PLOT:

Princess Principal is set in a fictional version of England called Albion. The less well-off of Albion grew angry at their country's ruling class for largely ignoring their problems, sparking the "London Revolution" in which the lower classes attempted to overthrow the royal family. Eventually, the two sides reached a stalemate, and a great wall was erected through the middle of London, splitting Albion into two nations: the Commonwealth and the Kingdom. 

Several years later, the Commonwealth launches "Operation Changeling", a plan to replace the Kingdom's Princess Charlotte with Ange, a girl who bears a strong resemblance to her, in order to have a highly placed agent within the royal family. However, the Princess turns the plan on the spies, offering to work with the Commonwealth if Ange and her friends will help her become the Queen of the Kingdom. So begins the story as five girls, including the Princess herself, serve as undercover spies in the Kingdom working for the Commonwealth, while enrolled as students at the prestigious Queen's Mayfair School.