Percy Jackson awakens with no memory, save for a single name: Annabeth. He's been on the run for months, hounded by persistent, snake-haired gorgons who refuse to stay dead. His only companion is a celestial bronze sword that appears from a pen named Riptide. Guided by instinct and the wolf goddess Lupa, he stumbles upon a hidden valley in California, a sanctuary for demigods, unlike anything he's ever known: Camp Jupiter.
Upon arrival, he encounters two young Roman demigods, Hazel Levesque and Frank Zhang, who help him escape the relentless gorgons. The camp is a stark contrast to his vague memories of another place. Here, stern discipline and military precision reign. An old woman, who reveals herself to be Juno, the queen of the gods, announces Percy's arrival and declares him a son of Neptune, a claim that raises eyebrows and suspicion among the Roman demigods, especially the ambitious augur, Octavian.
Soon after, Mars, the Roman god of war, descends upon the camp. He reveals a dire prophecy: Thanatos, the god of death, has been captured by the giant Alcyoneus in Alaska, a land where the gods' power is weak. Without Thanatos, death holds no sway, and monsters, once defeated, simply reform, making the war against the awakening Earth Mother, Gaea, impossible to win. Mars declares Frank his son and charges him with a quest to free Thanatos within four days, before the Feast of Fortuna. Frank, to his dismay and surprise, is ordered to lead the quest and chooses Percy and Hazel as his companions.
Their journey to Alaska is fraught with peril. Hazel carries a heavy secret: she died decades ago and was brought back by her half-brother, Nico di Angelo, and her past mistakes involving cursed jewels still haunt her. Frank, meanwhile, discovers his unique family legacy and the terrifying truth behind his life - a single piece of firewood, burning away slowly, is tied to his very existence. Along the way, they seek out Phineas, a blind seer, in Portland. Percy challenges Phineas to a gamble involving gorgon's blood – one vial containing poison, the other a cure. Percy's victory not only frees a harpy named Ella but also slowly begins to restore his fractured memories, revealing his true identity as a Greek demigod and the son of Poseidon.
Their quest leads them through Amazon territory, where they encounter Hylla, Reyna's sister, and gain the aid of Arion, a super-fast, magical horse. They brave attacks from gryphons and ogres, pushing deeper into the frozen wilderness of Alaska, a land teeming with Gaea's awakened forces. Percy's memories continue to return in flashes, reminding him of his friends at Camp Half-Blood and the goddess Hera's scheme to unite the Greek and Roman demigods. He understands the gravity of their mission, not just for Camp Jupiter, but for both camps and the world.
Finally, they reach the Hubbard Glacier, the lair of Alcyoneus. They find Thanatos chained and guarded by an army of undead Roman soldiers. To free the god of death, Frank must make the ultimate sacrifice, using the flames from his life-giving piece of wood. In a climactic battle, Percy confronts the ghostly legion, while Hazel faces the giant Alcyoneus, realizing that only by luring him out of Alaska can he be defeated. Frank, embracing his family's shapeshifting abilities, ignites his firewood, freeing Thanatos, and narrowly escaping with a mere ember of his life-force.
With Thanatos freed, the balance of life and death is restored, and the monsters they battle can now truly perish. They race back to Camp Jupiter, arriving just as Polybotes, another giant, leads a massive army of monsters, including Earthborn, Cyclops, and centaurs, to attack the Roman camp. Percy, with his memories fully returned and his leadership recognized, rallies the Roman demigods and leads them into a fierce battle, demonstrating his unparalleled tactical prowess and power as a son of Neptune. He is named praetor, a leader of the Roman camp, as they prepare for the inevitable war to come, a war that will require both Greek and Roman heroes to face Gaea's awakened might.
