Elena puts herself in a dangerous position as she searches for the truth about Katherine's past and what her own future may hold. Knowing that Stefan would never agree to her plan, Elena swears Caroline to secrecy. Damon uses a new-found confidant to help him try to discover the real purpose and power of the moonstone. Jeremy and Bonnie meet Luka, a new student with a surprising family history.
Ancient Elijah is able to regain conciousness and heal himself, despite being staked seemingly fatally. He attacks after Rose takes Damon to vampire expert, Slater, a closet vampire himself, who doesn't survive after explaining about the Original Vampires. Meanwhile, Caroline reluctantly helps Elena enter the tomb, so she can bribe Katherine with blood to elaborate on the Bulgarian origins of her Petrova bloodline. When new Salem witches Jonas and Luka arrive in town, Damon finds that the moonstone is part of a way to undo the Sun and the Moon curse requiring bloody sacrifices, including the human doppelgänger Elena.
Rose meets with Elena, Stefan and Damon warning them that the dangerous Original vampire Klaus is coming to Mystic Falls for her. Elena meets Katherine who is locked in the tomb and offers blood to her if she reveals information about Klaus and the power of the moonstone. Katherine accepts and tells the story of her life since 1490, when she delivered a baby in Bulgaria that she never saw. Meanwhile, Bonnie meets Jeremy at the Mystic Grill to play pool. There, she meets the newcomers to town: Luka and his father Dr. Jonas Martin, who lived in Louisiana. Luka discloses a family secret to Bonnie.
- Bulgaria, 1490. A young Katerina Petrova, surrounded by her family, is giving birth to a baby girl. She asks to hold the baby but her father says that she has shamed their family enough and takes the baby away, leaving Katerina sobbing.
In the present day Mystic Falls, Elena skips school and goes to the tomb, where she offers Katherine a bottle of blood in exchange for the truth about the doppelgängers and the Petrova curse. Katherine reveals that when back in 1490, she was kicked out of Bulgaria for being unmarried and having an illegimate child, she fled to England, took the English name of Katherine Pierce, and assimilated into English society. It is here where the original vampires Elijah and Trevor found her. They recognized her as a Petrova dopplegänger and Elijah tried to kill her, but Trevor (who had fallen in love with her) helped her escape. Trevor then made Katherine a vampire to escape Klaus, the ultimate leader of all vampires, and incriminated Trevor and his sister Rose in the process, thus causing Elijah and the Originals to hunt them for the next 500 years. Katherine reveals that a dopplegänger from the Petrova line can be used to break the 1,500-year-old vampire/werewolf curse. Elena, as Katherine's descendant, is the current Petrova dopplegänger. Katherine says that she came back to Mystic Falls to offer Klaus what he needed to break the curse: the dopplegänger (Elena), the moonstone, a witch (Bonnie), a werewolf (first Mason, then Tyler), and a vampire (Caroline). She hoped that in exchange, Klaus would spare her life. Stefan finds Elena talking to Katherine and tells her not to believe anything Katherine says, saying that Katherine is telling lies to free herself from the tomb. Katherine tells Stefan that when Klaus comes to kill them, and all the other humans in Mystic Falls, she will be safest in the tomb because no vampire will enter it knowing they'll be trapped in it.
Meanwhile, Bonnie meets a warlock named Luca and his father, and Jeremy gets jealous over the attention that Luca shows towards Bonnie.
Elsewhere, Stefan, Damon, and Rose try to find more information about Klaus, and while they are discussing how to break the curse Elijah walks up, fully healed, and overhears them. Damon and Rose sleep together, and at the end of the episode it is revealed that Luca's father is working for Elijah.