when will they find out that katherine is in elena's body
No Exit (The Vampire Diaries) Katherine (Nina Dobrev), still pretending to be Elena, tries to get closer to Stefan (Paul Wesley) so they can be back together. When Caroline (Candice Accola) arrives and tells them about Damon, Stefan wants to find him and Katherine offers to go with him. Caroline warns him that it is weird for him to be so close to "Elena" after everything that happened but he reassures her that they are just friends. Before Stefan and Katherine go, Caroline gets a phone call from Tyler (Michael Trevino) who tells her that he worries about Matt (Zach Roerig), who is vanished for two days, and that Nadia (Olga Fonda) was compelling him. Katherine hears Caroline's conversation with Tyler and she calls Nadia to ask if everything is fine. When Nadia tells her that Matt found out that she is not Elena, Katherine asks her to kill him. Caroline leaves to go to Tyler's while Katherine and Stefan leave to find Damon.
Death and the Maiden (The Vampire Diaries) Now that everything is done, Katherine finds Qetsiyah to ask for her cure only to find that Qetsiyah changed her mind and all she wants now is to die and meet her true love on the Other Side. She kills herself without finding a cure for Katherine. Katherine is once again alone since now that Bonnie is back, Caroline asks her to move out of their dorm. Nadia (Olga Fonda) appears to ask her to go back with her to Prague, but Katherine turns her down. Caroline is shocked by the fact that Nadia is Katherine's daughter.