Heres my written summary of the start of the plot, and dont worry there arent any spoilers:
The first episode starts with a bit of a prologue telling the watchers what happened up untill the story.
Basically the Holy Empire of Britannia[show above] (what Britain would have become if we lost the revolutionary war) set it's sights on the country of Japan. Now despite Britannia's massive size and strength Japan was able to hold it's ground. But not for long. Britannia soon brought out it's secret weapons: Knightmares.
Basically they are gundums with grappling hooks and sh*t. These knightmares crushed the japanese armies and Britannia emerged victorious. Britannia decided to continue to belittle the people of humble japan by renaming the country Area Jyuuichi (11) and calling the citizens Elevens.
The story follows a Britannian student named Lelouch who gets caught up in a terrorist plot. He gets surrounded by Britannian soldiers in a basement when a strange woman, who comes out of a capsle thought to have been filled with poison gas, gives him a strange ability. The Geass.
The Geass contains the power to enforce absolute command on anyone who the user looks at. Lelouch quickly uses this power and orders the group of soldiers to die. And with a final sentance of "Yes Your Majesty" the soldiers all commit suicide. With this newfound power Lelouch decides to put it to use. He is going to destroy the Britannian Empire.