It's hard to talk about the plot of Code Geass without spoiling it, but basically, you have Lelouch, who is a Britannian teenager living in Japan. Various circumstances lead him to gain a power called Geass, basically he can give anyone an absolute command which they are obligated to follow, but he can only give any one person one command, ever. For various spoilery reasons, Lelouch decides to use this power to launch a rebellion against Britannia, disguising himself as "Zero", and forming a group called the Order of the Black Knights. Then ALL SORTS OF AWESOME STUFF HAPPENS.