Arthur Penhaligon is stuck in the Lower House because someone - or something - has assumed his identity and is taking over his life. But before Arthur can take action, he is drafted by Sir Thursday and forced to join the Glorious Army of the Architect.
The Army has its headquarters in the Great Maze, a defensive area of the House. Half of the Maze has already been dissolved by Nothing, and hordes of Nithlings emerge regularly to attack the rest. If the Nithling invasion can overcome the Army and the Great Maze, the House will be lost and the whole universe destroyed.
While Leaf tries to banish Arthur's doppelganger on Earth, Arthur must survive his basic training, avoid getting posted to the Front, and work out how to free Part Four of the Will. Somehow he must gain the Fourth Key from Sir Thursday ... if the latest, strongest and most dangerous Nithling offensive doesn't break through first.
Sir Thursday is the fourth book in the Keys to the Kingdom series.