The Crown of Ptolemy is the sequel to The Staff of Serapis and is told in the perspective of Percy Jackson. Percy, Annabeth Chase, Sadie Kane, and Carter Kane will appear together. The paperback version of the story was released on March 31, 2015, while an e-book is slated for release on May 12, 2015. The preview of the short is available for everyone exclusively with Hypable. It takes place after The Blood of Olympus.
In their first encounter, demigod Percy Jackson and magician Carter Kane had to battle a giant crocodile on Long Island. A month later, Annabeth Chase ran into Carter's sister, Sadie, on the A train to Rockaway, where the pair fought a god named Serapis. Now trouble is brewing again, this time on Governor's Island. An ancient Egyptian magician named Setne has come back from the dead and is experimenting with Egyptian and Greek magic, trying to become a god himself. He's so powerful and tricky that all four - Percy, Annabeth, Carter, and Sadie - have to team up against him. But their usual weapons and spells aren't going to cut it this time. Will the heroes be taken down by a wannabe god who looks like Elvis, or will they rise to the challenge? Told from Percy’s point of view, this third demigod-magician crossover story has all of the spunk and action that Rick Riordan fans crave. 
Annabeth has a dream from her mother, Athena, telling her that there is trouble brewing in Manhattan. Annabeth goes to Percy's apartment and they take a ferry to Governor's Island where a freak hurricane has caused all the mortals to evacuate. While they are trapped on the island surrounded by snakes, they attempt to contact Carter and Sadie but fail. Annabeth and Percy decide to face Setne, who is reading from the Book of Thoth, by themselves. They make a plan to use the invisibility cap to sneak up on Setne, while Percy distracts him. Before running off into battle, Percy gives Annabeth a kiss in case they die. While Percy is sneaking up on Setne, he trips and falls on his butt which makes him noticeable to the magician. Setne knows that Annabeth is using her invisibility cap and captures her, stating that he's been using invisibility magic for as long as the pyramids have been existing. Setne then uses a magic spell that keeps Percy on the ground. Setne reveals that he was watching Annabeth and Sadie when they were battling Serapis and that it was an experiment to see the powers that Annabeth and Sadie possessed.
Setne summons Wadjet briefly so that he can consume her essence and take the Crown of lower Egypt that she wears. Before he destroys her, he takes a selfie with her to "remember" the moment.
Their goal is to find Setne, the magician and Egyptian god, who is brewing up trouble. Annabeth concludes that once he puts two crowns of lower and upper Egypt that he will destroy the world. Setne claims he learned that a "little" demigod blood is good for starting a war, but Percy stops him by hitting him in the gut with Riptide. With Riptide protruding from him, he absorbs the sword's essence and learns about all of Annabeth and Percy's adventures. Suddenly, Sadie and Carter attack Setne with a camel, but he escapes.
The squad decide to regroup and decide to plan while heading south of the Governor's Island. Carter and Annabeth decide that they need to combine attacks to defeat Setne. Carter gives Percy his wand turning into a kopis and Sadie teaches Annabeth the Egyptian word for explode. In exchange, Annabeth teaches Sadie the Greek word for explode and gives her invisibility cap to Carter. When the squad find Setne, he is trying to summon Nekhbet. They all charge in, but fail at attacking Setne. Nekhbet is successfully revived and tries to attack Setne, but Percy and Annabeth pull her away since Setne is trying to consume her essence. Setne drops the Book of Thoth when Carter yells "stahp", and then he disappears with Nekhbet's Crown of Upper Egypt.
Nekhbet decides to stay with the four teenagers to get her crown back and help destroy Setne. She suggests that she merge with Percy since mixing Egyptian and Greek powers worked earlier. Percy reluctantly agrees to it, but only because it's the only way to defeat Setne. After merging with the goddess, they go off to find Setne again. After finding him, the quintuplet go after the crown so he doesn't turn in the ground. Setne talks about Carter's dad, and Carter takes off his hat of invisibility and goes to attack Setne in his avatar form, but Setne blasts Carter to the ground. Setne then rambles on about why he wants to be immortal including getting souvenirs after him such as snow globes. After Carter gets blasted, Percy turns into his vulture avatar form, and Sadie whispers something into Annabeth's ear. Percy grabs Setne and swoops him into the air. While in the air, Setne tells Percy he was a fool for giving up immortality. Nekhbet agrees with Setne that Percy was shouldn't have turned down immortality, but Percy fights back then plummeting to the ocean. Once in the ocean, Percy regains all of his strength, drops his avatar form, grabs Setne by the throat, and drags him to the island. A newly bandaged Carter greets Percy, along with Annabeth and Sadie who have drawn a circle on the ground. Percy decides to make him suffer for eternity instead of destroying him. While sealing him off, Carter thinks of Setne's snow globe idea and traps him in a snow globe. Sadie reveals that she told Annabeth her secret name earlier, but Annabeth states she's already forgetting all the magic she learned. Percy and Annabeth decide to not tell Camp Half-Blood. The four determine that they will remain in contact. Sadie and Carter fly off, and Percy and Annabeth go on a date afterward.
- It is included in the Indian version of The Sword of Summer.
- It is one of the three stories in Demigods and Magicians.