In Alexandria in 125 B.C., Bartimaeus, a demon known as a djinn, saves his current master, Ptolemy, from four assassins.
In modern day London, the British government is facing chaos. They sent troops to America to put down rebellion there. On the home front, the Spanish, French and Czech’s are fighting against the British. Bartimaeus is once again enslaved to the magician named Nathaniel, known to most by his professional name, John Mandrake. The djinn has grown weak from too much time on Earth and needs to return to the Other Place, his home dimension.
Mandrake has risen further in the government, and at the young age of 17, is the Information Minister. He spends his days monitoring rebel activity and creating war propaganda to encourage people to fight in America. One of his demons tells him of a group of children who recognized it as a demon. This incident is just one of many where a commoner has shown resilience to magic. The commoners with resilience are able to see through demon disguises and repel certain magical attacks.
Wanting to know how Bartimaeus fared fighting against a group of foreign demons, Mandrake commands one of his demons to bring Bartimaeus to him. Bartimaeus refuses to come. He hopes that his refusal will force Mandrake to summon him. If he is summoned, Bartimaeus plans to plead to be returned to the Other Place.
Mandrake is annoyed by Bartimaeus’ refusal but is distracted when Jane Farra, the Deputy Police Chief, arrives. With all the turmoil in London, she’s been assigned to watch for any conspirators within the British government. One of her demons followed a magician named Clive Jenkins and heard him say that he’s meeting with a man named Hopkins. Hopkins was implicated in a golem attack against the government two years prior.
Farra asks Mandrake to send a demon to follow Jenkins. The only demon he has free is Bartimaeus, which prompts him to summon the djinn. Mandrake agrees to dismiss Bartimaeus and allow him to return to the Other Place if he brings back information about Hopkins. He tells Bartimaeus to return to him when he’s completed the mission.
Kitty Jones, a 19-year-old commoner thought to be dead, uses the aliases Clara Bell and Lizzie Temple to move about London unnoticed. In the two years since she was involved with the golem attacks, she found an apartment and a job at a pub, and is an assistant to an elderly magician named Mr. Button.
Mr. Button was injured in a faulty summoning and lost his leg. Kitty helps by bringing him books, making tea and eventually helps him with summoning. Working for Button, she has learned how to summon demons on her own and has access to libraries only for magicians. She uses this access to search for information about Bartimaeus.
Mandrake attends a Council meeting where the magicians argue about what to do in America. The British are losing and need a plan. Mandrake suggests using Gladstone’s Staff, an extremely powerful magical object that could turn the tides in the war. The other magicians object, especially the Prime Minister. They fear the staff’s power and the power of the one who would wield it.
Bartimaeus follows Jenkins as he visits a number of lower level magicians. Jenkins meets with a man known as the mercenary who both Bartimaeus and Mandrake have faced before. Bartimaeus follows the mercenary to where he is supposed to meet Hopkins. To avoid being seen, Bartimaeus tries to hide nearby but is spotted by a group of demons. Bartimaeus changes into a frog and tries to escape but is followed. Because of his weakened state, he feels himself dying.
Mandrake attends a masquerade at the Prime Minister’s home. Quentin Makepeace, a playwright and close friend of the Prime Minister, requests that Mandrake visit him in the coming week. Mandrake agrees before deciding to leave the party early. He runs into Farra, and they decide to leave together.
While waiting for Mandrake’s chauffer, something triggers the magical alarm around the house. A frog breaks through the glass and lands at Mandrake’s feet. Mandrake realizes the frog is Bartimaeus and dismisses the demon before he dies. He plans to give the djinn two days to recover before questioning him about his mission.
Kitty asks Mr. Button about Ptolemy, an ancient magician she knows was one of Bartimaeus’ masters. He gives her little information and waves her away when she asks about something called Ptolemy’s Gate. Kitty goes to her job at the pub. A group of commoners who use the pub as a meeting place for their resistance group arrives. At the end of the meeting, they discover a woman new to the group is a demon. Kitty and the pub’s owner kill the demon before it escapes. A man named Nicholas Drew recognizes Kitty from their days together in a rebel group known as the Resistance.
Bartimaeus is summoned and is surprised to find that Kitty has summoned him. Kitty tells him that she wants the demons to help the commoners fight the magicians. He tells her that equality is impossible between demons and humans. On a hunch, she asks him about Ptolemy. Bartimaeus reveals that Ptolemy was an exception because the magician trusted Bartimaeus enough to let the demon have control.
Kitty doesn’t understand what this means. Bartimaeus challenges Kitty to give him control by stepping out of her protective pentacle. She refuses and asks him about Ptolemy’s Gate. Bartimaeus won’t answer. Kitty dismisses him, feeling disheartened.
After Bartimaeus caused such a stir at the Prime Minister’s home, Mandrake is out of favor. He goes to Makepeace’s house. Makepeace shows Mandrake an experiment he’s been working on. In a pentacle, Makepeace has placed a commoner named Nicholas Drew, the same man who Kitty worked with in the Resistance.
Makepeace summons a demon into Nicholas. The demon begins to make him go mad. Nicholas reveals that he saw Kitty at the pub. Mandrake, who was told by Bartimaeus that Kitty had died saving him from the golem, leaves before Makepeace can speak to him about the experiment.
Mandrake goes to the pub and finds it closed. He returns home and tries to summon Bartimaeus to ask him about Kitty. He is unable to summon the djinn and discovers that someone else has summoned Bartimaeus.
Realizing he’s alienated his allies on the Council and now his best demon is in service to someone else, Mandrake is disheartened. Wanting to get away from magic, he contacts his old art teacher, Mrs. Lutyens. The teacher knew him by his birth name, Nathaniel. Mandrake thanks her for trying to save him from a magician who attacked him when he was young.
Seeing the man he’s become, Lutyens tells Mandrake that he doesn’t speak for Nathaniel, the boy he used to be. She tells him the man he’s become has nothing in common with the boy he once was. The encounter leaves Mandrake feeling unsettled about his life and the choices he’s made.
Mandrake tries to summon Bartimaeus again and succeeds. He demands to know who summoned Bartimaeus, but the djinn refuses to tell. Mandrake tells Bartimaeus he knows that Kitty is alive and asks what Bartimaeus found out about Hopkins. Bartimaeus found out that Jenkins and the magicians he met with are choosing demons for something. He also found out that Hopkins is staying at the Ambassador Hotel. Mandrake summons his three most powerful demons and tells them to go to the hotel with Bartimaeus, find Hopkins and bring him back.
While Kitty is at work, Mandrake comes looking for her. She goes with him to prevent him from hurting anyone. Mandrake tells Kitty he hasn’t come as a government official but a private citizen. He wants answers about the golem incident, where she’s been for the past two years and her involvement with Bartimaeus. A magical orb bringing a message for Mandrake interrupts them. Makepeace’s new play premieres that night, and he’s requesting Mandrake’s presence. Mandrake brings Kitty with him to the theater.
Before the performance begins, Makepeace tells everyone to dismiss their demons and take out their magic lenses. The lenses allow the magician’s to see demons and a portion of the magical realm. Back in his private box with Kitty and Mandrake, Makepeace tells Mandrake to put his lenses back in.
As the play begins, Mandrake watches hundreds of demons swarm through the crowd. They bind and gag all the people in the theater. Makepeace holds a knife to Mandrake’s neck, preventing him from doing anything to stop the attack. Kitty tries to help Mandrake and is hit by a magical attack and rendered unconscious.
Bartimaeus and his fellow demons manage to get inside the hotel. While trying to break into Hopkins’ room, the man himself returns. He runs from the demons and ends up in the kitchen. Bartimaeus realizes that Hopkins isn’t a simple commoner. He’s been possessed by a demon that is controlling him. Hopkins kills all of Mandrake’s demons except for Bartimaeus. He places the weak djinn inside a silver soup bowl and puts the lid on it. The silver will eventually kill Bartimaeus if he can’t get out.
Kitty comes to and is tied up with Mandrake. They learn that Makepeace is planning to summon demons to inhabit himself and the other magicians loyal to him. They want to overthrow the Council and take over. Makepeace summons a demon named Nouda. Once the demon is in his body and the other magicians have summoned their own demons, Makepeace asks Mandrake to join them.
Not realizing that Bartimaeus is his only surviving demon, Mandrake tries to summon his demons to fight. Bartimaeus arrives and is nearly dead. Before Makepeace and his friends can react, the demons inside them take over their bodies.
Nouda, still in Makepeace’s body, asks Bartimaeus to join the demons as they take over Earth. Bartimaeus asks to be returned to the Other Place to heal. Nouda agrees, and Bartimaeus asks for one more favor. Nouda agrees without hearing the request. Bartimaeus wants Mandrake and Kitty spared from death. Nouda is unhappy but won’t go back on his word.
Mandrake dismisses Bartimaeus. The two humans are taken to a separate room to wait until the demons are ready for them. Kitty asks Mandrake about Ptolemy’s Gate. He tells her that it’s a way for magicians to reverse the normal summoning and travel to the Other Place. He also shares the method used for the reverse summoning. Kitty tells Mandrake that she has summoned Bartimaeus before and plans to do it again, but this time she wants to go to the Other Place.
She thinks if she gives Bartimaeus control by going to his realm and being at his mercy, he might help them fight the demons. Mandrake thinks it’s crazy. He plans to break into the vault where Gladstone’s Staff is locked up and use it to kill the demons. The staff is very powerful, and Mandrake isn’t sure he can control it. Mandrake tells Kitty to call him by his birth name, Nathaniel. Before either can put their plans into action, the mercenary enters the room. He takes Nathaniel and Kitty to see the demons.
Nouda is giving each remaining magician the choice to die or be possessed by a demon. When one of the ministers is faced with the choice, she summons her demon to fight. In the uproar, both Kitty and Nathaniel escape. Kitty finds a room with a pentacle and begins the reverse summoning. She is successful and travels to the Other Place.
Bartimaeus finds her, and she convinces him to help them. She believes if Nathaniel summons Bartimaeus into his body, the two of them using Gladstone’s Staff could defeat the demons. Bartimaeus agrees to try.
While Kitty is in the Other Place, Nathaniel makes it to the treasure vault where the staff is kept. He manages to get inside, but is injured by a magical Pestilence protecting the vault. He takes the staff along with the Amulet of Samarkand. The amulet has the power to absorb magic. The mercenary, who followed Nathaniel, is killed by the Pestilence, and Nathaniel takes the man’s seven league boots, boots that make one travel at great speeds. He returns to Kitty and finds her body in the pentacle.
Kitty wakes to find her body, which remained on Earth, extremely weak and aged 50 years. Kitty tells Nathaniel about her plan for him to summon Bartimaeus into his body. Nathaniel isn’t happy about the plan, but he agrees to try. It takes the magician and the demon time to acclimate to sharing a body.
Once they figure out a system for controlling Nathaniel’s body, they go to destroy demons. Before leaving, Nathaniel gives the amulet to Kitty to protect her. Nathaniel and Bartimaeus fight their way to St. James’ Park where Nouda is.
Kitty finds a group of magicians still tied up. She frees them, and together they go out into the London streets and try to save commoners from the attacking demons. She ends up in the park where she and the magicians find Nathaniel and Bartimaeus. Kitty and the magicians will try to help the commoners in the park while Nathaniel and Bartimaeus fight Nouda.
Nathaniel and Bartimaeus use the staff to attack Nouda. Nathaniel can’t get it to use enough power while keeping it from killing him. Nouda absorbs the attack from the staff, and Nathaniel is hit by a magical Detonation. His injury is severe and he nearly losing consciousness. The wound is too bad for Nathaniel to recover.
Kitty arrives and tries to convince Nathaniel to leave and regroup. He knows he won’t survive his injury, so He convinces her to leave. Nathaniel tells Bartimaeus that he was a good servant and that he’s dismissing him so he won’t die. Bartimaeus returns to the Other Place just before Nathaniel breaks the staff, killing himself and Nouda.
The morning after the demon attack, a few of the remaining magicians meet with influential commoners. They’ve created a joint governing council. Kitty was asked to join, but she refuses. When asked if she will continue her magical studies, Kitty reveals that she only studied magic so that she could summon Bartimaeus. She’s decided to travel the world and try to regain her health after her trip to the Other Place.