Bartimaeus and many djinni who are summoned by John Mandrake (Also known as Nathaniel) fight off an attack by foreign spirits. Bartimaeus is severely weakened after two years of continual service, and is easily defeated by a minor djinn. Despite their victory, they do not learn anything about where the attack came from, to Mandrake's displeasure. Currently in London, the America campaign is causing heavier casualties than expected. This, along with the increasing amount of resilience among the commoners, sparks talk of rebellion and demonstrations against the magicians. Mandrake continues to believe in the virtues of the magicians, and spends almost all of his time summoning demons to spread propaganda and spy on suspected rebels.
Jane Farrar visits Mandrake to discuss an occurrence a spy observed: several lower-level members of the government planning rebellion. Among them are Jenkins and Hopkins. Farrar's spy failed to find out specifics about the betrayal, so she asks Mandrake to spy on the traitors. Mandrake sends a weakened Bartimaeus to observe them.
Mandrake goes to council to discuss the recent attacks and America campaign. Ms. Whitwell advises pulling out of America, as they are having too much trouble there and could lose control over European countries as a result. Mr. Deveraux rejects the proposal, and Mandrake offers another: that they use the Staff of Gladstone to conquer America. Mr. Deveraux becomes outraged at the suggestion as he believes it is a challenge to his authority. Because of the recent attempts at rebellion, Mr. Deveraux is paranoid and delusional now; he distrusts everyone and only cares about his luxuries.
Bartimaeus successfully follows Jenkins where he meets at a cafe with several other conspirators, including the Mercenary. He learns that Hopkins is currently staying at the Ambassador hotel, but is then discovered by the group. He is pursued by many foliots who nearly kill him as he flees. Government demons destroy the foliots who pursue him, and he lands by Bartimaeus, passed out and unable to tell the magicians anything he learned. Farrar wants him to shock the djinni and force him to tell what he learned, which would kill him. Mandrake instead dismisses him, enraging Farrar.
Mr. Makepeace makes a surprise visit to Mandrake, and asks him to observe an experiment. Makepeace has captured Nick, the former Resistance member, and questions him by asking about the Shriveling Fire, something he knows nothing about. Then Makepeace summons a demon into Nick's head and repeats the question, which he successfully answers. The demon torments Nick, and he begs for it to be dismissed, saying that he knows about one last Resistance member who is alive. Makepeace tries to quiet him but Mandrake asks him who it is, and Nick answers that Kitty is alive and works at The Frog bar.
Kitty Jones meets a magician named Mr. Button who is extremely powerful, but not involved in politics. She becomes his assistant, and begins learning about magic herself. After witnessing summonings by Mr. Button, she successfully summons Bartimaeus to speak with him, and ask for help. Kitty talks to Bartimaeus about how the magicians, commoners, and spirits are in an endless cycle of suffering, and that the humans do not understand the nature of spirits. Bartimaeus agrees but is hostile towards being summoned by her. She asks Bartimaeus to help them fight against the magicians, and Bartimaeus laughs at her, saying she is naive for believing that a magician and a human could ever be allies.
Kitty brings up the example of Ptolemy, which shocks Bartimaeus. Bartimaeus acts like he does not know what she's talking about, and Kitty reveals that she researched him and found that Ptolemy had a strong bond with his slaves, and died when he was young. She deduced that Bartimaeus took his form out of respect. Bartimaeus tells Kitty that their bond was unique because Ptolemy went to the Other Place, and that such a bond will never happen again. He tells Kitty to forget her desires to team up with the spirits and focus on survival. Kitty dismisses him in disgust.
Mandrake tries to summon Bartimaeus and is horrified to learn that he has already been summoned. By using a scrying glass, he confirms that Kitty is living to his great shock. He then uses the glass to find Mrs. Lutchens, his old art instructor. He finds her and attempts to start a conservation, but Mrs. Lutchens is closed off from him; she tells him that she liked the boy he was, but that he is no longer that boy and they have nothing in common. Mandrake offers her a better job at the government, but she refuses and he leaves, distraught.
Mandrake successfully summons Bartimaeus and questions him about who summoned him, but he refuses to answer to not put Kitty in danger. Bartimaeus observes that Mandrake lost all his composure that he usually prides himself on, and mocks him for it, which only makes Mandrake more angry. Finally Mandrake agrees to dismiss him if Bartimaeus can provide good information. Bartimaeus informs him that Hopkins is at The Ambassador, but Mandrake does not dismiss him, and instead sends him to go with the other demons. Mandrake visits The Frog and captures Kitty, but then is found by Makepeace's search sphere, which requests them both to come at once to his new play.
Bartimaeus and the other demons move to The Ambassador and quickly corner Mr. Hopkins. Mr. Hopkins flees to the kitchen and picks up a meat cleaver, which disturbs Bartimaeus, and then reveals to Bartimaeus that he is actually Faquarl, inside Hopkins. Faquarl easily destroys the others and imprisons Bartimaeus in a silver pot, intending to kill him. Meanwhile at the theater, the magicians dismiss all their captive demons as per Makepeace's requests to not disturb the players. Makepeace then summons a great amount of imps to tie up all the magicians in the rows. To the private boxes, he sends djinni to subdue to major government figures. Mr. Pinn puts up resistance, killing one of the three demons send to him, but the others are easily captured, loaded into vans, and imprisoned.
Makepeace reveals his plan: he and the other conspirators plan to summon demons into themselves to boost their power, and use magic themselves. Mandrake pretends to go along with this plan, and requests that Kitty be spared when the Mercenary is about to kill her. Hopkins arrives and - pretending to still be human - gives a demonstration of his magic. The conspirators quickly summon demons themselves, Makepeace summoning Nouda the Terrible. Nathaniel attempts to summon all of his djinni, but they are all dead besides Bartimaeus, who is so weak that he appears as slime. Bartimaeus observes the situation and realises that the humans won't be in charge for long, just as the demons all take over the magicians' bodies.
Faquarl is about to kill Bartimaeus when Nouda orders him to be spared, as he is a spirit. Then, Bartimaeus asks Mandrake and Kitty to be spared, which Nouda grudgingly grants. Bartimaeus is dismissed. The demons begin calling the magicians one by one and forcing them to summon a demon into themselves. Ms. Whitwell is killed by Nouda when she resists, and in the chaos that follows, Mandrake and Kitty escape. The Mercenary follows Mandrake, who went to retrieve the Staff of Gladstone. Several magical traps delay him, but he manages to stab Mandrake's shoulder. Mandrake rushes into the treasure room and grabs the Amulet of Samarkand before the pestilence inside can kill him. The Mercenary follows, but the pestilence proves to be too strong, killing the Mercenary. Mandrake collects the Staff. Nathaniel also tells Kitty his birth name, completing his transformation back into the morally-good person he was when he was a child.
Kitty asks Nathaniel for information about how to perform Ptolemy's Gate and reach the Other Place. Nathaniel is bewildered, but tells her anyways. She visits the Other Place and talks with Bartimaeus. He expresses regret that the commoners will die in the upcoming spirit rebellion, but says there is nothing he can do. Kitty proposes that Bartimaeus enter Nathaniel's body, as the other demons did, but not destroy his mind. This would allow Bartimaeus to be shielded from pain and help use the Staff. After some arguing, he agrees. After leaving the Other Place, Kitty finds that she has forgotten how to use her body, which has been damaged by the foreign essence in the Other Place. She is hardly able to move, but starts recovering with time.
Bartimeus is successfully summoned into Nathaniel, and they begin hunting down the rogue spirits. Eventually they encounter Faquarl, who blasts them with a detonation, making them lose the staff. Bartimaeus talks to him and tells him that he betrayed the Other Place by staying willingly on Earth, and Faquarl responds that vengeance is his new master. Despite this, Faquarl admits to a discomfort in his new body that Bartimaeus claims is the lost link to his home. Faquarl realises the truth of this and is demoralized; he chooses not to kill Bartimaeus/Nathaniel, and then does not fight back as they retireve the staff and use it to kill him. They proceed to kill most of the demon army, but Nathaniel is mortally wounded in the side with a detonation in the fight.
Bartimaeus/Nathaniel attack Nouda with the staff, but to no effect; Nouda is too powerful. However, Nouda is in a palace made of glass and iron, and Nathaniel realises that the only way to kill Nouda is to take the safeguards off the Staff, releasing all the energy at once. Kitty offers him the Amulet, which he refuses, and they go to confront Nouda. After goading Nouda into coming to them, Nathaniel tells Bartimaeus that he was a good slave, but he doesn't trust him to release the staff at the right moment, so Nathaniel will dismiss Bartimaeus, automatically breaking the Staff - and also saving Bartimaeus's life. The resulting explosion kills Nouda and Nathaniel. In his final dialog, Bartimaeus says that he would have liked to tell Nathaniel what he thought of him at the end but that he rather thinks he knows anyways, as they were in the same mind. Nathaniel's sacrifice mirrors Ptolemy's sacrifice, so it is implied that Bartimaeus loves him in the end, despite calling him a typical magician, right up to the end.
Ptolemy's Gate is a winner of the following awards:
- A 2006 Booklist Editors’ Choice
- A 2006 Child Magazine Best Book of the Year
- A Horn Book Fanfare: Best Book of 2006
- Book Sense National Best Seller
- USA Today Bestseller
- New York Times Bestselling Series