Finding demigods and bringing them to Camp Half-Blood is more a satyr thing than a demigod thing. But Percy, Annabeth, and Thalia Grace—recently recovered from her years as a pine tree thanks to the Golden Fleece—don’t hesitate to travel to Bar Harbor, Maine, when Grover calls for help. He’s found two demigods in one place: Nico di Angelo and his older sister, Bianca, both students at Westover Hall. He needs assistance because the vice principal is a real monster—specifically, a manticore, with a fast-action tail that shoots poison spikes.

One of those spikes finds its way into Percy’s shoulder. While he slowly succumbs to the poison, he gets the pleasure of hearing Thorn gloat about the Great Stirring, a massive uprising of deadly monsters. There’s even one powerful enough to bring down Olympus itself.

His gloating would likely have continued if Annabeth didn’t interrupt with a brilliant sneak attack that sends Percy and the di Angelos out of the line of poison-dart fire. Thalia and Grover join the melee, but they are no match for Thorn’s deadly spikes.

Luckily, reinforcements arrive: the Hunters of Artemis, led by the goddess herself. While the Hunters fill Thorn like a pincushion with their arrows, Annabeth leaps on his back and stabs him with her dagger. Roaring with rage, the manticore leaps off a cliff, taking Annabeth with him. Percy wants to dive after her, but the Hunters restrain him. Artemis senses that Annabeth is gone, but not lost. Wherever she is, Artemis will find her. And she’ll find the monster Thorn mentioned. Find it, and destroy it before it can destroy Olympus. But first she’ll get them a ride back to Camp Half-Blood, courtesy of her twin brother Apollo, the sun god.

Back at camp that night, Percy dreams that Annabeth is in a dark cavern, struggling to hold up an impossibly heavy weight. Her face twists in agony. Nearby, her once-trusted friend Luke Castellan sneers, then turns and walks away.

The dream convinces Percy that Annabeth is alive but in grave danger. He wants to leave camp to find her, but to do that he’ll need a quest. He gets one in the weirdest way possible. He’s arguing with Thalia over whose fault it was they lost capture the flag to the Hunters (obviously, Thalia was to blame), when the desiccated mummy that houses the Oracle of Delphi strolls into the clearing and spouts a prophecy amid a cloud of green smoke.

The prophecy sends Percy on his way across the country, with Nico, Bianca, Thalia, Grover, and a Hunter named Zoë as backup. They battle zombies, ride a giant pig, and scout out an enormous pile of godly metal discards. Tragedy strikes along their journey, a devastating blow that will come back to haunt Percy. Finally they skirt an acid-breathing dragon and climb a mountain to the cavern Percy saw in his dreams. Luke is there, waiting. So is Annabeth. Her burden has been assumed by another: the goddess Artemis, kidnapped while hunting for the deadly monster and now trapped beneath the crushing weight to prevent her from returning to Olympus. But return she must, for she is the deciding vote in the most important decision the gods have ever made. A decision that could save them or doom them forever.