Ep 17. Know Thy Enemy

In "Know Thy Enemy," the 17th episode of the second season of The Vampire Diaries, the Mystic Falls gang is still reeling from the discovery that Katherine is out of her tomb and back in town. Elena and Stefan are still struggling with their relationship after Elena learned that Stefan used to be a blood-thirsty killer and had a tumultuous past with Katherine. Elena seeks some guidance from Alaric, who has his own issues to deal with when he realizes that Isobel, his dead wife and the mother of his child, is still alive and living with Klaus and Elijah, the Original vampires.

Meanwhile, Damon and Stefan work together to try and figure out what Katherine is up to and how they can stop her. They enlist the help of Caroline, who has been turned into a vampire and is now struggling to adjust to her new life. Meanwhile, Bonnie tries to do her part by using her witch powers to protect Elena and the others from Katherine.

As the gang races to figure out Katherine's next move, they uncover a shocking secret about her past and how it relates to the Original vampires. With time running out, the gang must come together to stop Katherine and save Mystic Falls from destruction.

Overall, "Know Thy Enemy" is a thrilling and suspenseful episode that keeps the audience on the edge of their seat. With the introduction of new plot points and the deepening of existing storylines, this episode keeps viewers engaged and invested in the characters and their journeys.