The Vampire Diaries Season 1, Ep. 21 “Isobel”

Isobel returns to town and surprises Alaric with her new cynical and cruel attitude and her demand that he arrange for her to meet Elena; despite her seeming indifference toward him, it appears that she still loves him and that she is not truly happy with her life as a vampire. When Elena and Isobel finally meet, Isobel refuses to answer most of Elena's questions, but states that she is also looking for the mysterious invention that John Gilbert has been seeking. Upon learning that Damon has the device, Isobel orders Elena to obtain the device from Damon and threatens to kill everyone Elena knows if she refuses. It soon becomes apparent that both Isobel and John are working for Katherine. Isobel attempts to intimidate Damon into giving her the device and offers to reveal Katherine's whereabouts in return, but he makes it clear that he will not tolerate her threatening him or the people he cares about, and that he no longer wants anything to do with Katherine. Isobel later kidnaps Jeremy in order to force Elena's hand, and Elena implores Damon to give her the device, which he does. Bonnie offers to remove the spell from the device, which can supposedly kill vampires; it is later revealed that Bonnie only pretended to deactivate the device, due to her vendetta against all vampires.

Elena gives the device to Isobel, who reveals that she knew Damon would surrender the device to Elena because he's in love with her. It is revealed that John is Elena's biological father, and that he and Isobel had dated in high school. John plans to use the device to kill all the vampires in Mystic Falls, including Stefan and Damon. Before leaving town, Isobel, who appears to care for Elena after all, expresses her fear that the Salvatore brothers may eventually turn Elena into a vampire - a life that she doesn't want for Elena - and that getting rid of them is the best thing she and John could do for their daughter. Anna appears and tells Jeremy that her mother was killed, and that she knows she should not be there, but she has nowhere else to go. Jeremy hugs her and attempts to comfort her.