The Originals. The most feared vampires out there. They can control vampires with a simple suggestion, make witches and warlocks work for them despite the animosity between the two. Indestructible with no known weakness. Except one. A dagger dipped in the fossilized ashes of the White Oak Tree, a dagger that will kill any vampire who wields it as a safety precaution. A dagger kept by a witch in the Originals' fold, Alexandra, to ensure it's safe keeping so that it doesn't fall into the wrong hands.
Katherine Pierce knows the dangers of the Originals, having been running from them for five hundred years. They wanted to use her to break a curse, but her becoming a vampire ruined their plans and now they want revenge. So Katherine did what made sense. She seduced Alexandra and stole the dagger. She knows that she can't kill the leader of the Originals without dying herself and she has fought too hard to survive. But she knows whom she can go to to do the deed for her. And this is why she returned to Mystic Falls. To save herself and to regain Stefan's affections.
Meanwhile in Mystic Falls, the town is reeling from the Founder's Day fiasco. The Tomb vampires are dead. Caroline is in the hospital with internal bleeding. Tyler is trying to figure out what he heard, not knowing his father is dead. And amidst all of this, Katherine is back, bringing with her a myriad of problems for everyone involved.