|This article is about Klaus, an episode. You may be looking for Klaus, a character on both shows.|
KNOW THY ENEMY— Stefan and Damon are furious when they discover that Elena has defied them both to make a new ally and take control of the plan to out-maneuver Klaus. Disagreeing over how they should protect Elena leads to growing tension between the Salvatore brothers. Meanwhile, Stefan has his hands full trying to keep a confused and frightened Jenna safe. Flashbacks to 1492 reveal Katherine’s introduction to Elijah and Klaus along with the origin of the moonstone curse. Elena learns shocking new information about Klaus and his motives.
Elena sits as Elijah suddenly comes back to life and his first word is "Katerina" when he sees her, thinking at first that she is Katherine. Right into the first flashback set in 1492, England, where Trevor introduces Katerina to the Lord Elijah. In present day, Stefan realizes both Elena and Elijah are gone.
He gets Elena on the phone and she tells him that she needs time alone with Elijah and Stefan needs to respect her decision. She gives Elijah the dagger back to show that he can trust her. After a quick stop at the Lockwood mansion for clothes, Elijah continues to reveal history and drops the bomb that he and Klaus are brothers. "I'm a bit behind on the times, but I believe the term you're searching for is OMG," Elijah says.
It turns out Klaus and Elijah are part of the Original Family, from which all the vampires were created. He explains that nothing can kill an original — save for wood from one tree which his family made sure to destroy. So if the Originals can walk in the sun, then why does Klaus want to break the curse of the sun and moon? Well, turns out it's fake. Jenna has returned home, but unfortunately Klaus (in Alaric's body) has joined her. "Do you believe in vampires, Jenna?" Klaus says before spewing words about being obsessed with them. Stefan screams at Jenna to leave and gets his vampire face on before he starts beating him.
The damage is done and Jenna now knows the truth. Elena leaves Elijah briefly to comfort her aunt and admit that not only do vampires exist, but basically everyone around her has known but her. Back at the Lockwood mansion, Elijah goes on with the Story, revealing Klaus does need the doppelgänger, but created the myth in order to have every member of two warring species on the look-out for her. But the true curse is on Klaus. Elijah's mother had an affair with another man (Klaus' father).
When Elijah's dad learned of the infidelity, he killed her lover and his entire family, igniting a war between two species that still wages on today. Klaus' dad was a werewolf, making him a hybrid. But the curse is that the witches made his werewolf side dormant and he wants to ignite it. The curse can be broken during a full moon when he's in transition, which is also when he's the weakest and can be killed. At the Salvatore house, the brothers are at odds once again.
Their battle truly mirrors that of Klaus and Elijah. "Thank you for being in love with my girlfriend... but I have one thing you never will. Her respect," Stefan says, setting Damon off in outrage. They stop fighting when Elena and Elijah arrive to tell them that the sacrifice will happen and Bonnie will be able to kill Klaus. "You can all go to hell," Damon responds before walking out. In a heart-wrenching scene he tries to get his "girlfriend" Andie to leave, but instead she tells him she cares. He plunges at her neck before saying, "Get out of here before I kill you." As Andie looks back before exiting, she sees a teary-eyed Damon with blood running down his mouth.
- Antagonist: Klaus.
- On March 1, Dawn Olivieri tweeted part of the script that was in fact not supposed to be on there, as it was a leak and the producers do not like spoilers to be ruined. This is the reason why the image was removed shortly afterwards, and the original is no longer on Twitter.
- This episode features Klaus' first appearance in his own body. It also features a re-appearance of Trevor in flashbacks. 
- This is the seventh flashback episode of the series.
- This is the first episode of Season Two to feature only five main characters (Elena, Stefan, Damon, Jenna and Alaric).
- This is Greta Martin's first appearance on the show.
- The existence of vampires is revealed to Jenna by Klaus and inadvertently, Stefan, in this episode. With this revelation, all of the main characters of this season now know the truth or at least part of the truth about the Salvatore brothers.
- Elijah reveals three shocking secrets:
- He and Klaus are half-brothers.
- Klaus is a vampire-werewolf hybrid.
- The Sun and Moon Curse is fake. The real "curse" is that the witches suppressed Klaus' werewolf side so he wouldn't be an over-powerful hybrid with all the powers of both species and none of their weaknesses. Werewolves can mate with each other or humans so Klaus would have the ability to procreate and hence the ability to sire an unstoppable and nearly-invulnerable hybrid race, which would destroy the witch-enforced balance of Nature.
- Elijah and Klaus' struggle over Katherine's affections clearly mirrors the one between Stefan and Damon, which reiterates the series' recurring theme of history repeating itself.
- There is a bit of foreshadowing - when Elijah meets Katherine for the first time he says "Forgive me. You remind me of someone". We later find out that this "someone" was Tatia, the first Petrova doppelgänger and Elijah and Klaus' first love.
- Elijah told Elena a bit about his family whereabouts - He told her that his father was: "...a wealthy land owner in a village in Eastern Europe". But in the second season of The Originals, we learned that the Mikealson family actually originated in Norway, as both Mikael and Esther were born there. And that Mikael was not a land owner, but in fact a Viking warrior.
- When Klaus says "Niklaus is the name my father gave me. Please. Call me Klaus" he is indicating that he wants to distance himself from his step-father Mikael by rejecting the name he was given.
- In the "Previously On The Vampire Diaries" section, they foreshadow the revelation of Klaus being the subject of the curse himself, rather than vampires and werewolves in general, when just after talking about the Sun and Moon Curse, Rose says "Klaus is the real deal." This particular part was possibly deliberate as "Klaus is the real deal" here may mean both that Klaus is the most powerful vampire around and that he is the subject of the curse rather than curse cast on werewolves and vampires by an Aztec shaman.
- Alaric meets Katherine for the first time, but he initially mistakes her for Elena.
Behind the Scenes
- This is the first episode ever of Joseph Morgan portraying Klaus on the TVD\TO universe.
- This episode had about 2.70 million viewers in the USA, which was 0.11 million less than the previous episode.
- When faced with two terrible choices, Elena takes the third option and makes an alliance with Elijah.
- Elijah: "Yes. Klaus is my brother."
- Elena: "I— I heard that. I— I'm still processing.
- Elijah: "I'm a bit behind on the times, but I believe the term you're searching for is... OMG."
- Klaus: (in Alaric's body) "Do you believe in vampires, Jenna?"
- Klaus: (after returning to his true body) "That's more like it."
- Elijah: (to Elena) "I need some air. I'm still feeling a tad... dead."
- Elijah: (to Elena about Klaus) "I helped him because I loved him. That's changed. Now, he must die."
- Katherine: (to Elijah in 1492) "True love is not real unless it is returned. Do you agree?"
- Elijah: "I do not believe in love, Katerina."
- Katherine: (to Elijah in 1492) "Life is too cruel. If we cease to believe in love, why would we want to live?"
- Klaus: "The full moon is tomorrow, brother. After all these centuries, it is finally time."
- Elijah: "I have been to see the witches. They believe they may have found a way to spare the doppelgänger."
- Klaus: "What does it matter if she lives or not? She's a means to an end, that is all."
- Elijah: "What, she should die for your gain?"
- Klaus: "She's human. Her life means nothing."
- Elijah: "I beg you to consider this."
- Klaus: "Are you so foolish as to care for her?"
- Elijah: "Of course not."
- Klaus: "Love is a vampire's greatest weakness, and we are not weak, Elijah. We do not feel and we do not care."
- Elijah: "We did once."
- Klaus: "Too many lifetimes ago to matter. Tell the witches not to bother. The sacrifice will happen as planned."
- Stefan: (to Elijah) "I'm sorry for the part that I played in your death. I was protecting Elena. I will always protect Elena."
- Elena: (to Jenna) "I should've told you. I thought that if you didn't know. you'd be safe. But then things got so out of control, and everything I've done to keep you out of it all blew up in my face."
- Elijah: (to Elena) "Your phone wοn't stop its incessant buzzing. Answer it."