Plot

Arthur, a young boy, has a severe asthma attack one day as two mysterious people, one of which is called Monday, give him a 'Key', a magical artifact that saves his life, against Monday's intentions. He discovers there exists a world called the 'House', which is split into seven parts, each ruled by a 'Trustee', of which Monday is one. A divine being known as the 'Architect' has disappeared and left behind a 'Will', leaving the House and the seven Keys to her seven Trustees, but the Trustees have split up the Will and hidden the parts, intent on not letting the Will be fulfilled.

The first part of the Will has managed to break free and, by convenience, has chosen Arthur as the 'rightful heir' who must recover the other lost parts of the Will; this has led to Monday giving up half of his Key to him. In the first book, Arthur defeats Monday and takes control of the first part of the House. Books two to five follow a somewhat regular pattern: in each, Arthur tries to gain control over one part of the House, and ultimately succeeds after various challenges, setting free one part of the Will. Each book features one main antagonist, usually the Trustee of that part of the House, with the exception of one Trustee who wants the Will to be fulfilled but has been overthrown by the other Trustees. Sometimes, the Trustees sow chaos on Earth while he is away in the House.

One important recurring part of the story is Arthur limiting his use of the powerful Keys, as using them too much will turn him into an immortal, and the presence of an immortal on Earth causes harm.

At the end of the sixth book, Arthur has freed the sixth part of the Will but not yet claimed that part of the House, and the sixth Trustee is busy invading the domain of the seventh Trustee. In the seventh book, Arthur ultimately manages to free the last part of the Will, but then everything is destroyed and Arthur faces the divine being known as the 'Architect', who explains that the true purpose of the Will was to let her die and hand over the responsibility of being Architect to him. Having become the new Architect, he recreates the universe as it was, though the House will be made anew. He splits himself into the Architect and a human Arthur, whom he sends back to Earth.

Characters

• Arthur Penhaligon – A 12-year-old boy who is chosen to take back a magical world called the 'House' from the seven 'Trustees' and free each Trustee's part of the 'Will'
• Leaf – A 12-year-old human girl who befriends Arthur. She becomes involved with Arthur's adventures in the House.
• Suzy Turquoise Blue – A female inhabitant of the House, a 'Piper's Child' (meaning she was brought there by the 'Piper') who chooses to work with Arthur and becomes his friend. Has been in the House for centuries, but has the appearance and behaviour of a child.
• Fred Initial Numbers Gold - Another 'Piper's Child', a cheerful, talkative, happy-go-lucky boy who quickly befriends Arthur.
• Dame Primus – The personification of the parts of the 'Will' that Arthur has taken back so far at any point in the series.
• The Trustees – Each named after a day of the week and embodying one of the seven deadly sins; they are the rulers of the seven parts of the House and Arthur's antagonists.
• The Mariner – A powerful independent inhabitant of the House, an ally to Arthur for most of the series.
• The Piper – Another powerful independent House creature, boss of the 'Piper's Children'. Intends to take control of the House.
• The Architect - God figure. Creator of the House and the Secondary realms. Mother of the Mariner and the Piper.
• The Old One - Prometheus figure, condemned to the "coal cellar" to have his eyes gouged out every 12 hours. Husband of the architect and father of the Mariner, the Piper, and Lord Sunday.

World

Central to the story is the magical world called the House. The house is divided into seven parts, each with its Trustee:

• The Lower House – ruled by Mister Monday. Contains the prison of the divine being the 'Old One', contains the 'Front Door', which is a dome with many two-way portals including one connected with Earth, and the 'Seven Dials', which allows travel to anywhere in the universe.
• The Far Reaches – a grand cavern, ruled by Grim Tuesday.
• The Border Sea – an ocean, formally the domain of Drowned Wednesday but not under her control.
• The Great Maze – a gigantic, chessboard-like battlefield, ruled by Sir Thursday
• The Middle House – a giant, thrice-terraced mountain, ruled by Lady Friday
• The Upper House – the place where sorcerers are trained, ruled by Superior Saturday
• The Incomparable Gardens – the most important part of the House, a beautiful garden, ruled by Lord Sunday

House time is constant, while time in the universe is malleable; it is even possible to make it flow backwards with a sufficient power, such as a Key. Thus, a day may pass on Earth while half a year passes in the House. The House has 'Elevators' that allow travel between the different parts. There are also attachable 'Wings' which are used as an everyday means of short-distance travel. The House has an internal postal service, but it is notoriously poor.

The original inhabitants of the House are called 'Denizens' and are nearly immortal, though there are a few things that can kill them. Denizens are not supposed to interfere with the normal universe, but they are capable of this to some extent (a Trustee can only go to Earth on their "day") and they do. Apart from original House creatures, there are also the 'Piper's Children' and 'Raised Rats' (a group of anthropomorphic rats), which were both brought into the House by a House creature called the Piper, and so-called 'Nithlings'. Nithlings are creatures that come into existence out of the magical substance 'Nothing'; they can kill Denizens and go to the universe freely. Hordes of Nithlings fight with the Denizens, but there are also Nithlings that are allied with the Denizens and are used, for example, to wreak havoc on Earth.

The substance called 'Nothing' causes damage to things in the House and ultimately wipes it out when the Will is complete. One of the Trustees, Tuesday, makes various things out of it using his Key. The Piper creates an army of so-called 'Newniths' from it.