While investigating a series of murders in Dearborn, Michigan, Dean and Sam learn that the Egyptian god Osiris is responsible for the killings. A god with nothing but revenge on his mind, Osiris is putting random people on trial for any mistakes they've made in the past and then promptly killing them if they're found guilty. Upon sensing Dean's guilt, Osiris puts him on trial with Sam becoming Dean's lawyer. But both Sam and Dean are stunned when Osiris calls a witness, someone they thought they'd never see again: Jo. Jo, however, tries to assure Dean he had nothing to do with her death. Osiris's second witness is Sam, and he again tries to convince Dean that he had nothing to do with how his life turned out. Sam then calls Dean to the stand, and Osiris then poses a question to him, "Does he (Dean) want Osiris to call the final witness? Or would he like to hear the verdict?" Dean chooses hearing the verdict, leaving Sam stunned, as he doesn't know who this last witness is. Osiris then turns Jo into a vengeful spirit and forces her to try to kill Dean, however, just before she can, Sam stabs Osiris with a ram's horn, thus saving Dean. The episode ends with Sam telling a quizzical Dean that although he (Sam) has done a lot of bad things, he has paid his dues (with Hell), hence feels no more guilt.
Source: Wikipedia
Source: Wikipedia