The general plot has long been revealed. From the UESP:
The High King of Skyrim has been murdered, triggering a civil war among the province's people, the Nords. Many Nords wish to secede from the empire, which has been declining in power since the fall of the Septim Dynasty 200 years earlier, while others believe the empire is still worth preserving. A prophecy foretold by the Elder Scrolls is fully realized by this last event; dragons have returned to the land, and the Nordic god Alduin rises to consume the world. The player character is the last of the "Dovahkiin" (or "dragonborn"), mortals who are born with the soul of a dragon and can speak their language. Esbern serves as advisor, himself one of the last of the Blades, who swore to preserve the Empire. This prophecy is also spelled out on Alduin's Wall, a stone wall at Sky Haven Temple, the secret hidden refuge of the last of the Blades.