Arthur is a regular kid until he finds out he has inherited the keys to the secondary realm (a realm that lies parallel to the real world) and must now deal with all the political power plays between the beings that live there who don't want to let Arthur take control. Arthur is just your ordinary kid except one day he is visited by an other-worldly being that claims he is the Rightful Heir of the Keys to the Kingdom of the Secondary Realm from the Architect. The Secondary realm is a realm that exists next to the real world and the Architect was the being who created the realm. Arthur has already gone on an adventure in the Secondary Realms to claim the First Key from a being who calls himself Mister Monday in the first book of the series. Arthur has also already visited the Pit (it is a pit in the bottom of the Secondary Realm) which is owned by another being called Grim Tuesday (in the second book of the series). At the moment, Arthur has claimed the First and Second Key and has the companionship of the First and Second parts of the Will (the Will details what Arthur has inherited from the Architect but the Will is also a sentient creature) which have combined to create one creature that helps Arthur on the next stage of his adventures.
Arthur is recuperating from his adventures in Grim Tuesday's Pit at a hospital. He is visited by a friend, Leaf, and he decides to tell her the truth about everything he's been experiencing. She believes his story since she has also witnessed some of the odd things that results from these beings visiting their world. Suddenly, his hospital room is flooded with water and he and Leaf are swept away into a different world where they are floating along on an ocean. A big boat comes along and kidnaps Leaf but fails to get Arthur, who is swept away on his hospital bed boat. Arthur is adrift in the sea and he tries to use the Captain's Disc (a talisman of sorts that was given to him by a man named Captain Tom when Arthur was escaping from Grim Tuesday's Pit). He uses the disc to communicate with Captain Tom and try to get some help from him. One day, Arthur sees a floating buoy and he accidentally activates the buoy so that it releases some birds that fly away, accusing him of being a thief. Arthur has no idea what this could mean but has a bad feeling. A few hours later, another boat arrives with a bunch of scavengers (beings of the Secondary Realm that search for treasure). The scavengers think that Arthur has found treasure but they are afraid of who owns the treasure. Arthur convinces them to let him aboard their ship. They notice that Arthur has a red mark on his hand which is the mark of the pirate Feverfew. Everyone is afraid of Feverfew because he is a sorcerer of Nothing (Nothing is a substance that can destroy anything it touches but at the same time can be used by sorcerers to create things). Soon, Feverfew finds them and the chase is on. In order for everyone to escape, the scavenger crew's own sorcerer opens a transport portal.
Arthur and the ship travel through the portal only to become beached on an island – the ship is very damaged and requires lots of repairs so Arthur is stuck. No one knows how to remove Arthur's Feverfew mark. Arthur suggests to visit Drowned Wednesday who has invited Arthur to lunch. Drowned Wednesday is the apparent ruler of the Border Seas that Arthus has been traveling upon. Arthur learns that Drowned Wednesday loves to eat and has an insatiable hunger. She keeps her size in check through the power of the third key. Arthur is brought to Drowned Wednesday who tells him that she wanted to free the Will and cede her power to the true heir however she was betrayed by the heads of the other houses who stole the Will. Now she spends all her energy eating and trying to keep her size in check. She doesn't have enough power to control where her Border Sea goes and the Border Sea is leaking to all places of the Secondary Realm. She asks Arthur for his help to get the Will back. At the end of the conversation, she turns into a whale, the only form big enough to contain her insatiable hunger.
Arthur is dropped off with some talking ship rats who tell Arthur that Leaf has been enlisted to work as a member of the crew of another rat-manned ship but she is safe. The ship rats have an extensive network of spies all over the Border Seas so they know valuable information. They tell Arthur that the Will is likely hidden inside a protected "world bubble" inside Wednesday's stomach. Later on, Arthur finds out that Leaf's crew has been taken captive by Feverfew. The rats supply Arthur with a submarine and a crew of rats to help him man it and Suzy, one of Arthur's friends from the Secondary Realm, shows up to help him. They successfully get into Wednesday's whale belly and into the "world bubble". Feverfew and his crew are already there and Arthur and Suzy go into hiding. They meet with some locals of the world who call themselves the Followers of the Carp. Arthur finds out that the Carp is actually the third part of the Will in its sentient form.
Eventually, Arthur will succeed in saving Leaf and the Will along with all the Followers of the Carp from Feverfew. They will escape the "world bubble" before it bursts and Arthur will free Wednesday from her burden or non-stop eating. The power of the third key and the Border Sea will be transferred to Arthur.
Best part of story, including ending:
I like that Garth Nix is using the Seven Deadly Sins as an outline for the story.
Best scene in story:
My favorite scene was when Arthur and Leaf are swept away in a hospital bed after Arthur thought he could finally have a moment to rest.
Opinion about the main character:
I like that Arthur is smart and able to figure out how to trick Feverfew and rescue his friend Leaf as well as the third part of the Will.