Jason, Piper, and Annabeth infiltrate the palace of the giant Mimas in Sparta, disguised - Jason as an old man, Piper and Annabeth as Greek serving maidens. They seek information about the giants' plans to destroy the gods at the Parthenon in Athens.
Jason struggles with the magical ageing, his joints aching and lungs rattling. The infiltration goes wrong when the giant discovers them, and they must fight their way out. Jason demonstrates his growing mastery of wind and lightning.
Jason infiltrates a ghostly banquet at the ruins of Odysseus's palace on Ithaca, disguised as the old beggar Iros. The ghoul Antinous presides over a gathering of two hundred suitors and other spirits loyal to Gaea, who are waiting to ambush the Argo II.
Jason learns that Porphyrion has relocated to Athens, where Gaea plans to awaken at the Acropolis on 1 August. The ghost Eurymachus reveals details of the enemy defences before Antinous destroys him. Jason also encounters the ghost of former praetor Michael Varus, who sees through his disguise and summons the spirit of Jason's mother, Beryl Grace.
The ghost of Jason's mother appears before him, a mania sustained by her desperate longing and Gaea's power. She tries to lure Jason into joining the suitors, reminding him of the day she abandoned him at the Wolf House. Jason struggles with painful childhood memories but ultimately recognises the spirit for what it is.
Michael Varus and Antinous urge Jason to join them, but Piper uses her charmspeak to sow distrust among the suitors while Annabeth provides crucial distractions. Jason rejects his mother's ghost with an ancient sign of warding, declaring himself a child of both Greece and Rome. The suitors close in to kill all three of them.
At the Parthenon in Athens, the seven demigods face the assembled giants. Jason duels King Porphyrion, Percy battles other giants, and Leo rains fire from the Argo II overhead. The Olympian gods arrive to fight alongside their children - the only way giants can be killed.
Frank unleashes arrows, Hazel summons gems and precious metals to blind the giants, and Piper uses her charmspeak to confuse Gaea's forces. Annabeth fights alongside Athena. The gods and demigods together destroy the giants, but Gaea herself begins to wake.
Reyna shadow-travels with Nico and Coach Hedge, carrying the Athena Parthenos across Europe toward Camp Half-Blood. Each shadow-travel jump drains Nico's strength further, and Reyna must lend him her own energy through her praetor's ability.
They are pursued by Orion, a giant hunter, and must keep moving. Reyna reflects on her past - growing up on Circe's island with her sister Hylla, their father's madness, and the heavy burden of leadership. She is determined to deliver the statue and prevent civil war.
Reyna is tormented by nightmares as she sleeps in Pompeii. She dreams of a dark wave of earth destroying Camp Jupiter, and a mysterious hunter with glowing eyes who shoots her with an arrow. She also witnesses Octavian consolidating power at the legion's camp near Camp Half-Blood, reinstating the violent exile Bryce Lawrence and planning an attack on 1 August.
When she wakes, Coach Hedge informs her that tourists are flooding the ruins. Reyna learns that the Athena Parthenos is keeping ghosts at bay but radiating hostility, and that Nico has been sleeping for some time as the situation grows more urgent.
Reyna waits while tourists wander the ruins of Pompeii, reflecting on her past and her dashed hopes of visiting Italy with Jason. She studies the plaster casts of Pompeii's dead and loses track of time, unsettled by the site's connection to destruction.
After Nico finally wakes, Reyna tells him about her nightmares, including the hunter with glowing eyes and Octavian's plans. They discuss contacting Thalia Grace and Hylla for help. As night falls and they prepare to shadow-travel, the ghosts suddenly vanish. Coach Hedge begins speaking in Gaea's voice, warning that Reyna will die as earthen shells of Pompeii's dead rise around them.
Reyna fights the earthen ghosts of Pompeii while Nico frantically prepares for an emergency shadow-travel. She knocks out the possessed Coach Hedge and demands that Nico summon zombie legionnaires using the sceptre of Diocletian.
Reyna commands the undead soldiers with her praetor's authority, holding back wave after wave of earthen ghosts. The sceptre of Diocletian explodes from the conflicting magic, and her dog Argentum is damaged in the fight. Reyna's zombie reinforcements begin to crumble, and she takes a glancing blow to her ribs. With the last of her strength, she leaps towards Nico as he shadow-travels them away from Pompeii.
Nico arrives at Camp Half-Blood with Reyna and Coach Hedge, delivering the Athena Parthenos just as Octavian's Roman army prepares to attack the Greek camp. The statue's power dispels the hatred between the camps, and most Romans stand down.
Octavian refuses to stop, loading himself into a catapult in a desperate bid for glory. He launches himself at an enemy onager and is killed in the explosion of his own making. The battle ends, and Greeks and Romans begin to reconcile.
Leo, Percy, Frank and Hazel explore the ruins of Olympia in search of the goddess Nike, who has been driven mad by the Greek-Roman schism. Leo and Percy share an awkward silence, with Leo still harbouring resentment over Calypso.
Frank reads from a tourist pamphlet while Leo mocks the rubble, but an uneasy feeling grows. At the Temple of Zeus, Leo makes the connection that Nike's statue once stood here. When he jokes about Adidas shoes angering her, Nike appears in her golden chariot, towering over them and demanding they fight each other to determine a winner.
Nike confronts Leo, Percy, Frank and Hazel, splitting between her Greek and Roman forms as she insists they fight to the death. The goddess radiates competitive aggression that makes the demigods want to attack each other.
When they refuse her demands, Nike spawns four metallic Nikai warriors and herds them towards the ancient Olympic stadium. Hazel proposes using her Mist magic to raise obstacles from the Labyrinth, giving them cover while they pretend to fight. Leo distributes Archimedes sphere gadgets to his friends, and they prepare to enter the stadium and subdue the goddess.
Inside the ancient stadium, Hazel transforms the field into a shifting maze using her Mist and Labyrinth powers while the demigods pretend to fight. Leo and Percy take down Nikai by working together, using popcorn and Greek fire grenades as distractions. Percy apologises to Leo about Calypso and promises to help him find her.
When Hazel is wounded by the last Nikette, Leo melts its face with a fire blast. He then triggers a water-geyser trap beneath Nike's chariot, flipping it. Leo blinds the goddess with an Archimedes death ray and traps her in a Celestial bronze net. Nike prophesies that one of the four demigods present will die battling Gaea, and Frank gags her with his sock and duct tape. They take her captive and resolve to seek the physician's cure.
Nico passes out after shadow-travelling from Pompeii and sends his consciousness searching for Thalia Grace. Instead he stumbles into Clovis's dreamscape at Camp Half-Blood and asks the son of Hypnos to relay messages about the Athena Parthenos and the pursuing hunter.
Clovis shows him a vision of a Camp Half-Blood war council: Clarisse argues for a pre-emptive strike against the Romans, while Malcolm warns against it. Will Solace reports that Apollo's gift of prophecy has failed, and Grover warns that nature spirits are becoming unreliable as Gaea stirs. Connor Stoll argues they must hold out for the Athena Parthenos, trusting Annabeth's plan. Coach Hedge wakes Nico, telling him they have trouble.
Nico wakes in a Portuguese town called Evora, having overshot Spain during their shadow-travel and slept for thirty-six hours. The Athena Parthenos lies sideways across a Roman temple while Coach Hedge entertains locals with street performances.
A burning ghost of a mediaeval monk beckons Nico to a chapel made of five thousand monks' bones, where Hades appears in person. His father reveals that the hunter pursuing them is Orion, a giant with mechanical eyes who was once Artemis's companion. Hades warns that one of the Seven aboard the Argo II will die, and urges Nico to let go of his anger. He also cautions that the shadow-travel may destroy Nico. Wolves are approaching, and Nico must hurry back to his friends.
Nico shares what he learned from Hades with Reyna and Coach Hedge over a meal of pastries in Evora. They discuss Orion and the wolves, while Reyna wonders if the wolf goddess Lupa might be sending allies. Nico reflects on his complicated feelings about Percy and his painful loneliness.
Reyna reveals she cannot contact her sister Hylla, and the burden of the quest rests squarely on Nico's shoulders. As they prepare to attempt their longest shadow-jump across the Atlantic, wolves suddenly appear from every direction. Their leader transforms into a tall man with glowing red eyes and a crown of finger bones - Lycaon, the first werewolf, who has been employed by Orion to detain them.
Nico confronts Lycaon, the first werewolf, who reveals he is working for Orion and intends to hold them until the giant arrives. Reyna produces a silver pocketknife - the only weapon that can harm werewolves - while Coach Hedge bluffs about the Hunters of Artemis being nearby.
Nico erupts a massive wall of bones from the ground beneath Evora's ancient square, trapping most of the wolves. While the coach climbs to the statue and Reyna covers their retreat with her silver-coin-wrapped sword, Nico stabs Lycaon with the silver knife and dissolves him into shadow. He then uses the pool of Lycaon's shadow to shadow-travel them all away, just as Orion arrives on a nearby rooftop with his bow drawn.
Piper and Frank search the port of Pylos for the poison ingredient of the physician's cure, fighting monsters along the way. Piper is haunted by a secret conversation with her mother Aphrodite about what lies ahead.
Frank visits a cave where his shape-shifting relatives, descendants of Periclymenus, live. They receive him coldly but give him a tiny vial of Pylosian mint - a deadly poison. Back aboard the Argo II, Frank reports that the chained god they must find in Sparta is Ares, whose statue was kept in chains by the ancient Spartans. Piper shares visions from her dagger of ruins and a bronze warrior surrounded by flames. The crew debates who should investigate Sparta, and Leo announces he'll make tofu tacos as they set course.
Piper and Jason share a quiet evening together aboard the Argo II. Jason confesses that part of him wanted to give up after encountering his mother's spirit on Ithaca, and fears he might make the same destructive choices she did.
Piper tells him a Cherokee story about two hunters and a taboo, teaching him to trust his instincts and not feed his bad spirits. She then dreams of the giants' war council at the Acropolis, where Porphyrion gathers over twenty giants and hordes of monsters. The giant Thoon, born to kill the Fates, promises to destroy destiny itself. Annabeth wakes Piper as they arrive at Sparta.
Piper and Annabeth search the ruins of ancient Sparta for the chained god's heartbeat. Annabeth struggles with lingering trauma from Tartarus, confiding in Piper about her fear of Percy's darker impulses.
Piper comforts her friend, then notices three fire pits erupting randomly. Realising the pits respond to emotion rather than logic, Piper impulsively jumps into one and lands in an underground chamber. She decapitates three bronze fire-breathing dragon statues guarding the entrance. Annabeth follows by rope, and they find a chained bronze statue of Ares pulsing with a metallic heartbeat, flanked by two doorways carved with the faces of Phobos and Deimos - the temple of fear.
The giant Mimas ambushes Piper and Annabeth in the underground temple of fear. Born to slay Hephaestus, Mimas disrupts rational thought and planning. Annabeth is nearly paralysed by amplified terror, but Piper realises they must embrace fear rather than fight it with logic.
Piper cuts their escape rope and leads Annabeth through the dark tunnels by following emotion instead of reason. She slashes off Mimas's ear and drives him back with relentless attacks. At the chained statue of Ares, Piper sacrifices her cornucopia and beheads the statue, releasing the makhai - the spirits of battle. The makhai swirl around her and promise to answer her call once. Using the power of Phobos and Deimos, Piper offers the severed ear as tribute and escapes through a doorway as the temple collapses on Mimas.