You join the Dawnguard, do a quest, meet an awesome chick named Serana, and her father gives you an ultimatum of whether or not to join the Vampires. If you do, then you do this quest where you get high and you get some juicy blood for the Vampires. Then, you get this moth priest to read an Elder Scroll. Then you go to this place called the Soul Cairn where all the people ever soul trapped exists. Then, you meet Serana's mom and fight a giant dragon.
Then, you get two other Elder Scrolls, play in a field of daises with moths, and magically get the ability to read them. You go to this really stupid place called the Forgotten Vale, you get Holy Water, and then you get the Bow of the Sun God, Auriel. Then you fight Harkon, die a lot, and then you and Serana live happily ever after.
tl;dr: You do a bunch of stupid stuff and fight a big bad vampire at the end with a bow that can shoot Princess Celestia's poo.