Recovering a long-lost statue, trekking across Tartarus, journeying through enemy waters, closing the Doors of Death. The seven demigods of the Great Prophecy—Percy Jackson, Annabeth Chase, Piper McLean, and Leo Valdez from Camp Half-Blood, along with Jason Grace, Hazel Levesque, and Frank Zhang from Camp Jupiter—have done more in the past month than most gods do in a year. And they’re just getting warmed up.

Gaea, the primordial goddess of the earth, is close to waking from her millennia-long slumber. If she does, she will destroy the world with her army of giants, monsters, and undead. But to truly awaken, she needs something only two of the demigods can provide: their blood. A few drops spilled on the earth of the Acropolis in Athens on the Feast of Spes, the Day of Hope, will do trick.

The demigods are tempted to flee back to Camp Half-Blood, and not just to keep their blood inside them. Their friends are in danger. The Roman legions from Camp Jupiter are gearing up to attack. They’re led by the power-hungry augur Octavian, who craves victory over the Greeks and will stop at nothing to get it. Percy and the others could even the odds.

But they know they must stay in Greece for the ultimate showdown against Gaea’s giants. If they don’t, the giants will wreak havoc on the world, with or without their earth mother. Still, they can’t allow Camp Half-Blood to fall, so they send Nico and Reyna there with the Athena Parthenos statue. The statue radiates powerful energy, but more importantly, its arrival in the hands of both a Greek demigod and a Roman demigod symbolizes the potential for unification over civil war.

Getting the statue there won’t be easy, however. Nico and Reyna, with Coach Gleeson Hedge along for combat support, must shadow-travel the whole way. If that doesn’t kill them, then the mighty—and mighty insane—hunter Orion, pursuing them every step of the way, might. And if they don’t get the statue to Camp Half-Blood before Octavian launches his attack, their efforts will have been in vain.

Meanwhile, the seven demigods of the Great Prophecy are facing struggles of their own. Jason is dying from a stab wound that won’t heal. Leo is feverishly working on a secret project to save his friends, but at great personal cost. Piper and Annabeth are seeking a statue hidden in a deadly location. Frank is dealing with mean-spirited ghostly relatives who possess an ingredient Leo needs for his secret project. Hazel helps Leo and Frank find another ingredient, this one on the island of Delos where they encounter a pair of godly twins. And Percy, being Percy, is fighting an underwater battle—but this time, he’s losing.

If they survive their trial, they must still reach the Acropolis and defeat the giants on the Feast of Spes. But if two of the demigods shed even a drop of blood . . . then their struggles and their sacrifices will have been for nothing.

Gaea will waken. And she will not rest until she has destroyed the world.