The Vampire Diaries Season 3 Episode 7 (10/27/11)
Ghosts invaded Mystic Falls this week, causing some serious problems and even killing a history teacher in front of the entire town.
The only way to send them back was for Bonnie to destroy the talisman that anchored them here in the present day: Elena's necklace. But Elena didn't want this action to take place yet because Lexi was back and working some weird spell to get Stefan's humanity back. It maybe worked? Kind of? A little bit?
By the end, when Lexi was forced to leave, not a lot had been accomplished, but Elena possibly got through to Stefan with a threat that she wouldn't stick around forever.
Oh, and how did Lexi get forced to leave? Well... Anna stole Elena's necklace because she wanted to stick around and be reunited with her mom, but after she and Jeremy kissed (an action Bonnie later learned about and dumped him for), Anna realized it wasn't fair to be Jeremy's ghost girlfriend. So she gave the necklace back, Bonnie united with Grams to destroy it and all the ghosts left for The Other Side.
Except that the necklace then got spit out of the fire to close the episode.
Also, Mason led Damon in a search deep inside his family's dungeon. But Damon wasn't allowed to go all the way inside, and after Mason left he was forced to call Alaric to assist. He did so, and the hour concluded with Alaric stumbling upon some kind of ancient writing on the walls.