'Vampire Diaries' Shipper-cap: A Surprising Past Relationship Is Revealed

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"The Vampire Diaries" -- "Because the Night"
"Because the Night" -- Arielle Kebbel as Lexi and Ian Somerhalder as Damon

Of course the big, scary, mystical thing Silas wants is called the Triangle, since this show is full of them.

Actually, make that quadrilaterals and pentagons and maybe even an octagon. With the revelation that Damon and Lexi hooked up in the '70s (decades before he, you know, killed her), it seems like pretty much every character on "Vampire Diaries" has at one point been romantically entangled with everybody else.

It certainly makes for some juicy 'ship-watching. Here's what went down with the various couples and duos this week:

Damon and Elena

The Big Apple is about to get a big vampire bite taken out of it. Delena are ready to rock, roll, and feed in New York City. But it seems like Damon has an ulterior motive -- he wants to search the apartment of his friend Will (the one who was working with Katherine that Klaus bit and Damon mercy-killed) for info on Katherine's whereabouts.

But Robot Elena is now superlogical and rational and totally sees what Damon's up to. She's got a plan of her own: play along until she can take whatever it is that Damon's looking for. After he finds a list of Katherine's former addresses, Elena tries to seduce him on a rooftop to steal it. Of course, Damon is way too clever for that and totally sees what she's up to. Unfortunately, he does not see Rebecca coming up from behind him to snap his neck.

[Related: Ian Somerhalder Calls Out Stranger Who Broke into His Car]

Damon and Lexi

The last time Damon was in New York City, he had turned off his emotions and was Son of Sam-ing it in the '70s. Then Lexi shows up ("You got Lexi-ed?" Elena asks while he's telling this story) because word of his bloodthirsty deeds had made it to Mystic Falls. She's there to get him to turn his emotions back on.

After drinking, feeding, and dancing together, Damon reveals he flipped the switch because of his growing feelings for Lexi. They (OMG!) have sex, but when Lexi wakes up on the roof, she discovers Damon lied about everything. He wants to get back at her for trying to nag him into changing. And, of course, as we remember from Season 1, the next time they saw each other was at Stefan's birthday and he killed her.

His regrets over Lexi lead him to try to protect Elena from the same thing. When you flip the switch back on, Damon warns, all the bad things you did will come back to bite you.

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Bonnie and Silas

So, guess Bonnie totally knows who Silas is. She doesn't care, though, because he can help her get Jeremy back. Unfortunately, that entails killing 12 witches to complete the Triangle. Bonnie agrees to the plan and sets up a meeting under the guise that she needs to be "cleansed" of dark magic.

In the forest, she faces a coven of witches, who link together to perform the cleansing. When Stefan shows up to warn them about Silas, the head witch makes a move to kill Bonnie. Stefan is immobilized, but Caroline rushes in to save her friend. All 12 witches die at once.

Later, Bonnie wakes up with no memory of what happened. In fact, the last thing she does remember is Jeremy trying to pry the cure out of Silas's decrepit hands. Poor thing has to find out twice that her ex-boyfriend died.

Caroline and Klaus

A depressed Caroline is cleaning the Salvatore house and swigging leftover beer (ew) when Klaus shows up. She snarks about why he's there instead of out chasing Tyler, when Stefan tells her he asked for Klaus's help in tracking down Silas.

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After learning of the "Triangle," Caroline figures out where the massacre will take place, and she and Klaus set off to go stop it. When he needles her about being attracted to darkness, Caroline harshes on him: Yes, once, she was attracted to a dark person "when I thought he was worth it. But it turns out some people can't be fixed. … People who do terrible things are just terrible people." Ouch, Klaus looks genuinely hurt!

When they reach the area, Caroline sees that the witches are about to kill Bonnie so she snaps the head witch's neck and the entire coven dies. The Triangle of massacres is now complete, thanks to Caroline.

Later, when a guilt-stricken Caroline struggles to comprehend what she did, it appears that Klaus is about to comfort her. Instead, he coldly says, "Why don't you find someone less terrible you can relate to?"

Elena and Rebecca

No, they're still not friends, but they want the same thing: to find the cure before Damon and Stefan. After Rebecca breaks Damon's neck, they take his car and head out to track down Katherine.

[Related: Julianne Hough Hits the Beach for Friend 'Therapy' After Split From Ryan Seacrest]

Klaus and Silas

After a hurt Caroline leaves him, Klaus senses a presence -- it's Silas. He wants the cure, or at least information on where it is. Why should Klaus help him? Oh, because Silas has a big, fat white oak stake. He drives it into Klaus's back, just an inch from his heart, as a warning. Wow, even Originals have someone to fear.

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"The Vampire Diaries" airs Thursdays at 8 PM on The CW.