Know Thy Enemy

Know Thy Enemy is the seventeenth episode of the Second Season and the thirty-ninth episode in the series.

Summary
thumb|303px|right|Know Thy Enemy Promo HIDE IN PLAIN SIGHT — and  both lash out at John when Isobel’s unexpected arrival leaves Jenna devastated. works with and  to find the spell they’ll need to harness her ancestors’ power. A heartbroken doesn’t know what to do about. Stefan and Damon realize they have a new secret weapon.

Main Cast

 * Nina Dobrev as Elena Gilbert/Katherine Pierce
 * Paul Wesley as Stefan Salvatore
 * Ian Somerhalder as Damon Salvatore
 * Steven R. McQueen as Jeremy Gilbert
 * Katerina Graham as Bonnie Bennett
 * Candice Accola as Caroline Forbes
 * Zach Roerig as Matt Donovan
 * Michael Trevino as Tyler Lockwood (credit only)
 * Sara Canning as Jenna Sommers
 * Matt Davis as Alaric Saltzman/Niklaus Mikaelson

Recurring Cast

 * David Anders as John Gilbert
 * Marguerite MacIntyre as Elizabeth Forbes
 * Bryton James as Luka Martin (his corpse)
 * Susan Walters as Carol Lockwood

Guest Cast

 * Mia Kirshner as Isobel Flemming
 * Gino Anthony Pesi as Maddox
 * Michael Roark as Frank

Trivia

 * Antagonist: Niklaus Mikaelson
 * First appearance of Klaus though only in Alaric's body.
 * Isobel dies in this episode, making her the second character whose name is an episode title to die after Rose.
 * This episode marks the first time we see a vampire die from sun exposure. However Ben died from fire in Fool Me Once in season 1.
 * This is the first time we see a Lapis Lazuli stop working.
 * This is the first episode of a vampire killed by sunlight.

Production Notes

 * The name of the episode was revealed on Valentine's Day, by Ian Somerhalder.
 * This episode has only two songs on the soundtrack, so it is episode with the least songs featured.
 * Matt Davis becomes the second main cast member to have a dual role after Nina Dobrev.
 * All five characters from Season One that reappears in this episode:
 * Isobel Flemming
 * John Gilbert
 * Elizabeth Forbes
 * Frank
 * Carol Lockwood
 * This episode had 2.730 million viewers in the USA.

Quotes

 * Stefan: "Why don't you call Alaric and let him know that his wife just showed up on his girlfriend's doorstep."


 * Damon: "Don’t mistake the fact that we haven’t set you on fire in your sleep for trust."


 * Damon: "Great, we’ll have to put that on our list of things to do today. ‘Harness dead witch power."


 * Elena: "You screw up everything, John. Everything you touch just falls apart. But you’re the only parent I have left. So maybe I can learn not to hate you."


 * Klaus (in Alaric's body): Bulgarian: "Здравей, Катерина!" English: "Hello, Katherine!"


 * Elena: (About Isobel) "If that’s what you thought of her, then why did you ever put your trust in her at all?"


 * John: "Because I was there when she gave birth to you, Elena. I saw how heartbroken she was to give you up. And because she was the first girl I ever loved. And when she said she’d help keep you safe, I believed her."


 * John: (About Isobel) "I always knew that she and Katherine were close, but I never realized that Isobel and her were working against me. Two of the world’s most uncaring and selfish vampires, and yet they were genuinely friends."


 * Caroline: (About Matt) "You know, this wasn’t how it was supposed to happen. I was supposed to tell him at the right moment, in the right way, and he was supposed to be okay with it because he loves me.”