Episode 6 "Plan B"
"Now that Stefan's rejected her, she has no ties to the town anymore, so she'll do anything and everything," Dobrev says. "She doesn't care who she kills, even if it's in front of a room of people. There's almost no way to stop her."
When the official description said that Katherine is chooses a new victim, the new victim is possibly Aunt Jenna. She's one person who doesn't know anything about vampire and all supernatural things on the Mystic Falls. Katherine's action brings a friction between the aunt and the niece. Jenna would be in a mortal danger and would see Elena in a whole new way.
Episode 7 "Masquerade"
With Aunt Jenna in a mortal danger, it keeps Elena in attending the Masquerade ball. She has to solve her "family issues". In other hand, it gives Katherine an opportunity to get in. So, Katherine will disguise again as Elena. Now she not only tries mimicking her personality but also her hairstyle and fashion sense.And with her hair straightened, it takes a very trained eye to recognize that she's not Elena. "Most people in town don't realize that she's not Elena," Dobrev says, "So she's trying hard to impersonate Elena, but she gets so bored with it because she thinks Elena is dull."
This episode also the first time when the Salvatore brothers confront Katherine directly. It's going to get spicy -- she literally rips the masks off of their faces.
Another spoilers about what will happen with our favorite characters :
-Damon and Stefan :
It is the first time when both of them really need to cooperate with each other. Set aside the differences and the hatred, to kill Katherine. Stefan now changes into a new different vampire after years denies his nature instinct. The desire to bring down Katherine, forces him to a new direction :
"As psycho as Damon is, Stefan needs him as an ally," says Wesley. And Stefan is stepping up his own game, too. Now that he's accepted his fate as a vampire - and started drinking people blood again - he's got a new attitude. "Stefan can't keep being the drab individual he was in Season 1. He's on the same playing field as Damon now."
-New team form
Damon and Stefan also realize the fact that only them aren't enough to take down Katherine. So they let the others to join this new ally : Caroline and Bonnie. And then Alaric and Jeremy.
-Bonnie and her anti-vampire issue also a new witch on the town
Bonnie, of course, has her own issue to solve if she wants to get along with this team. She has to get over her hatred to vampires so she could preserve her friendships with Caroline and Elena, but that doesn't mean she doesn't have it out for Stefan, Damon, and Katherine. She's also going to be meeting Lucy, a new witch in town.
-Another a new love triangle :
There's a new triangle in town - Matt, Caroline, and Tyler (Michael Trevino). Though Matt will reconnect with Tyler in upcoming episodes, Tyler is also forging a connection with Matt's very recent ex-girlfriend, Caroline. Matt will definitely start to notice that something is sparking between the two.
Source : Zap2It