But anyway, you get a letter from Cassia asking you to come to a forgotten Dwemer ruin in the south west corner of Skyrim. Her brother has been gone in there for a long time and she wants you to come and save him.
The story is a mystery that needs to be solved. Time-travel is involved as well as paradoxes. What is the Dwarves' Law? Are the Dwemer busts on the walls really watching your every move? How does the sun under the ground work? And who will be the one to break the Dwarves' Law and condemn the whole city to slaughter? It's non-linear and there are multiple possible endings.
When you take the leap of faith you arrive in a deserted city. There are burned corpses everywhere and on the square in front of you, from a branch in the tree in the middle hangs the corpse of an old man. Approaching him he falls down and you pick up his suicide note. That's Altrius, Cassia's brother. How did he get old?! The letter mentions a loop of events and the key to the Lakehouse. The Citadel is the only door that can be opened and exploring it you find several letters in the inhabitants' rooms, as well as an indicator that one of them was clearly a member of the Dark Brotherhood. On the top floor you're attacked by Altrius' ghost and then you find the key to the Lakehouse on the Jarl's balcony. The balcony collapses once you pick up the key and you almost die. Then you head to the Lakehouse where you find a letter from the Jarl saying that he opened a portal to the past with his life energy as the price and asks the player to prevent the horror that happened.
So you go through the portal and end up in the past. You're taken to meet the Jarl and after explaining that you got there from the future and giving him his own letter he asks you to find out who is most likely to break the Dwarves' Law and doom them all.