Jason, Piper and Annabeth are teenage demigods with one mortal parent and one parent of mythological, divine origin. They are in Ithaca, Greece, to investigate the sudden gathering of Penelope’s suitors, who are evil spirits (characters from Homer’s Odyssey).
The evil spirits serve the dormant earth goddess Gaea, who wants to rise from sleep and overthrow the gods and demigods. Gaea’s other evil servants, the giants, intend to bring Gaea back to life at the Acropolis in Athens. Jason tries to infiltrate the ghostly suitors, but his identity is revealed when his mother’s ghost finds him.
Jason’s mother’s ghost has become a mania, a spirit of insanity. She asks him to join her in serving Gaea, but Jason chooses to remain loyal to his friends. In an ensuing fight with the ghostly suitors, Jason defeats most of them, but is wounded. Jason calls on the Roman goddess Juno to heal him, but she is only able to offer him advice; he and his six friends should take their boat, the Argo II, to Olympia and defeat the victory goddess Nike before they go to Athens to stop the giants from raising Gaea.
From a different point of view, demigods Reyna and Nico and their satyr mentor, Coach Hedge, are transporting the 40-foot-tall Athena Parthenos statue. The statue is a valuable relic stolen from the Greek demigods at Camp Half-Blood by the Roman demigods at Camp Jupiter. Since the statue’s disappearance, the Greek and Roman demigods have been on the verge of war. The gods themselves have been split between their Greek and Roman personalities, unable to maintain a solid form or help the demigods.
Nico is a son of the Greek death god Hades and can teleport himself and others through shadow-travel, but it drains his strength. Nico is already exhausted from previous quests and tasks, so the three friends and the statue crash-land in Pompeii. Reyna, daughter of Roman war goddess Bellona, shares some of her strength with Nico to sustain him.
Reyna has a vision of the Roman praetor Octavian preparing a legion of Roman demigods to attack the Greeks at Camp Half-Blood. In eleven 11 days, the giants plan to awaken Gaea, and Octavian, who is in league with Gaea, plans to simultaneously attack Camp Half-Blood. Reyna needs to return the statue before that happens, to restore peace between Romans and Greeks.
Suddenly Gaea gives bodies made of dirt to the ghosts of Pompeii so they can attack Reyna, Nico and Coach Hedge. Nico uses a magic scepter to raise a zombie army of old Roman soldiers, and Reyna commands them with her special power as a Roman praetor. The zombies cannot defeat the earth-ghosts, and Nico teleports himself, his friends and the Athena statue to safety just in time.
Leo is a son of Greek god Hephaestus and the designer/engineer of the Argo II, currently carrying Percy Jackson, Annabeth, Piper, and a wounded Jason to Athens, along with demigods Frank and Hazel. They sail to Olympia and confront the victory goddess Nike, who takes Frank, Hazel, Percy and Leo into the Olympic stadium to make them fight each other to the death. They pretend to fight each other until Leo manages to trap Nike with his inventions.
The crew of the Argo II is looking for ingredients for something called the physician’s cure, which Nike said they would need in order to cheat death. One of the seven of them is prophesied to die in the upcoming conflict, so they need the physician’s cure to revive whoever needs it. They travel to various cities, gathering ingredients.
Reyna, Nico and Coach Hedge shadow-travel to San Juan, Puerto Rico, which is Reyna’s hometown. The hunters of Artemis capture them, and Reyna is taken to the headquarters of the Amazons, female warriors who are working with the Hunters. Reyna is surprised to find that her older sister, Hylla, is now the Amazon Queen. Hylla is trying to trap Gaea’s great hunter Orion, and she wants to use her little sister as bait, since Orion is tracking Reyna. Instead, Orion ambushes the Amazons, forcing Hylla and Reyna to team up and fight him.
The giant Polybotes captures Percy, but Jason kills the giant with help from Kymopoleia, the goddess of sea storms. Kymopoleia is angry that she never gets acknowledgement as a goddess, but Jason promises her that he will ensure that she and all the minor gods are honored at Camp Jupiter and Camp Half-Blood in the future.
Bryce, a scout from Octavian, comes to capture Reyna, but Nico defeats Bryce by using his underworld powers to turn Bryce into a spirit, effectively killing him. Nico goes unconscious for three days, and when he wakes up, Coach Hedge has called an army of pegasi to come carry the statue because Nico is too weak to transport it further.
The crew of the Argo II finds Asclepius, the god of healers, and asks him to mix together the physician’s cure for them. Leo knows that one of them must die in the fight with Gaea, and he is determined to sacrifice himself for his friends. Only Frank and Hazel are aware of Leo’s plan. The Argo II flies to Athens so they can fight the giants.
The pegasi fly Reyna, Nico and Coach Hedge near Camp Half-Blood and leave them and the Athena Parthenos safely on a yacht docked in the Long Island Sound. Reyna fights Orion one-on-one while her friends team up with the Roman troops who are not loyal to Octavian.
Reyna throws a knife into Orion’s chest, and the goddess Athena extends a cloak of protection over Reyna to shield her from Orion’s explosive arrows. Her mother, Bellona, grants her extra strength, and Reyna kills Orion by strangling him with her cloak.
Piper, Annabeth and Percy sneak into the tunnels under Athens to stop the giants’ ritual to raise Gaea. The giants need to spill demigod blood onto the earth in order to raise Gaea, and they capture Annabeth and Percy. Piper rushes to their rescue, but in the ensuing fight, both Annabeth and Percy bleed onto the ground, which means Gaea will rise.
Nico goes to Camp Half-Blood and leads a mission to disable the catapults that Octavian wants to use to annihilate the camp. The Greek and Roman demigods are on the verge of a bloody battle with each other when Nico sees Reyna flying to Camp Half-Blood with pegasi carrying the Athena Parthenos statue. Returning the statue restores peace between the Greeks and Romans, but Gaea has already awakened.
When the Athena Parthenos statue is returned to Camp Half-Blood, it also restores the divided gods, who can now lend their help. In Athens, Zeus and the other Greek gods appear to help Jason and his friends fight the giants. Soon, the giants are vanquished, but the gods cannot defeat Gaea because a prophecy says that only half-bloods can.
Zeus sends the Argo II sailing all the way to Camp Half-Blood. Leo then uses his metal dragon Festus to lift Gaea into the air, and Jason flies up with Piper to join the fight. Piper puts Gaea to sleep with charmspeak. Leo sends Jason and Piper to safety while he makes his metal dragon explode, simultaneously killing Gaea and sacrificing his own life.
The Roman demigods return to Camp Jupiter, and Jason remains at Camp Half-Blood, knowing that he’ll be visiting back and forth between the two camps in his new role as pontifex maximus, a priest who makes sure all the gods are honored. Nico, Jason, Piper, Percy and Annabeth stay at Camp Half-Blood wile Reyna, Frank and Hazel return to Camp Jupiter.
Leo revives from death in midair after injecting himself with the physician’s cure. He discovers that his metal dragon, Festus, is damaged from the explosion and has flown Leo all the way to Ogygia, the island of Calypso. Leo fell in love with Calypso in a previous novel and promised he would return to her one day. Calypso wants to leave the island, so Leo takes her with him, and they fly away on his metal dragon.