<Tr> <Td> </Td> <Td> This section's plot summary may be too long or excessively detailed . Please help improve it by removing unnecessary details and making it more concise . (October 2011) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . </P> <P> 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 . </P> <P> 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 . </P>

When do the originals come into the vampire diaries