The Argo II sails over the Italian mountains, under constant attack from mountain gods. Hazel narrowly avoids being hit by a boulder. Nico di Angelo falls from the foremast when it collapses, but is unhurt.
Leo struggles to keep the ship flying while Jason, Piper, and Frank defend against aerial attacks. The crew is demoralised by the loss of Percy and Annabeth to Tartarus but pushes onward toward the House of Hades in Epirus.
Hazel is overjoyed when her magical horse Arion appears on the Italian hillside. Despite warnings from Nico and Leo, she mounts Arion and races toward a sinister black tornado forming over some nearby ruins, sensing that her father Pluto may have sent help.
Arion carries her straight into the storm, which Hazel recognises is supernatural in origin. She is determined to take the lead for once rather than feeling useless aboard the Argo II.
Hazel arrives at the summit inside the storm and finds herself in a colourless, eerie crossroads. Three ghostly forms of the same woman converge into one - the goddess Hecate, who appears with her polecat Gale and black Labrador Hecuba.
Hecate reveals that she helped Hazel's mother summon Pluto years ago and sees even greater potential in Hazel. She announces that Hazel stands at a crossroads and must make a critical choice about the quest's path forward.
Hecate shows Hazel three possible futures in the gateways of the crossroads: a demigod civil war, the destruction of the Argo II in the mountains, and Percy and Annabeth dying before the Doors of Death. She reveals a secret northern pass through the Apennines - the only route that gives their friends in Tartarus any chance of survival.
Hecate instructs Hazel to learn to manipulate the Mist and to seek out two dwarfs in Bologna who guard a treasure vital to the quest. She warns that a formidable sorceress awaits in the House of Hades whom only Hazel can defeat through magic. Hazel boldly declares she will find her own path and save everyone, then returns to the ship to share the new route with Nico and Leo.
Annabeth and Percy fall through the darkness of Tartarus. After what feels like an eternity, they land in the underworld's deepest pit - a landscape of jagged black glass, burning rivers, and poisonous air.
They drink from the River Phlegethon to survive, the liquid fire healing their wounds but tasting terrible. Annabeth's broken ankle mends. They spot empousai - vampire demons - heading toward the Doors of Death and decide to follow them, staying hidden in the hellish landscape.
Annabeth and Percy land in the River Cocytus - the River of Lamentation - after their fall into Tartarus. The river's voices of pure misery nearly drown Percy, but Annabeth fights the despair by making him talk about their future together in New Rome.
They drag themselves ashore onto a beach of jagged black glass. The air itself is acidic, their wounds won't heal, and Annabeth has lost her dagger and backpack. She realises they must find the River Phlegethon if they want to survive.
Annabeth and Percy climb down a canyon to reach the River Phlegethon, their bodies deteriorating from Tartarus's poisonous atmosphere. Annabeth forces herself to drink the liquid fire, which tastes agonising but heals her blisters. She pours firewater into Percy's mouth when he passes out, reviving him.
They decide to follow the Phlegethon downstream toward the Doors of Death, staying close to it as a source of healing. Arachne attacks from behind, but Percy destroys her instantly with Riptide, and they press onward into the depths.
Annabeth and Percy overhear a group of empousai - vampire demons - marching along the Phlegethon toward the Doors of Death. Their leader is Kelli, a cheerleader-demon whom Annabeth killed years ago, now re-formed and heading to the mortal world to fight for Gaea.
Kelli specifically mentions wanting to devour Percy Jackson. Though terrified, Annabeth realises the empousai are heading exactly where they need to go - to the Doors of Death. She and Percy decide to follow them at a distance.
Leo spends the night crawling over the Athena Parthenos in the Argo II's hold, trying to find hidden mechanisms, but the statue operates on pure magic with no moving parts. He feels guilty about Percy and Annabeth falling into Tartarus, blaming himself.
He falls asleep and has a nightmare: Gaea taunts him while a massive dark giant - Clytius - pursues him through his mother's old workshop, then through a ruined Camp Half-Blood. He encounters a hateful sorceress at Half-Blood Hill who offers him only death. Jason wakes him with news that they've reached Bologna and Nico has information.
The crew gathers in the mess hall to discuss strategy. Nico reveals that he communed with a dead priest of Hecate and learned that the giant Clytius - designed as the anti-Hecate - guards the Doors of Death and can be killed by fire. Everyone looks at Leo, who describes his dream of Clytius and the sorceress.
Hazel confirms that Hecate warned her about a witch only she can defeat. Frank mentions that July is the Chinese ghost month, when the spirit and human worlds are closest. Suddenly the ship lurches - two mischievous dwarf creatures called Kerkopes have boarded, tied up Piper and Coach Hedge, and begun stealing everything they can grab.
Leo and Jason chase the Kerkopes - Akmon and Passalos - through Bologna. The dwarfs are incredibly fast and love stealing shiny things, having taken Leo's tool belt, Piper's knife, and the Archimedes sphere. Jason gets trapped upside down by golden cords from a Neptune statue while Leo recovers the Archimedes sphere.
Leo chases the dwarfs alone to the top of a medieval tower. He improvises flash-bang grenades from grocery store supplies and manages to capture both Kerkopes, recovering his tool belt and the stolen items.
Leo rifles through the Kerkopes' treasure and finds two significant items: a broken astrolabe made by Odysseus and a leather-bound book stolen from a minor god in Venice. The dwarfs reveal the god lives at La Casa Nera on Calle Frezzeria but can't pronounce his name.
Jason arrives dramatically through the window. Rather than killing the dwarfs, Leo strikes a deal - he spares them in exchange for their oath on the River Styx to go to New York and harass the Roman army marching on Camp Half-Blood, slowing Octavian's advance.
Percy and Annabeth trek through Tartarus, following the empousai toward the Doors of Death. They encounter Bob, a Titan formerly known as Iapetus, whom Percy had defeated years ago by wiping his memory in the River Lethe. Bob renamed himself and now works as a janitor in the Underworld.
Bob remembers Percy as a friend and agrees to help them. His skeleton kitten, Small Bob, rides on his shoulder. The Titan's broom becomes their most powerful weapon against the horrors of Tartarus as they journey onward through the realm of nightmares.
Percy and Annabeth descend deeper into Tartarus, following the empousai's trail across a nightmarish wasteland of monster blisters and cave entrances. Percy begins to sense that the entire landscape is a living thing - the body of the god Tartarus himself, with ground like skin and ridges like teeth.
They lose sight of the empousai and realise too late that they've walked into a trap. Kelli and four other vampires emerge from the rocks, surrounding them. Kelli gloats, eager for revenge against the demigods who killed her.
Percy and Annabeth try to bluff the empousai, claiming the Greeks and Romans have united and that Hecate is on their side. Annabeth manipulates one empousa named Serephone into challenging Kelli's leadership, but Kelli kills Serephone and rallies her remaining sisters.
The fight begins - Percy cuts down one empousa, but two others grab him while Kelli lunges at Annabeth. Percy is bitten in the neck, drops his sword, and seems doomed. Then a massive shadow falls over the battlefield, and a Titan drops from above with a war cry.
Bob - the Titan formerly known as Iapetus - crushes Kelli and destroys the remaining empousai with devastating efficiency, using his broom's hidden spearhead. He is ten feet tall, with silver Einstein hair and a janitor's uniform, and is delighted to help.
Bob heals Annabeth's wounds and Percy's injuries with a touch. He explains that he jumped into Tartarus from Hades's palace because he heard Percy say his name. He warns that other monsters are pursuing them and urges them to hurry, promising he knows a safe route.
Frank wakes up as a python, struggling with the warring voices of Mars and Ares constantly screaming in his head since the fighting broke out at Camp Jupiter. He forces himself to focus by staring at a flame, determined to be useful now that Annabeth is gone.
The Argo II docks in Venice, where dozens of strange cow-like monsters called katoblepones roam the streets, invisible to mortals. Frank, Hazel, and Nico volunteer to go ashore and find the owner of the stolen book at La Casa Nera, navigating through the monster-infested city.
Frank, Hazel, and Nico navigate Venice, where Nico explains that the city is filled with lemures - angry Roman ghosts. They find La Casa Nera, but the katoblepones attack when poisonous roots sprout around the demigods' feet.
Frank transforms into a lion and fights off several monsters, but one blasts him with poisonous green gas. Hazel takes a full blast in the face and collapses. They flee to the Black House, where a young man in denim - the minor god Triptolemus - opens the door and offers to cure Hazel, but only if they come inside quickly.
Triptolemus, the god of farming, explains that the katoblepones were accidentally imported from Egypt centuries ago. He can cure Hazel, but when he learns that she and Nico are children of Hades, he turns hostile - Demeter, his patron, despises Hades. He transforms Nico into a corn plant.
Frank notices that Trip's winged chariot is missing one of its two serpent-wheels. Frank desperately offers to fix the chariot in exchange for healing his friends, invoking Hecate's name. Triptolemus agrees - but warns that if Frank fails, he'll be turned into sorghum.
Frank realises he needs Mars to turn an enemy into a serpent, just as Ares once transformed the demigod Cadmus. To prove himself worthy, he must match the legendary feat of Horatius - a single warrior defending a bridge against an entire army.
Frank harvests poisonous roots as bait and leads every katobleps in Venice on a chase through the city, shapeshifting between lion, dolphin, eagle, and human to stay alive. He lures the entire herd onto a wooden bridge and makes his stand, slaughtering hundreds of monsters in a frenzied battle. Mars himself appears, impressed, and transforms the last katobleps into a python. Frank has grown physically - taller, broader, and more confident. He returns the python to Triptolemus, who heals Hazel, restores Nico from his corn-plant form, and shares the secret of surviving the House of Hades: eat barley cakes before drinking the temple's poison chalice.