Princess Principal is set in a fictional analogue of England
called Albion. During the early 19th Century,
the Kingdom of Albion
monopolized a mysterious substance called "Cavorite" to construct a fleet of heavily-armed airships that made Albion the dominant power in the world. However, the proletariat of Albion grew angry at their country's ruling class for largely ignoring their plight, sparking the "London Revolution" where 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.