The End of the Affair

Klaus and Stefan arrive in Chicago, where they knew each other during one of Stefan's previous periods as a Ripper in the 1920s. It is revealed that Klaus knew Stefan in the 1920s. Stefan met Klaus and Rebekah in a bar where Stefan fell in love with Rebekah and become like a brother to Klaus. Klaus and Rebekah had to leave and Klaus compelled Stefan to forget them. Klaus then daggers Rebekah when she chooses Stefan. In present day, he removes the dagger keeping her demobilized as she has what Gloria (a powerful witch) needs to help Klaus create hybrids. It is revealed that what Gloria needs is the necklace Stefan had given Elena. Damon gets a tip on Stefan and Klaus' whereabouts from Katherine so he and Elena follow the trail. They go to Stefan's old apartment, where Elena sees Stefan's list of victims. While there, Stefan and Klaus arrive. Stefan sees Elena hiding in his closet, but does not tell Klaus. Tyler tries to rescue Caroline, who is being tortured by her father, who believes he can change her urge to feed on blood by burning her in sunlight every time she feels the urge.

Plot summary from Wikipedia (CC BY-SA 4.0). Source