Lord Sunday is the first son of The Old One and The Architect. He wields the seventh Key, and rules The Incomparable Gardens. He has ultimate authority in the House, but tends to leave the charge of the day-to-day affairs to his next-in-command, Superior Saturday, who has sought to prevent the implementation of the Architect's Will.
One of the Seven Trustees, Lord Sunday defied the Seventh Part of the Will of his mother and, along with six other Trustees, keeping his demesne and Key for himself rather than yielding it to a human heir, believing that the transition of the Architect's powers and authority to a mortal to be inappropriate. He reveals that he only defied the Architect so that he could keep on tending to his Garden.
Each of the Trustees is afflicted with a deadly sin. Six of the seven have been portrayed, leaving only pride for Lord Sunday.
It is revealed in Superior Saturday that even if all the realms of the House were to be destroyed by Nothing, except the Incomparable Gardens, the universe will still exist and will not be destroyed as Arthur previously believed. Arthur believed that the entire House was created before the universe and that its partial destruction would thus cause the Secondary Realms to cease to exist. This is incorrect as The Incomparable Gardens alone were in fact the first thing the Architect created from Nothing; the Secondary Realms were created secondly, followed closely by the other Demesnes of the House for the purpose of study and documentation. This therefore makes the Incomparable Gardens the most important part of the House, and indeed, the epicentre of all creation. The Incomparable Gardens are 'defended' by a variety of bug-like creatures (believed to be Nithlings), mostly giant worms, beetles, and similar. The Gardens' supporting Drasil Trees, which are situated in the four corners of the Upper House, are also defended by beetle-shaped Nithlings. Past attempts to invade have failed due to these creatures, and Superior Saturday's attempts to build a tower tall enough to reach Sunday failed due to the Drasil trees' growth. However, she was able to slow them down by destroying the two lowest realms of the house with Nothing. Thus, she was able to launch her invasion.
Superior Saturday's defining Deadly Sin is Envy, particularly of Lord Sunday. She even attempted to take over his reign of the Incomparable Gardens as she considers it her right since she was the first Denizen to be created by the Architect.
Falling from the Incomparable Gardens in Superior Saturday, Arthur, having won the Sixth Key, escapes impalement on Saturday's Tower by entering the Improbable Stair. His uncontrollable falling leads the Stair to spit him out somewhere completely unexpected - he is under attack from sentient insects somewhere in the Secondary Realms, and is unable to concentrate to use the Fifth Key to escape.
Meanwhile, Suzy Turquoise Blue, Arthur's friend, plots to escape from her prison in Saturday's Tower while battle rages above and below her. Saturday's forces are pressing into the Incomparable Gardens, but are also engaged in a fierce struggle to keep the Piper and his army of Newniths at the bottom of the Upper House. Suzy is being held by the intelligent but forgetful Sorcerous Supernumerary (meaning he failed his final sorcery exams), Giac. She, with some help from the sixth part of the Will, persuades Giac to free her and accompany her to the Citadel in the Incomparable Gardens.
On Earth, Leaf, responsible for the Sleepers from Lady Friday, struggles to cope with the aftermath of a nuclear strike and desperately needs help, especially since she herself has become a target for intruders from the House, namely Lord Sunday's Dusk and his 'pet'. Within the House, Nothing continues to rise and must be stopped before it can destroy the entire House and Universe. Sunday intends to hold Arthur to ransom by controlling Leaf and his mother; to this end, he dispatches the "Reaper" (Sunday's Dusk) to take Leaf through the Front Door. It is then revealed that the Door is filled with Nithlings (and is, indeed, collapsing through contamination by Nothing). The Lieutenant-Keeper of the Door is vanquished in battle with them; the Reaper comes to his aid a few minutes too late, and the Keeper dies (handing over the post to an unwilling Leaf). Leaf, being the new Lieutenant-Keeper, cannot be compelled to leave the Front Door, so the Reaper goes back to Lord Sunday to report his failure.
Arthur returns from the unknown planet in the Secondary Realms by a supreme effort of will and control over the Improbable Stair, because his ear was shot off before he fully entered the Stair; once healed, he believes he has returned to his bedroom on Earth, but as he searches his house he finds his mother (who went missing during the events of "Lady Friday") in the living room. As he approaches her he realizes that she can't see him. It is later revealed that Arthur's mother is trapped in a time loop and is being displayed as an exhibit in the Incomparable Gardens. He cannot interact with her; when he looks out of the windows he can see nothing but green leaves draped against them. Soon after, a Piper's child (employed as a gardener) enters the house with a flaming pitchfork; Arthur grabs and deactivates it, and forces the boy to lead him out of the house. The boy professes not to have heard of the upheavals among the Trustees; he thinks Arthur is the Reaper (leading Arthur to suspect that the Reaper is Sunday’s Dawn, Noon or Dusk).
As they leave Arthur's house, they are ambushed by Sunday's Dawn and Noon; and the Piper's child reveals himself to be Lord Sunday in disguise. After chaining him up by his arms to a colossal dragonfly, (the form of transport commonly used in the Gardens) they fly him over the Gardens to a smaller replica of the clock face that the Old One is bound to. Arthur, his Keys overcome by the superior power of the Seventh Key, tries to escape by using his own innate powers to form something out of Nothing; he fails to create anything, but he brings his childhood toy, Elephant, to life. He sets it to search for his confiscated Keys; meanwhile, he calls the Mariner to come and rescue him. Hours later, the Keys are sent to him and Arthur is able to free himself.
Leaf has ventured to the Middle House via portal and has joined forces with Suzy's band of motley Raiders in an attempt to infiltrate the Upper House, in order to prepare elevators for the Army of the Architect to get through to the Gardens. Fred Initial Numbers Gold, during an attack by the Piper's army, accidentally picks up Leaf's sword, and with it the post of Lieutenant-Keeper of the Front Door.
Arthur battles with Lord Sunday, who also has to contend with an invasion by the combined forces of the Piper and the subservient Saturday. The battle is taken to the Elysium, the epicenter of creation; it is here that the seventh part of the Will (a withered apple tree) is kept in a gilt cage. The Mariner opens it at Arthur's behest, but is killed in the process. The freed Will traps Sunday while Arthur claims the Seventh Key (in the form of a small key). When Dame Primus bites into an apple from the manifestation of Part Seven of the Will, the Will is made whole; Arthur unknowingly becomes its channel for its intended purpose - the destruction of the House. The Old One is freed, and He steps into the Will; it is explained that the Architect split Herself in two at the beginning of Creation to speed the evolutionary process, and the Old One is in fact a part of Her (but he had to be chained up when his views grew distinct from Hers), and the manifestation of the Will is the Architect in Her entirety. She reveals that, bored with life, She wished to return to Nothing, to see what it was like to die. She could not do so, however, without destroying the House, because in her anger She had chained the Old One to the fate of the House, and the Old One was anchoring her to existence. The Architect destroys the remnants of Creation with the powers of the Keys. Arthur, now omnipotent, survives and is anointed "New Architect" by the fading thoughts of the Architect. This is why She needed a mortal Heir, to provide a creative spirit to renew the Universe. Arthur decides to remake the Universe exactly as it was, using the Compleat Atlas of the House to provide the template. Because the Atlas holds a record of the House and Realms a few seconds before the destruction, Arthur is unable to save his mother, but he does split himself into the New Architect and the human Arthur, before his transformation into a denizen of the house. The new Arthur is not quite the same, being immune to all disease and with all the knowledge he acquired on his journey. He sends Arthur and Leaf back to Earth, by recreating the Seven Dials.
In the new Creation of the Universe, Suzy reveals her desire to be the new Lady Sunday, and pressures the New Architect to remake the House. The New Architect reveals he will soon bring back Doctor Scamandros, Fred, and Giac, the sorcerer, who could all help with the creation of new Denizens.