Mister Monday is the first novel in the Keys to the Kingdom series by Garth Nix. The other books in the series are: Grim Tuesday, Drowned Wednesday, Sir Thursday, Lady Friday, Superior Saturday and Lord Sunday. Mister Monday is afflicted with the deadly sin of Sloth.
Meanwhile, on Earth, a boy named Arthur Penhaligon is struggling through his first Monday at a new school. He suffers from severe asthma, but despite his pleas to his teacher, Mr. Weightman, he is forced to run the cross-country course. He collapses due to an asthma attack. Two of his schoolmates, Ed and Leaf, stop to help him use his inhaler and then run to get help.
While waiting for help, Arthur notices two strange-looking men materializing out of thin air. The first, known as Sneezer, is old, tall, grey-haired, and untidily dressed. The other is in a three-wheeled bath-chair which is being pushed by Sneezer. This man is much younger, handsome, but still bizarrely dressed. He is known as Mister Monday. They come close to Arthur, discussing the question of whether or not to give 'the Key' to Arthur. Monday is disinclined, but Sneezer assures him that by giving the Key to Arthur, who is meant to die shortly, Monday will fulfill his part of the Will. After Arthur dies, he will once again regain control of the Key. Reluctantly, Monday drops the Key into Arthur's hand. The Key is a thin, metal spike like the blade of a knife, but appears to have the shape of the minute hand of an old clock. Sneezer instructs Monday to give Arthur the other half, but Monday refuses, claiming that with the full Key, Arthur might live. Sneezer becomes angry; his voice changes as he spits out both words and a line of type. Monday points a metallic spike at Sneezer; Arthur hears a scream; then both Monday and Sneezer are gone, while the swirling type remains. The type becomes a small book which drops onto Arthur's head. Arthur puts the book in his pocket, picks up the Key and suddenly regains his breath. He notices his Coach and the School Nurse rushing across the field towards him. He buries the Key and immediately feel his lungs tighten, before he passes out.
Arthur wakes up in a hospital bed, tired. He is visited by the two schoolmates who helped him on the cross-country track. Their names are Ed and Leaf, and they are brother and sister respectively. Leaf comments that she had seen an old man pushing a bath-chair with a young man in it; obviously Sneezer and Mister Monday. Ed has not seen Monday and Sneezer, but claims to have seen a bunch of men with dog-like faces digging up the field. Leaf confirms this. Arthur, who desperately wanted the event to have been a dream, asks Leaf to retrieve the small book he placed in his school shirt pocket. She does; Arthur is disappointed to discover that the events were real after all. Ed and Leaf are ushered out by a nurse who then proceeds to give Arthur an injection. In pain, Arthur pushes his hand under the pillow, only to have his fingers touch the Key, which has mysteriously appeared there.
A week later, Arthur returns home. When he gets there, he uses the key to open the book, which is called the The Compleat Atlas of the House and Immediate Environs, and learns that the House (a giant, polyglot-fashioned building that he passed when going home) has an entrance called Monday's Postern. That night, he is visited by the dog-like "Fetchers," noted by Ed earlier. He is saved by the ceramic Komodo dragon in which he had placed the key. During the next day, he finds himself being pursued by Monday's Noon, who proves to be more powerful than the relatively vulnerable Fetchers.
He escapes, using the Key, and enters the House as directed by a mysterious message. There, he finds that the House is a world unto itself, around which the Universe is organized, whose purpose is, or was, to observe and record all that occurs in the infinity. In his travels through the House, he finds that he is the Rightful Heir, a person to whom the Will referred. If he fulfills this function, the Architect of the World's original intention will be enacted. To fulfill it, he must and does seize control of the other half of Mister Monday's Key, which grants mastery of the demesne called the Lower House. He is accompanied during the most of his journey by a cockney girl-child named Suzy Turquoise Blue, who was brought to the House by the Piper (one of the immortal Denizens of the House) along with many other children.
There, too, he learns something of the House's history: wherein the seven Trustees known as the Morrow Days, being contaminated by the seven deadly sins, refused to obey the Architect's Will; wherein the Architect's consort, the Old One, had acted in the role of Prometheus, in that he had defied the Architect for some purpose of his own and been imprisoned as a result; wherein the House, under the inadequate rule of the Trustees, has become a dystopia.
Arthur, at the end of the book, heals Mister Monday of his sin, hands the responsibility of government over to the Will itself, manifest in the form of Dame Primus (a name almost literally meaning "First Lady"), returns to Earth, and tries to regain the threads of his old life.