In this segment of the Keys to the Kingdom series, Arthur confronts the fourth being of the Secondary Realm, a realm that borders the real world, and must claim the fourth magical key as well as the fourth piece of the Architect's Will. Arthur is just a regular kid trying to live a regular life but when the living form of the Will of an Architect (a being of the Secondary realm) decides to make him an heir of their realm, Arthur gets irrevocably embroiled in the politics and power-plays of another world.
The story begins with Arthur recuperating from his adventures claiming the third magical key as well as the third piece of the Architect's sentient Will from one of the beings that rule the seas of the Secondary Realm. Arthur and his friend, Leaf, try to return home to their own world however they find out that a Spirit-eater (similar to a Doppelganger) has entered his world and this Spirit-eater will eventually take over Arthur's life. Now, Arthur cannot return home otherwise like anti-matter meeting with matter, his return would create the destructive substance called Nothing. Arthur learns that the Doppelganger's source of power is a pocket that used to belong to Arthur. Leaf decides to return home, find the doppelganger and get the pocket back so the Doppelganger won't be able to take over Arthur's life. Arthur also gives leaf his third magical key - he finds out he shouldn't be using anymore magic as if he uses too much magic, he will not be able to return home because the magic will have transformed him into a denizen or being of the Secondary Realm. Arthur is given a magical ring that can track the amount of magic is in his system - if the level reaches past 50%, he will not be able to go home to his own world.
Arthur learns that one of the beings of the Secondary Realm is trying to destroy the Secondary Realm. Arthur is tasked with having to find Sir Thursday and wrest the fourth magical key from him so that he can control Sir Thursday's section of the Secondary Realm. Sir Thursday commands the Architect's Army. Arthur is tricked into accepting a gold coin that means that he accepts being drafted into Sir Thursday's Army. Because his keys don't work in Sir Thursday's domain, he gives stewardship of the keys to the sentient pieces of the Will that have joined into one being called Dame Primus.
Meanwhile, Leaf returns to the hospital in her own world. She finds out that the hospital is quarantined and that the Arthur Spirit-eater has been infecting everyone at the hospital with a mind-control mold. While she is chasing the fake Arthur, she fell down a staircase and knocked herself out. The fake Arthur then infects her with his mind control mold. When Leaf finds out, she knows her time is running out. She finds Arthur's pocket and escapes the hospital, however the entire city is under quarantine and no-one is allowed to be found on the streets as the government thinks there is some sort of infectious bio-weapon spreading around. Leaf eventually makes it to Arthur's house and calls for help from Suzy.
Meanwhile, Arthur is knee-deep in army training. He is forced to get his mind "washed", that is, all his memories wiped as part of the process of joining the army. Sir Thursday is under attack from creatures of the Second Realm and Arthur must fight at the front lines. During the battle, someone calls Arthur's real name and he is stimulated to remember who he is.
Meanwhile, Suzy takes Arthur's pocket from Leaf, who is fast losing control of herself. Suzy tries to return to the Secondary Realm, however, at the Front Door (the portal that leads to the secondary realm). She finds out that Superior Saturday is responsible for creating the Spirit-eater creature. She decides to join Arthur in Sir Thursday's domain at a place called the Great Maze, which is where the battle between Sir Thursday's army and his enemies. Arthur eventually tricks Sir Thursday into giving him the fourth magical key. Unfortunately, now that Arthur has taken over Sir Thursday's domain, he must now deal with the war. He finds out that the leader of the enemy is called the Piper and this being claims to be the son of the Architect and the true heir to the Secondary Realm. Arthur and the Piper meet under the White Flag of truce. The Piper wants the fourth magical key as well as all of Arthur's magical keys but Arthur refuses. Arthur eventually gets the help of Dame Primus, who brings reinforcement and the power of the third key to cause the Piper and his army to retreat.
Best part of story, including ending:
I like that Leaf had a bigger role to play in this story because she's just an ordinary girl with no special powers but she was crucial to Arthur getting his pocket back and being allowed to return home.
Best scene in story:
My favorite scene was when Leaf got an old lady to pretend to have a heart attack to call up an ambulance and give her a ride to Arthur's house during the quarantine. It was a nice humorous scene.
Opinion about the main character:
I like that Arthur is not obsessed with the powers being given to him but in fact wishes he could just go home and rest - he's a good underdog hero for the series.