The big bust.
In Mystic Falls, they have giant parties almost every week, but it's only once in a blue moon that they actually throw a formal ball. Elena is startled when an invitation to it arrives for her. However, her dreams of the lavish event hit a snag once it's discovered that it is being thrown at Klaus' newly renovated mansion. Damon and Stefan do not like that one bit and insist on being chaperons.
The invitations work their way around town. Caroline and Matt each get one from people they were not expecting to give them. When she arrives at the ball, she receives quite a bit of attention from Niklaus, who proves that he has more to offer her than his blood. As they get to know each other, she realizes that there is a side to him she didn't know existed. That obviously means she hasn't just noticed what a sexy beast he is, but that he has a heart that beats for reasons other than pumping the blood of the people he butchers.
Elena doesn't manage to enjoy the big shindig too much because she stumbles on a rather gruesome plan. Even more disturbing than the possibility of impending doom is contemplating who she has to turn to for help trying to stop it. After the night ends in misery and bloodshed, Damon finds a new way to deal with his problems.