Teenage boy Lelouch is an exiled prince of the Holy Britannian Empire and he has a problem. He wants revenge on the Empire, but to get it, he will have to battle through an army of their mechs. His solution is to command giant mech armies of his own to wrest control of Britannia from those who wronged him.
Code Geass takes place in an alternate timeline where King Henry VIII of England had a male heir. This led to a chain of events where Napoleon defeated Britain and the British royal family fled to North America to set up the Britannian Empire. Japan was conquered by the Empire and turned into Area 11 (its citizens grimly referred to as “elevens”).
Knightmare Frames are the tools of conquest in Code Geass. They are the terrifying battle mechs that unleash death and destruction on the Empire’s enemies. Their origins date back to earlier battles when emergency survival cockpits would detach from main battle tanks to preserve their crews in the event the tank was destroyed. These cockpits were built with artificial legs to run away from the battlefield.
Eventually, some bright spark realized that fixing weapons to robotic legs would make a neat killing machine, and Knightmare Frames were born. The first mass-produced frame, the RPI-11 Glasgow, was so successful in the Britannian invasion of Japan that every other superpower on Earth began to copy the design.
Next came the RPI-13 Sutherland, a frame that Lelouch uses in his quest to win back the Empire. The destructive potential of Knightmare Frames in Code Geass led them to become the most important weapon deployed by any army.