The year is 4E 23. 23 years ago, Martin Septim failed to keep back Mehrunes Dagon and the Gates of Oblivion. A battle took place with Martin and Mehrunes Dagon, in which Mehrunes killed Martin before he summoned the power of Akatosh. The Daedra took over Cyrodiil, the best of the Deadric gods holding a city. The Elder Council recieved reinforcements from other nations, but they all failed. 23 years later a group of refugees (us, the RPers) have formulated a plan to destory the Deadra and claim back Cyrodiil; before they enslave Tamriel.
OR that the Deadric War machine destroyed Tamriel, making it a Wasteland similar to Washington DC in Fallout 3. Creatures have been have had their powers boosted by fire and the Deadra are attempting to take over Tamriel (like the Enclave.) A group of bandits (us) see a greater good and attempt to kill the Deadra.