A midwinter rescue mission goes sideways when Percy, Annabeth, and Thalia travel to a remote boarding school to retrieve two powerful new demigods, siblings Nico and Bianca di Angelo. The encounter ends in disaster: the manticore Dr. Thorn attacks, the goddess Artemis and her Hunters intervene, and by the time the dust settles Annabeth has vanished. Artemis, hunting a monster with the power to destroy Olympus, is soon taken captive herself, and with her gone the balance among the gods tips toward danger. A quest is launched to find her before the winter solstice council convenes.
The questers, drawn from Camp Half-Blood and Artemis's immortal band of Hunters and led by the Hunter Zoë Nightshade, journey across the country toward Mount Othrys, where an ancient Titan plots his return. Along the way Percy navigates uneasy alliances, the sting of being left off the quest, and a prophecy that promises loss.
This third installment darkens the saga's tone considerably, raising the stakes of the looming war with the Titans and confronting its young heroes with genuine sacrifice. New characters expand the mythology, old loyalties are tested, and the cost of being a hero becomes painfully concrete.