At the conclusion of The Mark of Athena, Annabeth and Percy tumble into a pit leading straight to the Underworld. The other five demigods have to put aside their grief and follow Percy's instructions to find the mortal side of the Doors of Death. If they can fight their way through the Gaea's forces, and Percy and Annabeth can survive the House of Hades, then the Seven will be able to seal the Doors from both sides and prevent the giants from raising Gaea. But, Leo wonders, if the Doors are sealed, how will Percy and Annabeth be able to escape?

They have no choice. If the demigods don't succeed, Gaea's armies will never die. They have no time. In about a month, the Romans will march on Camp Half-Blood. The stakes are higher than ever in this adventure that dives into the depths of Tartarus.

Though the Greek and Roman crewmembers of the Argo II have made progress in their many quests, they still seem no closer to defeating the earth mother, Gaea. Her giants have risen—all of them, and they're stronger than ever. They must be stopped before the Feast of Spes, when Gaea plans to have two demigods sacrificed in Athens. She needs their blood—the blood of Olympus—in order to wake.

The demigods are having more frequent visions of a terrible battle at Camp Half-Blood. The Roman legion from Camp Jupiter, led by Octavian, is almost within striking distance. Though it is tempting to take the Athena Parthenos to Athens to use as a secret weapon, the friends know that the huge statue belongs back on Long Island, where it might be able to stop a war between the two camps.

The Athena Parthenos will go west; the Argo II will go east. The gods, still suffering from multiple personality disorder, are useless. How can a handful of young demigods hope to persevere against Gaea's army of powerful giants? As dangerous as it is to head to Athens, they have no other option. They have sacrificed too much already. And if Gaea wakes, it is game over.