Dean and Sam investigate a series of deaths in Calumet City, Illinois where the victims had everybody telling them the cruel truth about themselves, leading them to commit suicide. Dean also finally speaks to Lisa who breaks up with him, telling him that his life is too dangerous and that she is angry Dean shoved Ben. Dean is now wary of his brother ever since he remembers seeing Sam allowing the vampire to attack him. The spell affects Dean and he takes advantage of the situation to question Sam, but Sam says he just froze and is appalled Dean doesn't trust him. As Sam appears to have told the truth, they continue their hunt and discover it is the goddess of truth, Veritas who is invoking the spell. But things don't go as planned as brothers are taken captive by Veritas and she is shocked to see that Sam is unaffected by her spell and he has been lying to Dean all along. As Veritas prepares to eat them, the brothers break free and kill her before Dean confronts Sam. Sam finally admits he has been lying and that he allowed Dean to get turned, telling him he knew about the cure. Sam explains that he doesn't know why, but he just doesn't care about anything anymore. Enraged, Dean beats Sam unconscious.
Source: Wikipedia
Source: Wikipedia