I'm sorry I'm being vague, but I don't want to include any spoilers by giving names or locations and whatnot.
It's a Fantastic show. One might even say a work of art. I highly recommend it.
Japan gets invaded by the Brittannian Empire and Japan is now renamed as Area 11. Young Lelouch, a Brittannian student, gets caught up in a terrorist attack against Brittannia, reuniting with his childhood friend Suzaku. Lelouch also meets the mysterious green-haired girl, C.C. Lelouch accepts a mysterious power C.C. offers to him. Lelouch, using the new power, wages a war against the Brittannian Empire, attempting to destroy the Empire.