While on her way to her boyfriend's apartment, Annabeth Chase comes across a two-headed monster in the subway in a crab shell. To her surprise, the monster ignores her and boards a subway train. Annabeth follows it and notices the monster has a strange aura that pulls her towards it, but weakens her at the same time. The monster's aura also affects the mortal subway passengers, causing everyone who didn't simply pass out from the aura to abandon the train at the Fulton Street Station. Annabeth gets its attention, causing it to howl so loud it shatters the train's windows and the remaining mortals wake up and flee. Annabeth attempts to figure the monster out, noting that its two heads seem to be fused together. She realizes that they were statues that had come to life, the lion having been in the Metropolitan Museum of Art and the wolf an antiques store. The monster's aura intensifies as it travels, causing Annabeth to speculate that it's searching for another piece. After her questions about the monster possibly being from Egypt enrage it, she tricks the two heads into fighting each other in order to buy herself some time, since she doesn't have a weapon. Before she can figure out what to do next, the train pulls into the High Street Station in Brooklyn, where Sadie Kane is battling the third piece of the monster, a black Labrador retriever. During the fight, Sadie knocks the dog into the train and climbs aboard just before it leaves the station. As the two girls assess each other, the two monsters head towards each other to combine and Annabeth orders Sadie to stop that from happening. Annabeth tackles the two-headed monster and pins it down while Sadie uses mar to make the dog retch. Eventually, the spell wears off, so Sadie blasts it with her wand while Annabeth attempts to attack and restrain it with her heavy backpack. However, as the train continues further, the monster grows stronger, causing its aura to further weaken Annabeth. Eventually, the dog knocks down Sadie, and the distraction causes Annabeth to lose her fight with the other part of the monster. It then roars, causing a wave of energy and wrecking the train on Rockaway Beach, causing Annabeth to hit her head in the process. While the two-headed part of the monster gets away, Sadie is able to restrain the dog part and pull an injured Annabeth to safety.
After Annabeth wakes up and they introduce themselves to each other, Sadie heals Annabeth's injuries with a healing potion from Jaz, and the girls drag the dog monster away from where the mortals can find them to talk. When the two explain their respective histories, Sadie is stunned to find out that her brother and Annabeth's boyfriend fought a monster together a few weeks before as Carter hadn't told her about the boy he'd fought the monster with. Sadie explains that she got involved with the monster because it was part of a statue Cleo was examining at Brooklyn House before it came to life, shredded their defenses, and escaped. The two realize that the dog monster is connected to the two-headed monster since the two appeared to be trying to meet. Annabeth theorizes that it is a three-headed scepter belonging to a god that was somehow both Greek and Egyptian, though she doesn't remember his name. Sadie realizes that it's Alexandria, the capital of Egypt when the Greeks ruled it and combined the gods of the two cultures. Annabeth remembers that one of the gods that was created during that time owned the three-headed scepter and realizes that the monster isn't just trying to put itself back together, it's trying to reach this god. As they realize they need to keep the dog monster from finding the god and its other part, it finally breaks free from the supposedly unbreakable ropes and takes off across the beach. While trying to find a weapon, Annabeth grabs Sadie's wand, which changes into a Celestial bronze dagger similar to her old one. She points Sadie towards the direction the dog monster went, only to be surprised when Sadie tells her that there is a storm in that direction that she cannot see. Sadie puts a paste on Annabeth's eyelids that allows her to see into the Duat. After growing accustomed to it, Annabeth sees that Sadie is right: there's a storm gathering over a derelict apartment building, drawing in red streams of energy. The two realize that the dog monster has gone to the building to reunite with its other part and its master and head to the building to stop it.
Arriving at the building, they enter through the basement to find that the god is on a ledge on the thirtieth floor and is cannibalizing the building to build something from a storm of debris. While they aren't sure what, they decide to find out by asking the god when the monster, now with three heads, enters the building and then the maelstrom which starts to carry it up to the god. To protect them from the storm, Sadie cast N'dah, creating a shield around them. After testing it by stepping into the stairwell, Sadie gives Annabeth her staff to maintain the shield while she turns into a kite and stops the monster from reaching the god, ordering Annabeth to distract the god so she can do it. Sadie is able to fly through the maelstrom by flying into the Duat to protect herself. Annabeth makes her way to the ground floor with the shield protecting her from the debris, but growing weaker with each hit. She attempts to draw the god's attention, but fails until she remembers that his name is Serapis and calls him by it. The distraction causes Serapis to bring the storm to a stop, making everything hang suspended in mid-air, including the monster. Sadie takes the opportunity to drop one of Walt's camel amulets onto the monster and make it come to life, causing both to fall out of the air and the monster to be pinned down by the camel. Sadie's actions enrage Serapis and Annabeth takes credit for them to buy Sadie some time, demanding Serapis come face her. Serapis jumps down and she is stunned that he has a wicker basket for a hat. Serapis explains that the hat represents his former rule over the Underworld which he states he will rule again. He then demands to know what Annabeth is as she smells like a demigod and wields a celestial bronze weapon but also the staff of a magician. Seeing that Sadie wants her to continue distracting Serapis, Annabeth claims to be both and demands to know why Serapis is there. Serapis is just amused, thinking she's there to be his high priestess. Annabeth claims that she is there to be Serapis' high priestess and demands to know if he's worthy of her service. Amused, Serapis sends a bathtub into Annabeth's shield to test her defenses. The shield protects her, but the force of the blow destroys it and Sadie's staff. Seeing that Sadie is creating a protective circle and has something planned, Annabeth continues to distract Serapis by bluffing that her powers are so strong she doesn't need a shield and demands to know how she can serve him, assuming he's worthy of her service. Serapis is enraged by her audacity and tells her she can serve him by being the first who's power, life and soul he will consume. Serapis then destroys the camel and starts to summon his staff, but Annabeth tells him she should give it to him personally as his new high priestess and restrains it with her bare hands, demanding to know Serapis' plan. Despite his staff trying to warn him of Sadie's actions, Serapis doesn't notice and shows Annabeth what he's building: the Lighthouse of Alexandria. Serapis tells Annabeth that he plans to build his new capital on Rockaway Beach (though he intends to change the name) and that the lighthouse is to draw in the Greek and Egyptian gods so he can consume their essence, incorporate it and become the most powerful god of all. As Sadie continues to create her protection circle and Annabeth's hold on the staff weakens, Serapis explains that when Alexander the Great conquered Egypt, he tried to merge the two religions together, but the Greeks and Egyptians didn't like it, especially when he chose Amun a sun god to be the new main god. When Ptolemy took over, he decided to create a new chief god and discovered the statue of Serapis, formerly a minor village god and brought it to Alexandria where the Greek and Egyptian priests performed auguries and incantations that increased Serapis' power and made him the great god Serapis, which both cultures agreed to. As Sadie finishes her protective circle and starts swinging her tyet necklace, Serapis continues to explain that he became the Greek and Egyptians most important god, making him grow stronger and as more people worshipped him, allowing him to drain the power of the older gods and eventually replace them. As part of this, Serapis took over the Underworld replacing both Hades and Osiris at which point Cerberus transformed into his staff which allows him to control the past, present and future. Serapis reaches out for his staff which starts to break free of Annabeth's hold. Seeing that Sadie needs a little more time to prepare her spell, Annabeth asks how Serapis returned since the Ptolemaic dynasty fell long ago and his cult was forgotten. Serapis tells her that its not important but that he was awakened by one who has delusions of grandeur and thinks he can control Serapis since he found some old spells in the Book of Thoth. From this comment, Sadie realizes that Setne woke Serapis since he has the Book. Serapis goes on to explain that it wasn't enough for Ptolemy to make Serapis a major god, he wanted to be one too but his attempt failed and his family was cursed as a result, leading to them growing weaker over the centuries until Cleopatra committed suicide and handed Egypt over to the Romans. Setne believes he can do better and has been experimenting with hybrid Greek and Egyptian magic in his own efforts to become a god, but Serapis proclaims that Setne went too far and that as he is awake now, he will rule the universe. Serapis attempts to tempt Annabeth with the prospect of what his plans mean: once he has consumed the gods after the lighthouse draw them in, he will build a new city and a new Library of Alexandria with all the knowledge of the ancient world and tells her that he could grant her great power as his high priestess. The prospect of a new Alexandria library tempts Annabeth, but when Serapis demands she turn over his staff, she stabs it in its shell instead.
Despite Annabeth's hopes, stabbing the staff in the shell does not destroy it, instead causing a tiny fissure to open and red magic to spew out. Serapis is left enraged by Annabeth's betrayal of him while the staff tries to reach the knife with its three heads. Before Serapis can attack Annabeth, Sadie completes her spell by throwing her silver necklace and yelling tyet. The spell causes a giant hieroglyph of the tyet to surround Serapis and bind him. Sadie then names Serapis to complete the binding which she calls upon the power of Isis to do. This causes Serapis' lighthouse to fall to pieces and crash to the ground. When Annabeth notices the staff trying to reach Serapis, she attempts to stop it, but is hit in the head by a falling piece of lumber and is then buried in debris, preventing her from reaching the staff. By the time she breaks free of the rubble, the staff has reached Serapis' feet and Annabeth regrets not stabbing one of its heads which she didn't do because she didn't want to kill an animal, even one that was part of an evil monster. Serapis breaks free of his binding and claims his staff, enraged that Sadie would dare try to bind him. Having his staff makes Serapis' aura ten times more powerful to the point that Annabeth can sense it draining away her life force. Despite Serapis destroying her protective circle, Sadie stands her ground and tries ha-di on the god but Serapis simply snatches the hieroglyph out of the air and crushes it, causing smoke to shoot out between his fingers. Serapis explains to a stunned Sadie that his power is both Greek and Egyptian and it combines both, consumes both and replaces both. Serapis is also pleased to see Sadie's connection to Isis as Isis was once his wife. He explains that when he overthrew both Zeus and Osiris he bound Isis and forced her to serve him and now plans to do the same using Sadie as a gateway to draw Isis there. Serapis turns his staff on Sadie, surrounding her in red energy and causing her a lot of pain so Annabeth grabs a sheet of plywood, catches Serapis' attention and throws it into his face, breaking his nose and giving him two black eyes as well as breaking his concentration and releasing Sadie. Serapis tries to blast Annabeth with super-heated steam from his staff, but she dodges and hides. After seeing Sadie is recovering, Annabeth taunts Serapis about what she did to him to buy Sadie more time. Serapis tells her that he will consume her life force and use her to destroy Athena. He also tells her that she isn't as wise as Setne and even he doesn't understand the power he unleashed by awakening Serapis who won't let them gain the crown of immortality and controls the power of the past, present and future and will use it to rule the gods alone. Annabeth is able to use the time when he is distracted by his own speech to move away before Serapis can destroy her hiding place and Sadie takes the opportunity to cast Suh-fah, causing the wall behind Serapis to collapse on top of him and bury him in a thousand tons of rubble. An impressed Annabeth regroups with Sadie who explains she used the Divine Word for loosen, figuring it would be easy to cause the wall to collapse with that spell. As the two start to realize they need to leave as the building is becoming unstable, Serapis starts to break free of the rubble to Sadie's surprise. After the staff breaks free, Annabeth sees that the wound she inflicted is spewing hieroglyphs, Greek letters and English curse words like a timeline and realizes that they need to destroy the staff to defeat Serapis. Annabeth attempts to reach her dagger, still stuck in the staff, but before she can, Serapis breaks free. Sadie quickly casts another suh-fah, bringing another section of wall down on Serapis, but the effort of casting all that magic causes her to collapse. Annabeth quickly grabs Sadie and flees with her as the two loosening spells cause the entire building to collapse on top of Serapis.
Annabeth carries Sadie near the train tracks where she considers calling to the nearby emergency personnel for help due to Sadie's being hot and having steam coming off of her. However, Sadie wakes up before she can make up her mind and realizes that she used so much magic, she nearly combusted. Annabeth can sense that Serapis is not gone and will probably be able to free himself in a few minutes. While the two know they need help, Sadie has no energy to open a portal even if she could find one and Isis isn't helping her because she won't risk getting close to Serapis to have her essence consumed. Annabeth also has no way of contacting any other demigods to come help them out. As they try to figure out what to do, Annabeth realizes she still has her backpack and its lighter than it should be. Inside, she finds her architecture books have disappeared to be replaced by a square of ambrosia and her magic Yankees cap which surprises her as she hasn't carried it for sometime as it hasn't worked since she had an argument with Athena. Annabeth realizes that Athena sent her some help and it both encourages her and worries her that the situation is so bad that Athena got involved herself, meaning that Serapis must be a real threat to the gods. With the cap back, Annabeth is able to come up with a plan and teaches Sadie, who is unable to perform any more magic without burning up, some Greek words to confuse Serapis.
Just as they finish, Serapis finally breaks free of the rubble and rushes towards them. Annabeth is able to use her ambrosia to restore Sadie's strength then dons her Yankees cap which works again and makes her invisible. Sadie attracts Serapis' attention and draws him to the beach while Annabeth hides so that she won't risk Serapis detecting her, even invisible then goes after him once he's past her. Confronted by Serapis once again, Sadie chants some Greek, Egyptian and English magic words that she claims establishes a death link between them that will destroy Serapis if he kills her. While Serapis is distracted by this, Annabeth works her way towards him, waiting for him to lower his staff so she can get at it and her dagger. In his confusion, Serapis lowers his staff within Annabeth's reach who plans to get her dagger and kill the dog's head representing the future, but hesitates as the head is the least threatening and she worries that if she destroys the future part of the staff, she will be destroying her own future. Sadie angrily tells Serapis how her mother sacrificed herself to seal Apophis, who is a lot older and more powerful than Serapis in the Duat and she will not let a second-rate god take over after that. Serapis starts to see through the bluff as he senses no spell upon him and not even Setne had that sort of power. Sadie tells Serapis that she knows Setne and she is the reason he is still in the world before ordering Annabeth to "do it NOW." As Serapis goes to strike, Annabeth stops hesitating, grabs her dagger and drives it into the neck of the dog head on the staff, causing the dog head to implode into the scepter which Serapis can't let go of. Annabeth explains that by destroying the head representing the future on his staff, they've destroyed his future and he's nothing without it. The shell of the staff cracks open, heats up and the wolf and lion heads get sucked inside before it gets turned into a fireball. Despite Serapis' efforts to shake it off, the fireball starts consuming him, starting with the arm holding the fireball. As he is consumed, Serapis yells that without his guidance, they will never attain the crown and will all perish. Serapis is consumed by the fireball and destroyed and the fireball then winks out.
After Serapis is destroyed, Sadie and Annabeth relax and recover. The ambrosia Athena sent restores Sadie's energy and health and Annabeth asks her about Setne. Sadie explains that he is an evil magician who is back from the dead who escaped with the Book of Thoth after helping her and Carter. Sadie suggests that along with using the Book of Thoth to wake Serapis, the petsuchos Carter and Percy fought was another of Setne's experiments in combining Greek and Egyptian magic. The two know that Setne is trying to become a god and that waking Serapis will not be his last trick. Sadie speculates that what's happened so far is just Setne playing with them, testing his new magic and how the demigods and magicians will react to his actions before making his real move for power. The two hope that Setne can't actually become immortal as a god with the ability to use both Greek and Egyptian magic, can control both worlds and is as crafty as Setne would be a hard enemy to defeat. Annabeth recalls what Serapis said about them not being able to "attain the crown" without him and how Ptolemy had attempted to become immortal. Sadie recalls that Serapis mentioned a crown of immortality and suggests that Serapis meant a pschent, a Pharaoh's crown. Sadie guesses that if Setne is trying to recreate Ptolemy's efforts to become immortal, he's looking for Ptolemy's crown. Sadie decides to have Brooklyn House start researching Ptolemy in hopes of finding his crown in a museum somewhere while Annabeth promises to ask the kids in the Hecate Cabin about Ptolemy's magic and try to get Athena to help. The two realize that they have to find the crown and can't let Setne continue his experiments as they will rip their worlds apart. Before they part, Sadie and Annabeth exchange phone numbers to keep in touch and Sadie goes back to Brooklyn House while Annabeth goes to Camp Half-Blood.