Little is known yet about the plot, but players will vie for control of the throne of the Emperor in Cyrodiil.
All of Tamriel is in unrest due to the lack of a ruler. Races from certain provinces of Tamriel form alliances to defend their provinces and expand their territory. The Imperials form their own alliance with the noble Tharn family and Mannimarco to bring Tamriel back under Imperial Rule. However, Mannimarco is secretly working with the Daedric Prince Molag Bal to rule all of Tamriel themselves.
You will play through these events as an adventurer whose soul was stolen by Molag Bal. Fight to get your Soul back and bring order back to Tamriel.
After reaching your level cap and thwarting Molag Bal's plot to take over Tamriel, the Emperor's seat is still vacant. It's time to take the Imperial City with your faction! Taking the Imperial City will not be an easy task. The entire city will be overrun with Daedra sent by Molag Bal.
Once your faction takes the city, the player that contributed the most during the entire war will become the new Emperor of Tamriel. Details on contributions have not yet been released. Becoming the Emperor is more like being at the top of an online leaderboard.
Elder Scrolls Online will take place in the Second Era, the same period The Elder Scrolls Adventures: Redguard. During this era, there is no ruler of Tamriel, and Cyrodiil is in the midst of constant warfare.
Note: A little over 1000 years prior to the events of Skyrim, The last of the Akaviri Potentates was murdered and "Civil wars and insurrection dissolve the Tamrielic Empire"