 While on the road, Sam gets an e-mail from his old school friend Becky telling that her brother and also Sam s friend Zach is arrested, accused of murder of his girlfriend. Zach claims that he was in another place at the same time, but the surveillance camera had taped his image in the crime scene. Sam convinces Dean to drive to St. Louis, Missouri, to investigate the typical doppelganger case. Once there, they disclose that a mutant is killing and hurting people in the neighborhood. When the shape shifter takes the form of Dean, things get complicated for the Winchester brothers.