The Mark of Athena
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Following straight on from The Son of Neptune, the Argo II arrives bearing Greeks. Annabeth, Leo, Piper and Jason meet Percy, Hazel and Frank in the Roman Camp but, while Annabeth is meeting Reyna’s confidences with her own, Leo causes the Greek ship to use its weapons to fire on the Roman Camp. In the uproar, the seven demigods make it back onto the Argo and escape. Leo’s unaware of what he’s done and the other’s realise that he must have been temporarily possessed.
Looking to find materials to repair the ship, they find Narcissus gazing at himself in a pool of celestial bronze while his nymph fan club swoons nearby. The cursed nymph, Echo, helps Leo and Hazel to distract Narcissus and the fangirls and they leave her behind, determined to save Narcissus from himself. Piper sees a vision of Bacchus on a road in Kansas and they decide to go there. Bacchus tells them to go to Atlanta but then the meeting turns out to be a trap, and two Eidolons possess Percy & Jason who attempt to kill each other. Percy knocks Jason out and Piper uses her charmspeak to control the spirit possessing Percy until his pegasus, Blackjack, knocks him out.
On board the ship, Piper forces the Eidolons to leave Jason, Percy and Leo and never to enter them again. Percy dreams of Nico held prisoner with only a few days left to live. He and Annabeth talk, down in the quiet of the ship’s hold. He tells Annabeth that Camp Jupiter has generations and families of Romans living there and they dream of a future life together and fall asleep side by side.
In Atlanta Percy & Frank have to escape from the god and goddess of sea monsters having discovered that they need to go to Charleston to find a map. They send out a decoy to sidetrack the pursuing Roman eagles. Annabeth recalls seeing her mother’s Roman form the previous Spring, bitter and reduced, who gave her a coin to set her off on a quest of her own.
In Charleston, the girls meet Aphrodite, Piper’s mother, who tells them that the map is in Fort Sumter just as the Romans find them and attack. Percy helps them escape and he and Annabeth get the Argo II moving while the others defend themselves. Annabeth goes to the Fort and finds the map but is challenged by Reyna and her fighting dogs. Reyna lets her escape but warns her that when they next meet they will be enemies. As the Argo takes them away, Hazel shares a flashback with Leo and they discover that his great-grandfather was Hazel’s one-time friend Sammy Valdez.
A monstrous skolopendra attacks and, in fighting it off, Frank, Hazel and Leo are captured by underwater ichthyocentaurs. While they’re captive, Leo realises why Frank is scared of his ability to conjure fire. The underwater centaurs give them advice and send them back to the ship. Together they discuss the sources of Greek-Roman disharmony and Annabeth explains about the Athena Parthenos, the enormous statue of her mother at the centre of Athenian life which went missing, believed to have been stolen by the Romans. And about the coin her mother had given her with the Mark of Athena and the quest it implied.
At the Straits of Gibraltar, the gateway to the Mediterranean, they meet Hercules, an arrogant gatekeeper who forces them to steal the second horn from Achelous the river god, his rival in love. They unwillingly take the horn but then use its qualities as a cornucopia to escape into the Mare Nostrum. While Jason defends the flying ship from the attacks of various monsters Percy dreams of the twin giants, Otis and Ephialtes, who are holding Nico captive, and hears their plans to capture and torture Annabeth. The ship comes down on water and Percy takes over until they’re rammed by pirates, led by Chrysaor the human brother of Pegasus, who easily bests Percy in a fight and takes over the ship.
By a ruse, Percy persuades Chrysaor’s dolphin warriors that the demigods are under the protection of Bacchus. They are terrified and Chrysaor escapes. They sacrifice his entire ship to Bacchus, with its treasure onboard and weighed down with diet Pepsi from cornucopia. Jason takes over again and Gaia taunts Percy in his dreams about the imminent destruction of Camp Half-Blood.
They reach Rome and Annabeth goes off with Rhea Silvia and Tiberinus to start on her quest for the Athena Parthenos. She reaches the temple of Mithras and outwits its guardian spirits, only to fall down a shaft and break her ankle. In pain, she follows the marks of Athena through the underground passages, and constructs a woven bridge to cross a chasm, ending up in the cavern where she finds the missing statue, guarded by Arachne, her mother’s ancient enemy and now a monstrous spider.
Leo, Hazel and Frank, looking for Nico, find the entrance to an underground passageway in a Roman Church. They come across a workshop with some of Archimedes’ lost designs but are captured by some of his mechanisms possessed by the Eidolons. Leo manages to take over a control sphere and fries the attackers’ circuitry. Nico is not there but they find his sword.
Percy, Piper and Jason go looking for them but end up trapped in a nymphaeum, whose nymphs have become dried up and bitter over centuries of neglect. The nymphs pour out their liquid hatred to drown the demigods, but they manage to use Piper’s cornucopia to produce fresh water, overcoming the bitter liquid with an offering of their youth and love(!). They leave the nymphs by a tunnel which takes them under the Coliseum, where they find the giants holding Nico captive. They free Nico but can’t kill the giants without the help of a god. Bacchus turns up and forces the fight to continue above in the Coliseum. The demigods carry the day, helped by Coach Hedge who turns up in the Argo with Leo, Hazel and Frank.
Annabeth, underground with Arachne, has bought some time for herself by persuading the spider that her creations could become the artwork of the new Olympus. She tricks Arachne into weaving a trap for herself just as the Argo crashes a hole through from the ground above. They transport the huge statue into the ship, but as they are about to leave, Annabeth and Percy are pulled down below ground and into Tartarus by Arachne’s final thread.