Jason wakes on a school bus with no memory of who he is, holding hands with a girl he does not recognise. Piper acts as though she is his girlfriend, and Leo Valdez sits in front of them cracking jokes. Their teacher, Coach Hedge, a disguised satyr, watches the class with suspicion.
The bus arrives at the Grand Canyon Skywalk for a field trip. Jason discovers he cannot remember anything about his past - not even his surname. A classmate named Dylan begins acting strangely as storm clouds gather overhead.
A violent storm erupts at the Grand Canyon Skywalk. Dylan reveals himself as a ventus - a storm spirit - and attacks the group. Coach Hedge transforms, showing his true satyr form with a club, and fights to protect the demigods.
Jason discovers he can control lightning and summon a sword from a gold coin. He defeats the storm spirits, but Coach Hedge is swept into the sky by a tornado. Leo nearly falls off the skywalk but is saved by Jason, who discovers he can fly. A chariot drawn by pegasi arrives carrying Annabeth Chase, who is desperately searching for someone named Percy.
Piper rides the pegasus chariot with Jason, Leo, and Annabeth, heading east toward Camp Half-Blood. Piper is devastated that Jason has lost all memory of their relationship, which she believes was real.
Annabeth explains that they are demigods - half god, half mortal - and introduces Camp Half-Blood as a safe haven. More storm spirits attack the chariot, and the group crash-lands at camp. Annabeth is visibly distressed, as she had hoped the rescue mission would lead her to her missing boyfriend, Percy Jackson.
Annabeth gives Piper a tour of Camp Half-Blood, explaining the cabin system where each cabin represents a different Greek god. Piper learns about demigod life and how protectors like Coach Hedge are satyrs who find young demigods and bring them safely to camp.
Annabeth explains that the gods promised to claim their children by age thirteen. Piper wonders who her godly mother might be and struggles with the knowledge that a sinister voice in her dreams has been threatening her father's life, demanding her cooperation in exchange for his safety.
Leo receives a tour from Will Solace and learns he has been claimed by Hephaestus, god of fire and blacksmiths. He is assigned to Cabin Nine, which is rumoured to be cursed after the death of their previous head counsellor, Beckendorf.
Leo spots an old woman he recognises as his childhood babysitter, Tia Callida, watching him from the shadows of the Hera cabin. He is disturbed by the recognition, remembering how she placed him in a fireplace as a toddler. The woman vanishes before he can confront her.
Leo is introduced to the Hephaestus cabin forge and meets his half-siblings, including Nyssa and the injured Jake Mason. He learns about the Curse of Cabin Nine - a series of mechanical failures plaguing their work since Beckendorf's death.
Leo discovers that Cabin Nine has been trying to capture a rogue bronze dragon that has been terrorising the camp. The dragon was once an automaton built by the cabin but went haywire after its control disk malfunctioned. Leo is fascinated by the challenge and considers this his chance to prove himself.
Jason is escorted to the Big House by Drew, a daughter of Aphrodite. Every instinct tells Jason he does not belong at Camp Half-Blood. Drew flirts with him aggressively and dismisses Piper as beneath his notice.
At the Big House, Jason meets Chiron, the centaur activities director, and Rachel Elizabeth Dare, the camp's Oracle. When Jason enters the Hera cabin, time freezes and a mysterious woman in a black shawl appears in a cage of earth and roots, claiming to be Jason's patron and begging him to free her.
Piper collapses after Rachel speaks in a strange voice - not the Oracle's spirit, but something else channelling through her. Jason explains his vision of the imprisoned woman to Chiron and Annabeth.
Chiron is alarmed. Jason reveals his SPQR tattoo and knowledge of Roman terminology, which confuses the Greek demigods. Rachel delivers a new prophecy about a quest to free Hera before the winter solstice. Jason, Piper, and Leo are chosen for the quest, with Jason seeking his lost memories and answers about his sister - a girl in a photograph that Annabeth identifies as Thalia Grace.
Piper dreams of her last day with her father, the famous actor Tristan McLean, at their beach house near Big Sur. She recalls the close but complicated relationship they share, and the guilt she feels for stealing a BMW to get his attention.
The dream shifts to a sinister vision: her father is held captive by a giant, bound to a stake over a fiery pit. A voice demands that Piper sabotage her quest, or her father will die. Piper wakes in terror, knowing she faces an impossible choice between her friends and her family.
At the campfire, Chiron introduces the new arrivals and asks about the search for Percy Jackson. Annabeth reports that Percy has been missing for three days with no leads.
Piper is claimed by Aphrodite in a dramatic transformation that leaves her looking glamorous against her will. Jason volunteers for the quest to free Hera, and Piper and Leo join him. Rachel speaks a formal prophecy in the voice of the Oracle, warning of a giant's revenge and the need to answer a call.
Leo slips away from the campfire and ventures into the woods alone to find the bronze dragon. He recalls his troubled childhood, including his babysitter Tia Callida's dangerous tests and his mother's death in a workshop fire when he was eight - a fire Leo believes his own powers accidentally started.
Leo finds one of the traps set for the dragon and disables it. He uses his fire abilities for the first time since childhood, guided by an instinct he has been suppressing out of guilt. Deep in the forest, he hears the dragon approaching.
Leo encounters the bronze dragon in the woods. Rather than fighting it, he approaches with an engineer's empathy, recognising that the creature is malfunctioning rather than malicious. Using his innate understanding of machines, he manages to calm the dragon and examine its damaged control disk.
Leo names the dragon Festus (meaning "happy" in Latin) and works through the night to repair it and attach wings from Bunker 9 - a hidden workshop he discovers in the cliff face. He also finds a mysterious blueprint for a flying warship.
Jason dreams of being trained by the she-wolf Lupa, who taught him to fight at a ruined house in the redwoods. He wakes in the Zeus cabin, uncomfortable in this unfamiliar Greek camp. He sees burn marks on his arm - a tattoo reading SPQR with an eagle and twelve hash marks.
Jason examines a photograph of Thalia Grace and feels certain she is his sister, though he cannot explain how they were separated. He struggles with the paradox of being a child of Zeus at a Greek camp while his instincts and memories feel Roman.
Jason meets with Chiron privately and struggles to reconcile his fragmented memories. Chiron reveals little but seems deeply troubled by Jason's Roman background. Jason learns that Thalia is alive and leads a group called the Hunters of Artemis.
At the lake, Jason encounters the spirit of Annabeth's mother Athena, who warns that the coming threat is greater than anything the Greek demigods have faced before - an enemy who is older than the Olympian gods themselves.
Piper wakes in the Aphrodite cabin, still glamorously transformed by her mother's blessing and unable to undo it. She clashes with Drew, the cabin's bossy head counsellor, who mocks Piper and bullies the other cabin members.
Piper observes that Drew uses charmspeak - a magical ability to influence others with her voice - to control the cabin. Piper realises she may possess the same ability. She discovers a magical dagger called Katoptris that once belonged to Helen of Troy, which shows visions of the future in its blade.
Leo arrives at camp riding Festus the bronze dragon, now equipped with wings. The camp is both amazed and terrified. Jason and Piper are impressed, and Leo declares that Festus will be their ride for the quest.
The three questers depart on the dragon's back, flying west. Leo reveals his new magic tool belt from Bunker 9, which can produce almost any tool he needs. Annabeth watches them go, still grieving for Percy and hoping their quest will bring answers.
The trio flies north on Festus toward Quebec City, following a lead from the prophecy. Leo pilots while Jason and Piper rest. Leo worries about the dragon's control disk, which is not fully repaired.
Leo reflects on his past as a foster child who ran away from six different homes. He feels like the sidekick compared to Jason's heroism and Piper's beauty, but is determined to prove his worth through his engineering skills.
The group arrives in Quebec City and is intercepted by Boreas's children - Zethes, Calais, and Khione, the snow goddess. They are escorted into the palace of the North Wind, housed inside a luxury hotel.
Khione takes an unsettling interest in Jason, while viewing Leo with hostility because of his fire powers. Piper senses danger and uses her emerging charmspeak ability to keep the situation from escalating.
Jason leads his friends through the icy palace, holding Piper's hand for reassurance. Khione warns them against showing warmth in her domain. Jason thinks about the photograph of Thalia and his frustration at having a sister who might have answers but whom he cannot reach.
The group is escorted deeper into the palace. Jason feels the tension between the North Wind's children, who seem divided on whether to help or imprison the questers.
The questers face Boreas, king of the North Wind, in his frozen throne room. Piper uses her charmspeak and knowledge of French to address him respectfully. Boreas notices Jason's SPQR tattoo and recognises him as Roman, which changes his attitude.
Boreas reveals that dark forces are stirring - Gaea, the earth mother, is awakening. He gives them information about their next destination and warns that even the gods are divided. Khione attempts to keep Jason as part of her collection, but Piper's charmspeak convinces Boreas to let them go.