The second season mainly focuses on the arrival of Elena's doppelganger Katherine Pierce and the reasons she returned to Mystic Falls. It also introduces werewolves. Tyler's Uncle, Mason Lockwood, who is a werewolf, arrives after the death of his brother, Tyler's father, and tells Tyler about the so-called "Lockwood curse" in their family. To activate the "werewolf" gene, the person must kill someone. Katherine turns Caroline into a vampire so she can have a spy against Stefan and Elena. Katherine's main reason for returning to Mystic Falls is to find the moonstone, with the help of Mason, who is killed by Damon. Katherine calls in a favor from a witch named Lucy to get the stone. Lucy casts a spell so that when Katherine is hurt, it also affects Elena. When Lucy finds out another Bennett is at the party and Katherine had deceived her, she puts a spell on the moonstone, injuring Katherine. Damon then leaves her in the original tomb. She tells him the reason she hasn't killed Elena is because she is in danger. Elena is later kidnapped and held hostage by vampires Rose and Trevor, who have been on the run from the Originals-the first generation of vampires. Rose reveals to Elena that she is the Petrova doppelganger and needs to be sacrificed to break the sun and the moon curse. Katherine was also a Petrova doppelganger who used Trevor to help her escape from the Originals and Rose to turn her into a vampire to save herself. Elena is rescued by Stefan and Damon.
Klaus, the oldest vampire in history, is now hunting for Elena. Elijah has made a deal with Elena to protect her loved ones because he wants to use Elena as bait so he can lure Klaus and kill him. Jules, a friend of Mason, tells the Lockwoods that he never came back to Florida and is missing. Alaric is suspicious and calls Damon to the Grill. Jules reveals that she is a werewolf. Damon tells Jules she won't find Mason and to leave town. Jules breaks into the Salvatore mansion in wolf form and attacks Damon, but Rose pushes him out of the way, getting bitten instead. As Rose succumbs to the effects of her wolf bite, Damon stakes her into a peaceful death.
Uncle John's return to Mystic Falls comes as an unpleasant surprise to Elena, Damon, Jenna, and Alaric. John reveals to Damon that the only way to kill an Original is with a dagger dipped in white ash wood. Luka reveals that he and his dad are working for Elijah because Klaus has his sister and they want him dead as well. Luka says that Klaus will be weakened by the sacrificial ritual, meaning Elena has to die so Elijah can kill him and set his sister free. John gives Damon the special dagger so he can kill Elijah. Elena finds out that if a vampire uses the dagger, he will die. Stefan warns Damon and Alaric stabs Elijah, but he pulls the dagger out and tells Elena their deal is off. Elena stabs herself in the stomach, and Elijah begs her to let him heal her. She stabs him with the dagger and leaves it in him. Elijah's death means Katherine is no longer compelled to stay in the tomb. She helps kill the Martin witches. When Isobel returns to town, Katherine double crosses the Salvatores by kidnapping Elena and stealing the moonstone for Klaus in exchange for her freedom. But it is revealed that Klaus wanted Katherine and compelled Isobel to get her and the moonstone. Klaus enters Alaric's body and compels Katherine to stay in Alaric's house. Bonnie and Jeremy find the site of the witch massacre and Bonnie channels all the power. The Alaric-Klaus blends with Stefan, Damon, and Elena, and finds out that Bonnie has her powers. Damon tells Bonnie to put on a protection spell and make Alaric-Klaus believe that she is dead. Elena does not want Bonnie to die, so she pulls the dagger out of Elijah's heart. After Elijah comes to life, Elena learns the full history of Klaus. The Aztec curse is fake and there is a curse on Klaus as he is a vampire born in a werewolf bloodline.
Elena realizes that the real curse is on Klaus, therefore he wants to kill her. But Elijah says he has an elixir that can save Elena from dying.
Elena believes Elijah, but Damon says that there is another way to save her life and feeds her his blood. Stefan comes to stop him but he stakes the wooden dagger in his chest. Elena then cries that she will be a vampire but she does not want to become one. Maddox kidnaps Tyler and Caroline. Damon frees them and is bitten. Then he goes to Klaus to tell him that he does not have a werewolf and witch as he killed Maddox. Klaus says that he has backups. Klaus takes Elena from Stefan. Greta sees Jenna, who thinks she is dead, but Greta tells her she is transitioning. Damon asks Katherine about the vampire, and she says that Jenna is the backup vampire. He then checks his arm and sees that Tyler has bitten him.
