In 'Know Thy Enemy', there will be a complex relationship between Elena's mom Isobel and Elena's doppelganger Katherine.
Elena's biological mom Isobel appeared on Jenna's doorstep at the end of February 24 episode of "The Vampire Diaries". This serves as the midseason cliffhanger because the show is taking a five-week break and will only return on April 7 with the episode "Know Thy Enemy".
The synopsis for the episode is yet to be released but from the look of the preview, there's a kind of bond between Isobel and Katherine which may be bad news for Elena. Jenna, who has been the one oblivious of what has been going on Mystic Falls, invited the vampire in.
Speaking of Jenna, she would be more "in the know" when the show returns. Sara Canning told TV Guide, "In the next few episodes we're going to see something happen that is really going to rock her world and have her question if she is in such a steady and stable spot as she thought she was."
Meanwhile, Candice Accola who plays Caroline told E! Online about the rest of the season, saying "Every script is just an amazing cliffhanger and each one is pretty shocking, so the audience should be really excited for the end of the season."