<P> Nadia checks that finally the vervain is out of Matt's system so she can compel him again but before she makes him forget, Matt tries to warn Caroline about Katherine taking over Elena's body . He kisses Nadia who kisses him back and while they are kissing, he tries to text Caroline . He manages to write "Help K" and send it to Caroline before Nadia sees him and stops him . She is hurt by his behavior and trying to fool her, so she compels him and starts to exit the house . Caroline, who got the text, meets Nadia at the door and asks her what is she planning and they start fighting . Tyler gets there in time to stop Nadia from killing Caroline and he fights Nadia who runs away . </P> <P> Katherine and Stefan arrive at the house where Damon is but Damon warns them to not get into the house because they will not be able to get out and he will attack them . They tell him that they are not afraid of him and they both enter the house . Katherine, to help Damon fight his urge, cuts her hand with a glass and provokes him with her blood telling him that he can fight it . Damon starts feeding on her, Stefan tells him to stop and Katherine kicks a stake towards Stefan telling him that Damon is going to kill her . Stefan, instead of taking the stake, he takes a piece of glass and cuts himself to distract Damon . Damon leaves Katherine and Stefan snaps his neck . </P> <P> Katherine and Stefan bring Damon back to Mystic Falls and Katherine arranges immediately to meet Nadia . Katherine is happy because, despite her plan to kill Damon not working, she had a moment with Stefan . She stops smiling when Nadia tells her that she was bitten by Tyler Lockwood . </P> <P> 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 . </P>

When does damon find out elena is katherine