split up after Dean is injured. They lose track of the shapeshifter and meet back up, but Sam suspects that Dean might actually be the shapeshifter and tests him. When he fails, Sam confronts him but loses and ends up in the sewers, tied-up and being taunted.
While the shapeshifter returns to Becky's house to try and get close to her, Sam discovers that Dean is also tied up. They eventually escape and head to Becky's to save her.
The shapeshifter is about to kill Becky when the Police arrive, thanks to an anonymous tip. He escapes back to the sewers and sheds the Dean-skin he is wearing.
Dean continues trying to track the shapeshifter while Sam meets up with Becky. Both find Becky, but only one is the real one. The shapeshifter, as Becky,knocks Sam out. When he comes to, the shapeshifter is wearing Dean's skin again. It further taunts Sam, trying to make Sam believe that it really is Dean about to kill him, but the real Dean arrives in the nick of time, killing the shapeshifter.
With the shapeshifter destroyed and the murder of Zack's girlfriend now pinned on "Dean," the brothers head out of St. Louis, leaving the deceased "Dean Winchester" behind.