do they find out that katherine is in elena's body
No Exit (The Vampire Diaries) Damon wakes up chained on the Salvatore house's basement. Stefan is there and Damon tries to warn him that because of his cravings, one day he will kill him but Stefan tells him that they will find a solution. Damon points to Stefan that "Elena" provoked him to feed on her and then she kicked a stake towards him so he will kill him. Stefan does not believe that Elena would want that, he locks Damon up and gets upstairs where Caroline is. The two of them discuss what happened between Stefan and "Elena" and Caroline also tells him about Nadia and Matt and the text he sent to her. From Matt's text, who used "K" and not "E", they put the pieces together and they realize that Katherine is in Elena's body.
Mike Delfino At the end of the finale, Mike is seen marrying someone whose face is not shown.
Damon Salvatore In the third season, Damon helps Elena in bringing his brother, Stefan, back to Mystic Falls after Stefan becomes Klaus' henchman. The arrangement transpired after a bargain for his blood that would cure Damon of the werewolf bite he had received from Tyler. At first, he is reluctant to involve Elena in the rescue attempts, employing Alaric Saltzman, Elena's guardian, instead as Klaus does not know that Elena is alive after the sacrifice which frees Klaus' hybrid side. However, Elena involves herself, desperate to find Stefan. Damon, though hesitant at first, is unable to refuse her because of his love for her. He also points out to her that she once turned back from finding Stefan since she knew Damon would be in danger, clearly showing that she also has feelings for him. He tells her that "when (he) drag(s) (his) brother from the edge to deliver him back to (her), (he) wants her to remember the things (she) felt while he was gone." When Stefan finally returns to Mystic Falls, his attitude is different from that of the first and second seasons. This causes a rift between Elena and Stefan whereas the relationship between Damon and Elena becomes closer and more intimate. A still loyal Elena, however, refuses to admit her feelings for Damon. In 'Dangerous Liaisons', Elena, frustrated with her feelings for him, tells Damon that his love for her may be a problem, and that this could be causing all their troubles. This incenses Damon, causing him to revert to the uncaring and reckless Damon seen in the previous seasons. The rocky relationship between the two continues until the sexual tension hits the fan and in a moment of heated passion, Elena – for the first time in the three seasons – kisses Damon of her own accord. This kiss finally causes Elena to admit that she loves both brothers and realize that she must ultimately make her choice as her own ancestress, Katherine Pierce, who turned the brothers, once did. In assessment of her feelings for Damon, she states this: "Damon just sort of snuck up on me. He got under my skin and no matter what I do, I can't shake him." In the season finale, a trip designed to get her to safety forces Elena to make her choice: to go to Damon and possibly see him one last time; or to go to Stefan and her friends and see them one last time. She chooses the latter when she calls Damon to tell him her decision. Damon, who is trying to stop Alaric, accepts what she says and she tells him that maybe if she had met Damon before she had met Stefan, her choice may have been different. This statement causes Damon to remember the first night he did meet Elena which was, in fact, the night her parents died - before she had met Stefan. Not wanting anyone to know he was in town and after giving her some advice about life and love, Damon compels her to forget. He remembers this as he fights Alaric and seems accepting of his death when Alaric, whose life line is tied to Elena's, suddenly collapses in his arms. Damon is grief-stricken, knowing that this means that Elena has also died and yells, "No! You are not dead!" A heartbroken Damon then goes to the hospital demanding to see Elena when the doctor, Meredith Fell, tells him that she gave Elena vampire blood. The last shot of the season finale episode shows Elena in transition.
Killer Within Outside the prison, an unidentified individual lures a group of walkers towards the prison, breaking open the front gate's lock to allow them inside.