In the most important (and my personal favorite) MYTHOLOGY MOMENT of the episode, or possibly the season, we learn there is no frigging curse of the Sun and the Moon. Thank you, because I was really straining to accept an Aztec connection to this story that has largely played out in Virginia, England and Bulgaria -- you know what I'm saying? Had they used North American Native Americans from the outset, it would have made more sense, but I digress. The curse doesn't exist and that's good enough for me. It seems that for centuries, Klaus has been faking up legends about moonstones and the Petrova doppelganger and planting them all over the world, because while the Curse of the Sun and the Moon doesn't exist, the moonstone and the Petrova doppelganger are key to Klaus releasing his inner werewolf. He figures if werewolves and vampires all over the world are all seeking to break some curse they think will free them from their restrictions, someone is bound to happen upon the moonstone and La Petrova, and it'll be all over but the sacrificing and incanting.
Now at first, Elijah was willing to help Klaus in his quest to be his true, gruesome twosome monstery self, but then Katherine, or Katerina as she was then known, did what she does best -- got herself involved with a pair of gorgeous brothers. Because of his devotion to Katherine, Elijah set out to find a way to break the curse without killing La Petrova. And he found one, too. But when he brought the good news to Klaus, Klaus saw no reason to spare Katherine, even though it seems he was spearing her, if you know what I mean. Nope, Klaus was looking forward to sacrificing Katy, because he had bigger plans. Then Katherine fled (and got herself vamped). Naturally, Klaus suspected Elijah tipped her off about the sacrifice. In flashback, Elijah denies this and gives Klaus his word that he will find Katherine. Klaus threatens to kill Elijah if he fails. And his big plans? Well, it seems Klaus wants to start his own hybrid race. So, while Elijah loves/loved his brother, he will not be party to his hybrid procreational plot, especially since his brother threatened to kill him and all. The good news is, it looks Elijah might be partial, or at least amenable, to saving Petrova doppelgangers.
There's one thing I don't get, though. If Elijah is so set on not letting Klaus become a fully empowered hybrid, why was he (Elijah) seeking Elena? If it was just to protect her, wouldn't it have made sense to... I don't know... tell her that from the outset? His quest for Elena made sense when we thought that Elijah too wanted to be free of the Sun and Moon curse, but since that doesn't exist, what's his angle now? The easiest way to thwart Klaus would be to kill or vamp Elena, and yet Elijah has been pretty busy not killing her for weeks on end. The only way to kill Klaus is for a juiced up witch (hi Bonnie) to do it. Oh a little nitpick here: Elijah tells Elena that the only way to kill Klaus would be during a full moon, while he is in transition. It is then that a witch with enough power can kill Klaus (the "safely" is heavily implied). Klaus's lycanthropy is currently, magically dormant, so does this mean the curse has to be lifted before he is killed, so that he can transition and be vulnerable to killing?