Annabeth Chase has been searching for Percy for months. She finally gets a lead from Jason, of all people, whose memories have returned. He is not a child of Zeus, but of Jupiter, Zeus’s Roman aspect. Hera plucked him from Camp Jupiter, his home, stole his memories, and got him here. Her goal was for him to get to know the demigods of Camp Half-Blood so that when she returned his memories, he could bridge the gap between the Romans and the Greeks. Percy, Jason suspects, is being used in a similar way. If he’s right, then Percy is in Camp Jupiter.

Annabeth sets sail for California aboard an incredible flying ship built and captained by Leo Valdez. Piper and Jason go with them. They reach Camp Jupiter safely and after six long months, Annabeth and Percy are finally reunited. Their happiness is brief, however. Many of the Romans, particularly their augur, Octavian, view the Greeks with great suspicion.

Their praetor, Reyna, is willing to listen to the Greeks, however. She and Annabeth conclude that the Great Prophecy—or, as the Romans call it, the Prophecy of Seven—is about combining the forces of Camp Half-Blood and Camp Jupiter. The seven demigods must come from both camps.

Just then, Leo’s flying ship starts firing on the Roman camp. Furious, they retaliate. Percy and Annabeth race to their friends’ aid, with Jason, Piper, Frank, and Hazel close behind. Safely on board, the seven demigods, four Greek and three Roman, fly off on their most dangerous quest yet, one that will take them to Greece and Rome where their cultures began and where they were told never to go. Their goal? Follow the Mark of Athena and find what it leads to.

Quests usually call for three people, but on this one, Annabeth must go alone. The Mark of Athena leads her to an ancient forty-foot statue of her mother that exudes incredible power. Annabeth knows she must get the statue back to Camp Half-Blood, but it’s guarded by Athena’s greatest rival, Arachne, who has spun a web to hold the statue in place. Annabeth manages to free it and destroy Arachne at the same time. Percy and the other demigods arrive to witness her moment of victory. But triumph turns to tragedy. Annabeth is dragged into the depths of Tartarus . . . and Percy along with her. They have survived grave dangers before. But Tartarus is worse than anything they’ve ever encountered.