Chapter 3 The arrival of Elena's doppelganger

The second season focuses mainly on the arrival of Elena's doppelgänger Katherine Pierce and on her reasons for returning to Mystic Falls. It also introduces werewolves. Tyler's werewolf uncle, Mason Lockwood, arrives after the death of 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 as to 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 that another Bennett is at the party and that Katherine had deceived her, she puts a spell on the moonstone, injuring Katherine. Damon then leaves her in the original tomb where she is trapped until the spell has been removed from the tomb. She tells him that her reason for not killing Elena is that she is in danger. Elena is later kidnapped by a masked individual 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 doppelgänger and needs to be sacrificed to break the sun and moon curse. Katherine was also a Petrova doppelgänger, 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 by stabbing Elijah with a sharp wooden pole and they escape.

Klaus, one of the original vampires, arrives and is now hunting Elena. His brother Elijah has made a deal with Elena to protect her loved ones, because he wants to use her as bait to lure Klaus and kill him. Jules, a friend of Mason, tells the Lockwoods that Mason never came back to Florida and is missing. Alaric is suspicious of Jules and calls Damon to the Grill (local town hangout). 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 on the shoulder instead (werewolf bites are lethal to vampires). As Rose succumbs to the effects of the bite, Damon stakes her in a glorious dream they share, giving her a peaceful death.

Uncle John returns to Mystic Falls and 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. A boy named Luka settle in the town with his father (they are both warlocks) and reveals that they are working for Elijah because Klaus has his sister Greta and they want him (Luka) dead as well. Luka becomes friends with Bonnie and tells her that Klaus will be weakened by the sacrificial ritual, meaning that Elena has to die so that Elijah can kill him and free his sister. John gives Damon the special dagger to kill Elijah.

Elena finds out that if a vampire uses the dagger, that vampire 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 that 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 a witch massacre and Bonnie channels all the power.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. There is a ritual that involves killing a vampire and a werewolf and a Petrova doppelganger, that will unearth the werewolf side of Klaus making him a hybrid.

Elena realizes that the real curse is on Klaus, who therefore 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 tries to stop him, but he stabs in the chest with the wooden dagger. Elena then cries that she will be a vampire against her will. 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. Elena 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.

In the season's finale, Klaus is working on the ritual out in the woods and is near completion when he is about to kill Elena, but Stefan tries to work a deal with him. Klaus agrees to have Stefan work for him in exchange for Damon's protection. Elena is "killed" in the ritual but is shown to have survived. Regardless, Klaus's werewolf side is unleashed and he becomes the first hybrid, half vampire half werewolf. Bonnie arrives and subdues Klaus with her powers, giving Elijah the opportunity to strike, but Klaus talks him down offering to help find their siblings. Elena's survival is revealed as John had Bonnie cast a spell that had him die in Elena's place. The next morning John succumbs and dies. At his funeral, Elena reads a note from John saying he will always love her and she will always be his daughter, even if she becomes a vampire. Damon also shows where he was bitten and walks away.