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.
On Earth, a boy named Arthur Penhaligon (The main character) is at a new school. He collapses during an outdoor cross country run during gym because of his severe asthma. Two of his schoolmates, Ed and Leaf, stop to help him use his inhaler before running to get help.
While waiting for help, Arthur notices two strange-looking men materialising out of thin air. They discuss a key and whether or not to give it to Arthur. One of the men, Monday, doesn't want to give it up because he needs the key in order to continue his reign, but the other man, Sneezer, convinces him to as the Will states Monday must give the key to a suitable heir but after Arthur dies, Monday will once again regain control of the Key. Persuaded, Monday agrees to relinquish control of the key, which is shaped like the minute hand of an old clock, although he quickly becomes suspicious of Sneezer, who apparently never showed much intelligence before. Sneezer and Mister Monday then fight and disappear in a flash of light. In their place is a slim book which Arthur puts in his pocket. In his hand, he holds the key which Arthur finds helps him breathe. As his teacher and school nurse approach, Arthur hides the key and passes out.
Arthur wakes up in a hospital bed, tired. Ed and Leaf, the brother and sister who helped him, come and visit. 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, however, had not seen Monday and Sneezer, but claims to have seen a bunch of men with dog-like faces digging up the field. Arthur asks them if anyone had seen the key he buried in the field and they say no, but promise to visit him soon. Shortly afterward, they have to leave so Arthur can take a shot. In pain, Arthur pushes his hand under the pillow, only to have his fingers touch the key, which has appeared there magically.
A week later, Arthur returns home but Ed and Leaf have not kept their promise to visit him. When he gets there, he uses the key to open the book, which is called The Complete 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 one of the dog-like "Fetchers" mentioned by Ed earlier. He is saved by his ceramic Komodo dragon in which he had placed the Key. During the next day at school, he finds himself being pursued by the Fetchers again. Before being excused from gym class, a friend of Leaf's hands him a printed out email which explains her and her brother's absence. Apparently, they have caught some sort of virus and have been placed in quarantine. Leaf notes that she can smell the same stench from the Fetchers on those in her family but no one else can smell it. Arthur seeks sanctuary in the school library to figure out the Atlas and hide from the Fetchers. During this brief time, he reads up on the Fetchers and finds out that they are vulnerable to salt. His research is interrupted when Monday's Noon, a servant of Mister Monday who proves to be more powerful than the Fetchers, shows up. He attempts to attack Arthur and inadvertently starts a fire in the library.
However, Monday's Noon can only attack him between the hours of noon and one o'clock and Arthur manages to evade him until then and Noon disappears. Unfortunately, Arthur still has to contend with the Fetchers who have managed to steal away his Atlas during his struggle with Monday's Noon. He escapes to the kitchens and finds salt to destroy them. During this time, the fire department and the bio-containment and quarantine team, who believe the new virus that Ed and Leaf's family are suffering from has originated from the school, arrive. Arthur realises that the Fetchers must have caused the disease and notices that the smoke from the burning library is spelling out a message for him to get to the House. Arthur lets go of the key to induce his asthma which causes a paramedic to take him to the hospital. As they get closer to his neighbourhood, an unknown force hits the ambulance in the form of a violent rainstorm. Arthur takes advantage of this distraction to leave the paramedic and get to the House. Using the Key, Arthur enters the House and finds that the House is a world unto itself, around which the Universe is organised, whose purpose is, or was, to observe and record all that occurs in the infinity. He sets off on a journey discovering that while there he is in perfect health during his travels through the House, he finds that he is the Rightful Heir, a person to whom the Will referred. If he fulfils this function, the Architect of the World's original intention will be enacted. To save the Lower House, he must kill Mister Monday and steal the other half of the key, of which is the Hour Hand, from him. 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: The Architect and the Old One came from nothing to create the universe. Unfortunately, the Architect's consort, the Old One, acts in the role of Prometheus, in that he had defied the Architect for some purpose of his own and been imprisoned as a result. After a while, the Architect left the House and handed over control to the Morrow days. Unfortunately, these Trustees have each been contaminated with one of seven deadly sins and have disobeyed the Architect's will to observe and record the happenings of the Secondary Realms (Our universe) in favour of interfering to where it suits them. Because of their inadequate rule, the House has become a dystopia.
At the end of the book, Arthur defeats Mister Monday and takes control of the Hour Hand and gains the First of Seven Keys to the Kingdom. To the displeasure of the Will, he takes pity on Mister Monday and lets him go, after healing him mentally and physically. After going home, with the Will, he hands the responsibility of government over to the Will itself which manifests in the form of Dame Primus (a name almost literally meaning "First Lady" in Latin). He appoints Suzy as the second assistant to Noon who is actually Monday's Dusk who aided Arthur in his attempt to secure the key. Monday's former Noon is changed to the new Dusk while Monday's Dawn retains her position. Monday's Noon (AKA Monday's Dusk) gives Arthur a 'Nightsweeper' to get rid of the virus the dog faces brought with them. After waiting for ten hours, only to fall asleep and wake up at 11:56, Arthur puts the Nightsweeper (which looks like a toy horse) on his windowsill. It flies over his town, waking up everyone as it passes by. Arthur's family tells Arthur that his mother found a cure for the plague.
Then a phone rings, which none of Arthur's family can hear. He realizes it is the phone Dame Primus gave him in case of emergencies. The book ends with Arthur looking at the clock and seeing it is one minute past twelve, Tuesday morning.