The Mark of Athena

The Argo II arrives over New Rome with Annabeth Chase determined to prevent a war between Greeks and Romans. The Roman boundary god Terminus refuses to let an armed Greek warship land, but Annabeth negotiates a technical compromise: the ship will hover while the crew descends unarmed. In the forum, Annabeth reunites with Percy Jackson, now a Roman praetor, and meets Reyna Avila Ramírez-Arellano, Frank Zhang, and Hazel Levesque. A tense feast briefly suggests peace. The two sides compare prophecies and plans, agreeing that seven demigods must sail toward the ancient lands and close the Doors of Death, but Ella the harpy blurts a prophecy about the Mark of Athena, alarming Annabeth and Octavian.

Reyna privately warns Annabeth that Greek and Roman cooperation has historically ended in disaster and hints at a grievance involving Athena. Before she can explain fully, the Argo II attacks New Rome. Leo Valdez is discovered at the weapons controls, possessed and repeating a command to destroy the camp. The heroes flee under Roman fire, with Frank transforming into a dragon and Hazel creating a diversion. The attack makes diplomacy impossible, and Octavian uses it to inflame Roman anger against the Greeks.

The damaged Argo II lands near the Great Salt Lake for repairs. Leo and Hazel retrieve materials from an island, where Nemesis warns that Nico di Angelo has only days to live and gives Leo a fortune cookie that will provide help at a painful price. They also encounter Echo and Narcissus, and Leo secures Celestial bronze by distracting Narcissus while Hazel uses her powers. Meanwhile, the crew pieces together that Leo, Percy, and Jason have been influenced by eidolons, possessing spirits serving Gaea. Piper McLean uses charmspeak to force the spirits to swear on the River Styx to leave the crew and never return.

Guided by Piper’s visions and Bacchus’s advice, the crew travels through Kansas and Atlanta. Bacchus tells them to seek the sea god Phorcys for information about the twin giants Ephialtes and Otis. At the Georgia Aquarium, Percy, Frank, and Coach Hedge are trapped by Phorcys and Keto, who intend to display them as captives, but they escape from an enchanted tank. Phorcys reveals crucial clues: the twins are staging a spectacle in Rome, holding a prisoner in a bronze jar, and a map connected to the Mark of Athena lies in Charleston.

In Charleston, Annabeth, Piper, and Hazel meet Aphrodite, who confirms that the Mark of Athena leads to the lost Athena Parthenos, a statue stolen by Rome and central to the hatred between Greek and Roman demigods. Annabeth retrieves a bronze map disk from Fort Sumter during a Roman assault and confronts Reyna, who warns that the Greeks’ escape will make the camps enemies. The Argo II flees into the Atlantic, unable to fly after damage from the battle. There, Leo and Hazel share a vision revealing that Hazel once loved Sammy Valdez, Leo’s great-grandfather. Soon after, a sea monster attack throws Leo, Hazel, and Frank into the sea, where ichthyocentaurs Aphros and Bythos aid them, promise safe passage, and give Leo a letter to Tiberinus for Annabeth’s quest.

At the Pillars of Hercules, Hercules blocks the heroes from entering the Mediterranean unless Jason and Piper take the remaining horn of the river god Achelous. Piper defeats Achelous but refuses to reward Hercules’s cruelty. She uses the horn as a cornucopia to bury Hercules in food, allowing the Argo II to escape into the Mare Nostrum. Near Italy, the pirate Chrysaor captures the ship, but Percy exploits the dolphin warriors’ fear of Dionysus. Frank’s shapeshifting completes the ruse, and the crew sinks Chrysaor’s trireme as an offering to Bacchus before reaching Rome.

In Rome, the quest divides. Annabeth must follow the Mark alone, while Hazel, Leo, and Frank search for Nico, and Percy, Jason, and Piper later hurry to rescue him. Tiberinus and Rhea Silvia escort Annabeth to the beginning of her underground path. Using intelligence rather than powers, she survives hidden Roman shrines, Lares, a broken ankle, spiders, and collapsing tunnels. She reaches the Athena Parthenos, guarded by Arachne. Unable to beat the monster in combat, Annabeth manipulates Arachne’s pride, convincing her to weave a massive structure that becomes a trap like Chinese handcuffs. Arachne is immobilized, and the statue is freed, but the cavern begins to collapse.

At the same time, Leo, Hazel, and Frank find Archimedes’s hidden workshop beneath the Pantheon. Eidolons animate machines and nearly kill Hazel and Frank, forcing Leo to use Nemesis’s fortune cookie for the access code to the workshop’s control sphere. He saves his friends and gains Archimedes’s scrolls, but learns Nico’s sword was planted as a decoy. Percy, Jason, and Piper reach the giants’ hypogeum beneath the Forum and find Nico alive but weak inside a bronze jar. The twins unleash chaos, but Bacchus arrives, turns the battle into a Colosseum spectacle, and finally helps Percy and Jason destroy Ephialtes and Otis.

Nico reveals that the Doors of Death must be closed from both sides: the mortal entrance in Epirus and the Underworld side in Tartarus. The Argo II blasts into Arachne’s cavern and rescues Annabeth while the crew lashes the Athena Parthenos to the ship. As the chamber collapses, hidden spider silk attached to Annabeth drags her toward a chasm leading to Tartarus. Percy catches her but cannot pull her free. Rather than let her fall alone, he orders Nico to lead the others to Epirus, releases the ledge, and falls with Annabeth into Tartarus. The devastated crew escapes Rome with the statue, trusting that Percy and Annabeth are alive and resolving to meet them at the Doors of Death while Leo upgrades the Argo II for the next stage of the quest.