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Stefan and Damon decide on a new plan to deal with Katherine at the masquerade ball.

Katherine becomes a threat to Mystic Falls and Stefan and Damon decide to stop her at the Lockwood's masquerade ball. Bonnie, Jeremy, Alaric and Caroline help the Salvatore brothers, but Katherine brings the witch Lucy to protect herself. Meanwhile, Matt picks a fight with Tyler, but Caroline hits Matt avoiding the fight. Sarah is also compelled by Katherine to pick a fight with Tyler if Matt fails.—Claudio Carvalho, Rio de Janeiro, Brazil

Stefan and Damon decide on a new plan to deal with Katherine at the Lockwood's masquerade ball. Katherine calls on an old friend, Lucy, to attend the ball with her. Bonnie, Jeremy and Alaric all do what they can to help Stefan and Damon, but Katherine has a surprise planned that none of them could foresee. Things take an ugly turn when Matt and Tyler start doing shots with their friends.—CW Publicity

Stefan comes home to find a very shaken Caroline. She tells him she saw Katherine at the Grill.

We see her there, and Katherine tells her to tell the Salvatores that she wants the moonstone at the masquerade tonight or she'll rip the town apart.

Damon says he's going to kill her. Stefan says Damon's not going to -- because he is.

Elena, Jeremy and Matt bring Jenna home from the hospital. She thinks she walked into a knife (and not that Katherine compelled her to stab herself).

Elena tells Jeremy that things will be OK with Katherine because she and Stefan are over.

Katherine comes back to the B&B to find her friend Lucy the witch waiting for her. They're both going to the ball.

Bonnie arrives at the boys' house to find Jeremy, Ric, Caroline and the boys prepping. "We're going to kill Katherine," Stefan explains.

Back at the B&B Katherine straightens her hair to pass for Elena. Lucy thinks Katherine wants her to break the curse. She expects the boys won't just hand the moonstone over.

Stefan convinces Bonnie to help. She says she can do a spell to isolate Katherine.

Matt tells Elena he's going to the ball because there's something he has to do.

At the ball, the whole town is out in costume.

Tyler apologizes to his mom for his behavior. He asks her again about Mason. She's not surprised they haven't heard from him.

At the ball, Matt runs into Katherine and thinks she's Elena. She compels him and he recaps the plan to get Tyler drunk and start a fight with him, then beat on him until he snaps and not stop until Tyler kills him.

Outside the ball, Stefan and Damon look for Katherine, each checking the other's resolve.

In the office, Matt feeds Tyler shots.

Bonnie and Jeremy go upstairs with her bag of tricks. The spell she's doing is similar to the one that sealed the vampires in the tomb. She tells Jeremy she has pretty much only done small spells that do good. She says it never ends well for people like her, but she doesn't want anyone else hurt.

At home, Elena is surprised to learn that Jeremy went to the ball. She asks Ric what's going on. She's suspicious because she hasn't heard from anyone all day. He tells her Stefan asked him to keep an eye on her. She doesn't understand why Stefan would go.

With the room upstairs ready, Bonnie comes downstairs and gets a chill. She homes in on Lucy and asks her if she knows her.

Stefan sees Katherine. She demands he dance with her. She says no one will get hurt if he gives her the moonstone. He suggests they go get it together. She threatens to start killing if he doesn't hand it over but Amy interrupts them.

Katherine offers to fix her necklace then walks behind her. Amy goes limp, Katherine says she's paralyzed from the waist down. Then she does something else and Amy slumps over completely. Katherine says she's dead.

At home, Elena announces she's going to bed. But she grabs her keys and leaves.

At the party, Stefan tells Damon he put Amy's body in the trunk. He wants to call things off but Damon talks him back into it.

Jeremy makes conversation with Bonnie and asks if she wants to dance. Damon texts "now."

Lucy finds Katherine, angry that she didn't warn her there was another witch there. Jeremy interrupts, asking to talk to his "sister." He tells her Damon and Stefan want her to meet them at the edge of the lake.

As he's walking away, Elena grabs him and hauls him into the bushes, demanding to know what's going on.

Inside, Caroline watches Matt drinking with Tyler. She gets a text from Jeremy: "your turn."

She doesn't get far, when Katherine grabs her and chokes her, asking what the boys are up to. Katherine thinks she's being lured. Caroline tells her the boys are trying to kill her. She says Bonnie has the moonstone upstairs.

Outside, Jeremy and Bonnie tell Elena what everyone's up to.

Katherine drags a crying Caroline upstairs to the room Bonnie prepped. Katherine walks in and Caroline stops crying and announces she did it. "I really didn't think I'd be able to fool you, but I did it," Caroline says.

Stefan walks up behind her. Caroline says good bye.

Stefan has a stake. She's not worried. Stefan points out Damon behind her with a crossbow that shoots stakes. He nails her in the back.

Outside, Elena yelps in pain as a spot of blood starts spreading on her back.

Upstairs, Stefan stabs Katherine's arm and Elena starts bleeding. Bonnie realizes what's going on and tells Jeremy she's linked to Katherine and he has to get them to stop. He takes off running.

Upstairs, the boys both take on Katherine, firing stakes at her. She catches a few and throws them back, but everybody ducks. She grabs Damon's arm and turns the stake on him. Stefan knocks her over and holds onto her. Damon is about to drive the final stake home when Jeremy appears in the doorway and tells them to stop, that they're hurting Elena.

"You two think you're the only ones with a witch on your side? Wrong. Something tells me that my witch is better than your witch," she says.

Jeremy goes to check on Elena. Katherine picks up a stake and rakes it across her palm. Elena's hand starts bleeding. Bonnie says she can't break the spell, but she can try to take away the pain.

Jeremy reports back about Katherine's witch and Bonnie realizes who it is. She takes off to find her. Jeremy takes off his protection ring for Elena.

Back upstairs, Katherine again demands the moonstone. She tries to get them going by asking Stefan if it bothers him his brother is in love with his girlfriend.

Stefan asks what she wanted with Mason. She says he wasn't the only wolf in town.

Downstairs, a drunk Matt pours booze on Tyler's sofa and then on a picture of his dad. He reminds Tyler how his dad used to hit him, then he breaks the picture. When Tyler won't swing, Matt tackles him. Caroline hears them fighting and uses her vampire strength to break them apart. Matt says he can't stop so Caroline knocks him out with an elbow to the nose.

Sara suddenly goes trancelike, saying if Matt fails, she has to finish it. She picks up something and stabs Tyler in the shoulder and he reacts, whacking her away and into the desk. She hits her head and then the floor. Tyler tries to wake her up. He freaks out.

Caroline checks for a pulse and finds none.

Tyler starts writhing and warns Caroline to get away. His eyes turn wolf.

Upstairs, Stefan realizes that Katherine gave George Lockwood the moonstone to fake her death. It would have worked, except people found out she wasn't in the tomb because of Damon. "Have I mentioned how inconvenient your obsession with me has been?" she asks him.

Stefan wonders who she was running from in 1864.

Downstairs, Bonnie finds Lucy, who recognizes her as a Bennett. Lucy says she'll remove the spell if her friends turn over the moonstone. Bonnie threatens her and Lucy says she doesn't have a choice, Katherine saved her life and now she has to pay up.

Lucy grabs her and realizes Bonnie has the moonstone. She tells Bonnie she can trust her and to hand it over.

Upstairs, Damon pours Katherine a drink then slams her against the wall, telling her he's going to kill her the second the spell is broken.

Lucy joins them, holding the moonstone. She tells Katherine the spell keeping her in the room has been broken and that when she hands over the stone, her debt is done. Katherine agrees.

Lucy hands it over.

With the moonstone in hand, Katherine starts gasping for breath. Lucy tells Katherine she should have told her a Bennett witch was involved. She assures the boys Elena is fine and Katherine will go quickly. She apologizes for her involvement and leaves as Katherine gasps on the floor. Finally, she stops moving.

Caroline tells Carol Lockwood that Sara was dancing and tripped, it was a horrible accident. Caroline's mom is on the way. Tyler joins them, fine. Carol leaves and Caroline tells Tyler that Matt's in the car sleeping it off. He asks what she's doing and says she doesn't know what Sara being dead means. She says she does. He checks his wound. It's healed.

Outside, Lucy apologizes to Bonnie for her spell. Lucy asks about what Bonnie felt. Lucy says they're distantly related. She says meeting her was a wake-up call to stop letting vampires control her. She says Bonnie is one of the good ones and should stay in the middle of things. Lucy leaves.

Jeremy comes and offers Bonnie a ride. They sort of have a moment and she says yes.

Outside, Stefan finds Elena in one piece. He tells her Katherine is really gone. But she wants to feel safe before they get back together. He says he understands. She walks away.

Cut to Katherine, waking up in a tomb next to the moonstone. She tries to run but is trapped there. Damon comes by and tells her death would have been too kind. He starts to close her in the tomb but she tells him Elena's in danger. She says Elena's the doppelganger and has to be protected, that's why she hasn't killed her. Damon says he'll protect Elena as he slides shut the tomb.

Elena walks to her car, talking to Jeremy on the phone. She's feeling better. But then someone in a mask walks up behind her and puts their hand over her mouth.