Holeborn was a rambling system of tunnels located in the city of London, home to a community of mice, one of the most perfect that existed in all of England. Their civilization was an ancient one, and the inhabitants were governed by a mouse known as the Great Thane. Beneath him were the Ministers, who were mice running different guilds.
The community was brought to ruin when Morgan and his army of rats attacked and every mouse there was killed. However, after the defeat of Jupiter, Arthur Brown would eventually lead a group of restless young mice from the Cutty Sark to the city, where they restored Holeborn to its former glory. Arthur became the new Thane.