The book begins with Saturday discussing with her new Dusk (who was younger brother to Saturday's former Dusk) her plan to invade Sunday 's realm, the Incomparable Gardens , which she has been trying to reach since the Architect disappeared.
Arthur, returning to Earth with Friday's captives, discovers that the national army (under orders from General Pravuil, who is secretly Saturday's servant) is planning to destroy his hometown with nuclear bombs to prevent a disease outbreak, and uses the Fifth Key to stop time so that he can find a way to forestall this disaster. He then discovers that he is now more than 60% contaminated by sorcery, and is therefore irrevocably turning into an immortal Denizen of the House . Seeing no other choice, he returns to the Lower House (leaving his friend Leaf to deal with Lady Fridays' Sleepers) to find it empty and on the verge of being annihilated by Nothing . He barely escapes, but reaches the Deep Coal Cellar, which is partially protected by the sorceries surrounding the Old One 's prison. After finding his advisor Dr. Scamandros in the Cellar, Arthur confronts the Old One, who has grown in power and has destroyed his clockwork jailers. He is now surrounded by flowers, which he calls a sign of impending change. He has become much younger and has defeated the puppets which came out and tortured him for the past 10,000 years. After talking to the Old One (receiving no definite answers to his questions), Arthur returns to the Citadel in the Great Maze and plans a pre-emptive strike against Saturday.
Arthur hence learns that Saturday (as Dame Primus suspected in Lady Friday ) has deliberately destroyed the Lower House and Far Reaches to stunt the growth of the Drasil trees that lift Sunday's realm higher than Saturday can build. She believes that even if the rest of the House is destroyed, the Incomparable Gardens will remain. Dame Primus wishes for Arthur to fortify the rest of the House against the influx of Nothing , but is reluctantly convinced to split herself into two (Dame Quarto and Dame Septum) to administer the House while Arthur seizes the last Keys from Saturday and Sunday. Before she splits, Dame Primus tells Arthur that the Nothing which had destroyed the Far Reaches and Lower House cannot be held off for ever unless the Seventh Key is recovered. Arthur requests Dame Primus to give him the Compleat Atlas of the House, but Dame Primus evasively tells him that she does not have it and advises him to look for it in any bookshelves he may come accross. He suspects she is lying, but has no idea of her motive.
Arthur, accompanied by Suzy Turquoise Blue , who has appointed herself General of a small regiment consisting solely of Piper's Children, infiltrates the Upper House using the Simultaneous Bottles (see quantum entanglement) owned by the Raised Rats. There, they find that Saturday is preparing her assault on the Incomparable Gardens, which are now within reach of her Tower. Finding that the Sixth Part of the Will is dispersed in the rain that constantly falls in the Upper House, Arthur goes to a holding tank for the water to reconstitute the Will (whereupon it assumes the shape of a raven), but is separated from Suzy when they are attacked by Artful Loungers . Flushed down to the lowest levels of the Upper House, he encounters a force of New Nithlings , led by the Piper , preparing to attack the Upper House. Throughout all this, Arthur finds that this part of the Will is more agreeable to him than the first four, though not more so than the fifth, and more humble than any of them. His own metamorphosis is a subject of much attention in preceding chapters, wherein he discusses the changes with himself and Suzy and struggles to control a ferocious arrogance and accompanying impatience that seem to have become part of his personality as a result of his transformation. It is suggested several times that he is not a typical Denizen, on the basis that his blood is golden rather than blue and his eyes, unlike any Denizen's, are luminous.
With the Will's help, Arthur disguises himself as a Sorcerous Supernumerary (the lowest-ranked and most terminally depressed sorcerer) in hopes of rescuing Suzy, but is caught up in Saturday's assault force. In the Incomparable Gardens, Arthur tries to take advantage of Saturday's distraction by Sunday's insectile guards to call the Sixth Key to him, but is thrown off the edge of the Gardens by Superior Saturday's spell as he completes the necessary incantation. The outcome is unrevealed.
The epilogue shows Leaf trying to get as many people into the hospital's bomb shelter as she can before the bombs are launched. With the help of staff-member Martine, she brings at least fifty-three into safety; but the others are killed as the bombs arrive. As the book concludes, she calls for Arthur to return and set things right.