After sending Leo's video to Camp Jupiter, Argo II prepares to land on the Roman camp's border, but Terminus, the god of borders, stops them for no weapons. He let the ship stay in the air while Annabeth, Leo, Piper, and Jason meet the Roman campers. Percy Jackson, Hazel and Frank welcome the Greek demigods and Coach Hedge. A reunion is held, and during the conversations, Ella, the harpy, speaks about a prophecy that refers to Annabeth's quest:The Mark of Athena. Piper uses her charm speak in order to divert the topic. While Jason gives Piper a tour of the camp, Reyna talks to Annabeth alone. During the conversation, the Argo II, which is controlled by Leo Valdez who was possessed by Eidolons, fires at Camp Jupiter. The Romans defend their bases, and Annabeth, Piper, Jason, and Percy rush inside the Argo II to stop Leo from firing on the campaign. They start to leave the camp while Hazel and Frank delay the Roman campers. The Argo II becomes damaged, and Leo lands for repairs, encountering Echo and Narcissus. There they are told that they have to search for the Mark of Athena to bring the Romans and the Greeks together.
In their journey, Gaea appears to Piper and tells her that she gets to pick which boy dies with her, Percy or Jason. When she refuses to pick, Jason and Percy's eyes glaze over and they attempt to fight to the death until Piper knocks them both out. On the ship the seven discuss the strange behavior of Jason, Percy, and Leo. Piper tries out a theory and says,"Eidolons raise your hands." At first everything seems normal, than Leo, Jason, and Percy raise their hands, revealing they are possessed. Piper charmspeaks them into leaving their bodies and the boys [minus Frank] return to normal.Percy, Frank, and Coach Hedge fight Phorcys and Keto. The seven quickly leave for Charleston. Annabeth, Piper, and Hazel go to Battery Park to find a mysterious ghost, which happened to be Aphrodite. Jason, Leo, and Frank go to a museum in Fort Sumter. Only the girls' story is told. Aphrodite tells them about their love lives and the other Olympians that are suffering from being confused about their Roman or Greek identity. After their talk, the angry Romans come, and war breaks out. Annabeth meets Reyna in Fort Sumter who was there to retrieve a map which will save her friends. Reyna begs Annabeth to come with her and die a painful death to save her camp, but Annabeth tells Reyna that she will only follow the mark. The seven reach the ship and leave safely to go to Chiron's brothers, who live in the sea. During the ride, they get attacked by a shrimp monster. Leo, Frank, and Hazel go underwater and find the icthyocentaurs, Chiron's brothers. They find the three's story interesting and speed their way to Rome. They meet Hercules near the pillars, or the entrance to Rome, and Piper and Jason get the second horn of Achelous, the bull-headed river god. Even though it was Hercules who ordered them to cut it off, Piper decided that Hercules did not deserve the horn and keeps it for herself. She brings forth a pile of supplies, and Hercules falls. The seven demigods then cross the pillars and sail into Rome. Annabeth has to find the Athena Parthenos by following the Mark of Athena and battle its guardian Arachne. Percy, Jason, and Piper travel into an underground area beneath the Colosseum and battle the twin giants, Otis and Ephialtes, the anti-Dionysus. Frank, Hazel, and Leo travel in search of Nico, and find Archimedes' scrolls on spheres.
Towards the ending, Annabeth outwits Arachne by tricking her into weaving her own trap. Percy and Jason battle the giants and with (very little) help from Dionysus's roman counterpart, Bacchus, and they rescue Nico. Frank, Hazel, and Leo battle the Eidolons and rescue Archimedes' scrolls. They all meet up where Annabeth secures the Athena Parthenos. However, Arachne pulls Annabeth into Tartarus by some spider silk as the statue is lifted onto their ship. Percy manages to grab a hold of Annabeth, but soon realizes he will follow down with her if he doesn't let go. Despite Annabeth's insistence to let her go, Percy refuses to leave her again and the two fall into the darkness.
After Percy and Annabeth fall into Tartarus, the remaining five demigods and Coach Hedge are at a meeting. Nico says that Percy told him to lead the demigods to the Doors of Death, while he and Annabeth could meet them at the end of Tartarus on the Doors' other side. Nico and Hazel both say they are fairly sure Percy and Annabeth are not dead yet. The book closes with Leo telling Festus the Dragon figurehead to set sails: declaring that they have friends to save.
In their journey, Gaea appears to Piper and tells her that she gets to pick which boy dies with her, Percy or Jason. When she refuses to pick, Jason and Percy's eyes glaze over and they attempt to fight to the death until Piper knocks them both out. On the ship the seven discuss the strange behavior of Jason, Percy, and Leo. Piper tries out a theory and says,"Eidolons raise your hands." At first everything seems normal, than Leo, Jason, and Percy raise their hands, revealing they are possessed. Piper charmspeaks them into leaving their bodies and the boys [minus Frank] return to normal.Percy, Frank, and Coach Hedge fight Phorcys and Keto. The seven quickly leave for Charleston. Annabeth, Piper, and Hazel go to Battery Park to find a mysterious ghost, which happened to be Aphrodite. Jason, Leo, and Frank go to a museum in Fort Sumter. Only the girls' story is told. Aphrodite tells them about their love lives and the other Olympians that are suffering from being confused about their Roman or Greek identity. After their talk, the angry Romans come, and war breaks out. Annabeth meets Reyna in Fort Sumter who was there to retrieve a map which will save her friends. Reyna begs Annabeth to come with her and die a painful death to save her camp, but Annabeth tells Reyna that she will only follow the mark. The seven reach the ship and leave safely to go to Chiron's brothers, who live in the sea. During the ride, they get attacked by a shrimp monster. Leo, Frank, and Hazel go underwater and find the icthyocentaurs, Chiron's brothers. They find the three's story interesting and speed their way to Rome. They meet Hercules near the pillars, or the entrance to Rome, and Piper and Jason get the second horn of Achelous, the bull-headed river god. Even though it was Hercules who ordered them to cut it off, Piper decided that Hercules did not deserve the horn and keeps it for herself. She brings forth a pile of supplies, and Hercules falls. The seven demigods then cross the pillars and sail into Rome. Annabeth has to find the Athena Parthenos by following the Mark of Athena and battle its guardian Arachne. Percy, Jason, and Piper travel into an underground area beneath the Colosseum and battle the twin giants, Otis and Ephialtes, the anti-Dionysus. Frank, Hazel, and Leo travel in search of Nico, and find Archimedes' scrolls on spheres.
Towards the ending, Annabeth outwits Arachne by tricking her into weaving her own trap. Percy and Jason battle the giants and with (very little) help from Dionysus's roman counterpart, Bacchus, and they rescue Nico. Frank, Hazel, and Leo battle the Eidolons and rescue Archimedes' scrolls. They all meet up where Annabeth secures the Athena Parthenos. However, Arachne pulls Annabeth into Tartarus by some spider silk as the statue is lifted onto their ship. Percy manages to grab a hold of Annabeth, but soon realizes he will follow down with her if he doesn't let go. Despite Annabeth's insistence to let her go, Percy refuses to leave her again and the two fall into the darkness.
After Percy and Annabeth fall into Tartarus, the remaining five demigods and Coach Hedge are at a meeting. Nico says that Percy told him to lead the demigods to the Doors of Death, while he and Annabeth could meet them at the end of Tartarus on the Doors' other side. Nico and Hazel both say they are fairly sure Percy and Annabeth are not dead yet. The book closes with Leo telling Festus the Dragon figurehead to set sails: declaring that they have friends to save.