Elijah. He tells Elena the story of how Klaus met Katherine. We also learn:
Elijah is Klaus' brother.
The sun and the moon curse is BS.
The actual curse applies only to Klaus. His mother had an affair and Klaus came from a different bloodline. A werewolf bloodline.
The witches put curse on Klaus (and only him) to keep his werewolf half dormant. That's what he wants to break.
Klaus is immune to the dagger covered in white ash, so the only way to kill him is for a witch to do it during the Sacrifice.
Elijah found a way to perform the Sacrifice without killing the doppelganger, so at least Elena's safe.
Meanwhile, in the present:
Jenna finally finds out the truth about vampires.
The war between Stefan and Damon is in full force. The argue over whether Elena should trust Elijah, Stefan accuses Damon of being in love with Elena, Damon storms off and nearly kills Andie in a fit of rage.
The real Klaus shows up in Mystic Falls after his entourage un-possesses Alaric. Greta arrives and she seems very happy to be with Klaus.