he Vampire Diaries was officially renewed by The CW for a full 22-episode season on February 16, 2010. The first episode premiered on September 9, 2010, at 8 p.m. ET. The season picks up immediately after the events of the season one finale. All the series regulars returned. Season two focuses on the return of Elena Gilbert’s doppelgänger, Katherine Pierce, the introduction of werewolves, the sun and moon curse, and the arrival of the original vampires. Tyler Lockwood’s uncle, Mason Lockwood, arrives in town searching for the moonstone, a family heirloom. Tyler later learns of his family’s werewolf curse. Meanwhile, Caroline Forbes is killed by Katherine while having Damon Salvatore's blood in her system, turning her into a vampire. The arrival of the original vampires, Elijah and Klaus Mikaelson, also bring about complications. Klaus is a vampire-werewolf hybrid, but his werewolf side had been forced into dormancy by witches, as nature would not stand for such an imbalance in power. Therefore Klaus arrives in town with plans to break the curse and unleash his werewolf side by channelling the power of the full moon into the moonstone, sacrificing a vampire and a werewolf, and drinking the blood of the doppelgänger. The season became available on DVD and Blu-ray on August 30, 2011.

Overview from themoviedb.org