Percy Jackson has been wandering for weeks with no memory except the name Annabeth. He is pursued by two gorgons who keep reforming no matter how many times he kills them. He reaches the hills above the Caldecott Tunnel in Berkeley, sensing his destination is close.
An old woman appears - Juno in disguise - and demands Percy carry her to the Roman camp's entrance. Despite his exhaustion, Percy agrees. She warns him that he must choose between the ocean's safety and an unknown camp beyond the tunnel.
Percy slides down the hill on a serving platter, crash-landing near the tunnel entrance to Camp Jupiter. Two Roman guards - including Frank Zhang and Hazel Levesque - challenge him. The gorgons pursue him to the gates.
Percy carries the old woman across the Little Tiber river, which strips him of the Curse of Achilles he didn't know he had. Juno reveals her true form and introduces Percy to the Roman camp, announcing him as a son of Neptune. She disappears, leaving Percy drenched but alive. Percy controls the river to destroy the gorgons.
Percy is given a tour of Camp Jupiter by Hazel and Frank. He sees ghostly Lares, Roman barracks, and a city far larger than Camp Half-Blood. Percy is assigned to the Fifth Cohort, the camp's least prestigious unit.
He meets Reyna, the praetor, who interrogates him privately. She seems to recognise something about him but is guarded. Percy notices her two metal greyhound automatons and senses she is conflicted about his arrival.
Percy continues exploring Camp Jupiter with Hazel. She explains the cohort system and the legacy system where children of demigods can also attend camp. Percy meets Octavian, the camp's augur, a scheming descendant of Apollo who reads prophecies by gutting stuffed animals.
Octavian is suspicious of Percy and clearly covets the role of praetor, currently held by Reyna alone since Jason Grace mysteriously disappeared months ago. Percy is placed on probatio status in the Fifth Cohort.
Hazel introduces Percy to her half-brother Nico di Angelo, who is visiting camp as an ambassador of Pluto. Nico and Percy study each other warily, with Nico pretending not to know Percy despite clearly recognising him.
Hazel reflects on her own secret: she died in the 1940s and was brought back to life by Nico, who found her spirit in the Underworld. She has been hiding at Camp Jupiter, terrified that someone will discover she is a resurrected soul from another era. The arrival of Percy - another powerful child of the Big Three - both excites and frightens her.
Hazel has a flashback to her previous life in 1940s New Orleans. She remembers growing up with her mother, Queen Marie, who ran a fortune-telling shop. Her curse - precious metals and gems spring from the earth wherever she walks - made her an outcast.
Her only friend was Sammy Valdez, a Mexican American boy who made her laugh. The flashback reveals the beginning of Hazel's tragic first life, before her mother was corrupted by Gaea's influence and forced Hazel to help raise the giant Alcyoneus.
Hazel and Nico walk through camp. Hazel trips over a gold bar that spontaneously emerges from the ground - a manifestation of her curse. Nico helps her recover and they discuss her situation at camp.
Hazel worries that Percy's presence will expose her secrets. Nico privately tells her that Percy is important to a plan he cannot yet reveal, and asks Hazel to look after him. The conversation hints at Nico's deeper knowledge of both Greek and Roman demigod worlds.
At the dinner feast, Hazel sits with Percy, Frank, Nico, and centurion Dakota. Percy tries to get Nico to admit they have met before, but Nico deflects.
The camp prepares for war games. Percy impresses everyone by volunteering for the most dangerous position. Hazel observes the tension between Octavian's faction and Reyna's leadership, sensing that the camp is divided and vulnerable.
Frank marches to the war games, reflecting on his troubled past. His mother, a soldier, died in combat in Afghanistan. His grandmother revealed his demigod heritage and gave him a piece of firewood - his lifeline, tied to his fate. As long as the wood exists, Frank lives; if it burns completely, he dies.
Frank is a clumsy but earnest son of Mars who feels out of place among the Roman warriors. His secret gift from his mother's side - the ability to shapeshift - remains untested, as he is afraid to try it. He has a crush on Hazel but lacks the confidence to tell her.
The war games begin on the Field of Mars. Frank serves as a defender while Percy fights alongside the Fifth Cohort. Percy displays incredible combat skills, cutting through opponents with Riptide and using water from the aqueducts to devastating effect.
Frank watches in awe as Percy single-handedly turns the tide of the games, leading the Fifth Cohort to their first victory in years. The camp is stunned. Even Reyna looks impressed by the new recruit's abilities.
After the war games victory, Frank celebrates with the Fifth Cohort. Percy is hailed as a hero. The god Mars himself appears at the campfire in a column of fire, shocking the entire camp.
Mars claims Frank as his son and assigns a quest: Frank must travel north to Alaska, free the god of death Thanatos, and recover the legion's lost eagle standard. The quest must be completed before the Feast of Fortuna or the camp will be destroyed. Mars gives Frank an enchanted spear and tells him to bring Percy and one other companion.
Frank chooses Hazel as the third member of their quest. He is promoted to centurion. The trio prepares to depart for Alaska with supplies and a boat called the Pax.
Octavian tries to undermine the quest, arguing that Percy cannot be trusted. Reyna overrules him and authorises the mission. Percy, Frank, and Hazel set off by boat, heading north through San Francisco Bay toward Portland and beyond.
Percy sleeps on the boat and dreams of his old friends. Grover Underwood appears in his dream, trying to locate him. Annabeth also reaches out, begging him to stay put so Tyson can find him.
Percy's memories begin to return. He dreams of a giant's fortress on an icy mountain, where the giant Alcyoneus waits with the legion's eagle. Gaea taunts Percy, telling him he will be the key to the gods' destruction. He wakes knowing the stakes are higher than he imagined.
The senate convenes an emergency meeting to discuss the quest issued by Mars. Reyna promotes Frank to centurion so he can lead the quest, despite protests from the senators. Nico explains the difference between Pluto and Thanatos, and how the Doors of Death have been forced open by Gaea's forces. Hazel reveals the giant's name is Alcyoneus, who is immortal in his home territory.
Octavian manoeuvres the senate into providing no aid beyond a boat, putting all responsibility on Reyna while making himself look reasonable. Percy lays out the stakes plainly - the Fifth Cohort must take responsibility for the eagle lost in the 1980s expedition. The senate votes to grant access to the Roman navy at Alameda but nothing more. After the meeting, Octavian delivers a message that Reyna wants a private word with Percy before they leave.
Reyna meets privately with Percy in the principia. She reveals that she and her sister Hylla once lived on Circe's island, which Percy and Annabeth apparently destroyed years ago. Reyna doesn't hold a grudge - she believes Percy was sent by the gods to replace Jason as her partner in leadership.
Reyna confides that the situation is worse than the senate knows: Octavian's auguries show a massive army led by a giant marching south, too powerful even for Lupa's wolves. She subtly offers Percy the praetorship and a partnership, but he declines, explaining he has a girlfriend. Reyna gives Percy a silver ring bearing her mother Bellona's symbol and asks him to find Hylla in Seattle, hoping her sister can send reinforcements.
Percy, Hazel, and Frank pack their supplies and leave Camp Jupiter. They pass through Terminus's border checkpoint, where the god gives them directions to the BART station and warns them to watch out for someone called Polybotes. They ride public transit to Alameda, where they discover the mighty Roman navy consists of a single battered rowboat called the Pax.
Percy cleans the boat with his water powers and gets it moving without oars. As they head north toward the Golden Gate Bridge, Gaea's voice whispers in Percy's mind, taunting him that he could have chosen safety in the sea but is now part of her plan. Percy tells her to get off his ship, and they speed north at fifteen knots.
The trio sails north along the coast. Hazel battles seasickness while Percy navigates. They pass San Francisco, where Percy sees a homeless man among the sea lions who points at him menacingly.
Percy's memories continue to surface. He recalls visiting San Francisco before, on previous quests. Frank asks about Annabeth, and Percy shares what little he remembers. The group heads toward Portland, where they hope to find the blind seer Phineas for information about Death's location.
Hazel wakes from a blackout on a cliff near Mendocino, about a hundred and fifty miles north of the Golden Gate. Frank and Percy had been unable to rouse her and brought the boat ashore. Hazel confesses the truth about her blackouts - they are a side effect of her past, from wherever Nico found her.
Frank reveals he has already guessed Hazel came from the Underworld, having noticed how she talks about her life as though it happened long ago. He promises they'll figure it out and keep her alive. Their tender moment is interrupted when the wind stops but the grass keeps moving. Before they can react, something knocks Frank backward and a force like a grassy hurricane drags Hazel into the fields.
Hazel is carried through the fields at tremendous speed by the living grass. Using her underground senses, she summons a massive outcropping of schist rock, which disrupts her kidnappers and launches her into the air. She lands on the rock and confronts the karpoi - grain spirits that look like chubby, green-skinned Cupids with fangs.
The lead karpos, Wheat, reveals they are children of Gaea who want to return to growing wild. They were sent specifically to capture Hazel for their mistress's army, led by the giant Polybotes, which is camped just over the ridge. Hazel tries to trick them into revealing where Alcyoneus is keeping Thanatos, but they catch on. She learns that her island in Resurrection Bay sank long ago, meaning the giant's lair could be anywhere in Alaska. As the karpoi begin climbing her rock, an arrow pierces Wheat's chest and he dissolves into Chex Mix.
Frank and Percy burst from the woods and help Hazel massacre the karpoi, reducing them to piles of seeds and cereal. Percy threatens to set the field on fire with a lighter if they try to re-form. From the top of the schist rock, they spot Gaea's army marching south through a ravine below - Earthborn, horned centaurs, Cyclopes with a siege tower, and the giant Polybotes himself.
Percy is shaken by the sight of Cyclopes and centaurs fighting for the enemy, feeling his scrambled memories conflict with what he's seeing. The Cyclops Ma Gasket bellows threats against someone named Leo, Jason, and Piper, which tells Hazel that Jason may still be alive. Polybotes smells a son of the sea god nearby but is convinced to keep marching. Before leaving, he throws three venomous basilisk snakes at a rainbow-shielded convenience store on the hill. Once the army passes, the heroes decide to approach the store for help.