Seven half-bloods shall answer the call.
To storm or fire, the world must fall.
An oath to keep with a final breath,
And foes bear arms to the Doors of Death.

These are the words of the next Great Prophecy, uttered by Rachel Elizabeth Dare, the new Oracle of Delphi, at the conclusion of The Last Olympian, the final book in the Percy Jackson and the Olympians series. The demigods had just defeated Kronos and his army of Titans, so another prophecy about the world falling wasn’t exactly what they wanted to hear. But prophecies can take years to come true, and when days and months tick by with no catastrophic events, the demigods of Camp Half-Blood relax.

Then Percy Jackson goes missing and three new demigods arrive: Jason, Piper, and Leo. Piper and Leo fit in well enough at Camp Half-Blood, but Jason is…different. He’s lost his memory, for one thing. For another, his weapons are made of Imperial gold, not Celestial bronze. He has a strange tattoo—the letters SPQR—on his arm. He uses Roman and Latin terms instead of Ancient Greek. And he’s the son of Zeus, which breaks a pact not to have children made by Zeus, Poseidon, and Hades decades earlier.

Jason isn’t the son of Zeus, but of Jupiter, Zeus’s Roman aspect. Hera wiped his memories and arranged for him to come to Camp Half-Blood, the Greek demigod camp, so he could learn their ways. Soon, he will be a bridge between the people of his real home, Camp Jupiter, a Roman demigod camp, and Camp Half-Blood. She stole Percy Jackson’s memories, too, and sent him to Camp Jupiter for the same purpose. When the time comes, they will forge an alliance between the two demigod worlds. United, they might have a chance of defeating the enemy that is even now growing in power: Gaea, the primordial goddess of the earth and mother of the giants.

Gaea has been slumbering for millennia. But now she is stirring. She is manipulating events to increase her power. The Doors of Death, for instance, are now in her control, moved to a place where no demigod dare go: Tartarus. Monsters, giants, Titans, and the dead: all are escaping from that place to the land above. When she awakens fully, they will be at her side. She will rise up and destroy the world.
How can seven demigods possibly stop her? By going to places they were never meant to go and stretching their skills, their friendships, and their very selves to lengths they never thought possible. But their efforts and sacrifice could all be in vain, for back at Camp Half-Blood, a demigod civil war is brewing…