Percy Jackson knows how to make a dramatic entrance. After days killing the same gorgons over and over, he spies the entrance to a place of safety. But before he can reach it, an old hag calls to him. She gives him a choice: carry me through the entrance and over the river on the other side and you’ll get your missing memory back, or leave me here knowing the world will end because you did so.

As far as decisions go, it’s a no-brainer. Percy has no recollection of who he is and where he came from. If all he has to do to get his memory back is carry an old lady through a river, he’ll do it. He scoops her up and charges toward the entrance, which is flanked by two Roman guards. They yell at him to hurry because—oh, good, the gorgons are back.

With the guards taking potshots at the gorgons with arrows, Percy makes it through the tunnel. He’s partway through the river when the monsters get their talons into the baby-faced boy guard. People on the far side of the river come running, but it’s Percy who saves the day. He summons giant hands of water that force the gorgons below the surface, where they disintegrate to dust. If that wasn’t spectacular enough, the old lady Percy is carrying transforms into the goddess Juno and disappears, leaving him with a bunch of astonished Roman warriors.

Their leader, a fierce-looking girl named Reyna, steps forward. She calls him a son of Neptune, god of the sea, and orders him to come with her. She needs to decide whether he is an ally . . . or an enemy who must be killed.

Percy wasn’t expecting applause—though it would have been nice—but being called an enemy and threatened with death? That strikes him a little extreme.

Then again, everything about the place—Camp Jupiter, it’s called—seems designed for combat. Take the two guards, Frank and Hazel, for instance. They snap to like well-trained soldiers. The campgrounds are laid out like a military base. The campers play war games for fun. To Percy, it seems wrong. Why, he can’t say. But his gut tells him he doesn’t belong at Camp Jupiter.

Hazel Levesque knows all about not belonging. A child of Pluto, she’s been shunned by practically everybody. Frank Zhang is the only camper who accepts her despite who she is. Except he doesn’t really know who she is. There’s just one person who does: her brother Nico. The son of Pluto, Nico is the most powerful demigod she’s ever met. He rescued her from a place she thought she’d never leave and brought her here. If Frank ever found out what that place was, or why she’d landed there . . . That’s a secret she will take to her grave.

Frank Zhang has his own heavy secret, but he tries not to think about it. Instead he works at becoming a better warrior. That’s the only way to get ahead at Camp Jupiter. But so far he’s been a big fat bust, the laughingstock of the Fifth Cohort—and that’s saying something given that the Fifth is the worst fighting group in the camp.

That changes when Percy joins them for his first war games. After months of losses, the Fifth suddenly beats the other cohorts—with Frank leading the charge. That victory earns him the recognition of his godly parent, the war god Mars. Mars awards Frank a quest, but there’s one condition. He must take Percy Jackson with him. Their mission is to free Thanatos, the god of peaceful death, who has been imprisoned somewhere in Alaska. If he remains in chains, the Doors of Death will be opened, and the dead will return from the Underworld. They will join forces with the one who opened the doors: Gaea, the primordial goddess of the earth and mother of the Titans and the giants.

Gaea has been sleeping for millennia. But now she is stirring. Her power has already been felt; it was she who ordered the giants to capture Hera. If she awakes fully, she will gather her children and the army of the dead and destroy the world. And there won’t be anyone, god or demigod, who can stop her.