Stefan's True Nature:
Damon reveals that Stefan has been feeding on human blood again. Stefan struggles with the guilt and confronts Damon about his addiction, leading to a fight between the brothers.
Elena and Damon:
Elena becomes increasingly drawn to Damon despite her feelings for Stefan. Damon confronts Elena about her feelings for him, and they share a kiss.
The Lockwood Secret:
Tyler Lockwood's werewolf curse is revealed. He undergoes a painful transformation, and his father, Richard, tries to keep the Lockwood secret hidden.
Founders' Day Celebration:
The town of Mystic Falls holds its annual Founders' Day celebration. However, the event turns dangerous as vampires, werewolves, and witches converge.
Stefan's Sacrifice:
Stefan discovers Katherine, the vampire who turned him and Damon, is back in Mystic Falls. Katherine wants Stefan and Damon to become vampires again and tries to compel Stefan to do so. However, Stefan sacrifices himself to save Elena from Katherine's compulsion.
Elena's Choice:
In a heart-wrenching moment, Elena must choose between Stefan and Damon. She chooses Stefan, and he is given the cure for vampirism.
The Ending:
Stefan leaves Mystic Falls with Klaus, an original vampire, to break a curse. Elena and Damon mourn Stefan's departure and vow to find him.
The season 1 finale leaves many questions unanswered, setting the stage for the events of season 2.