Percy Jackson has spent three days battling the snake-haired ladies, and frankly, he wishes they'd just stay dead. No matter how many times he's killed the gorgons, they turn up several hours later, ready to battle him again. To top it off, he's found himself in San Francisco with no memory of how he got there and only a dim memory of his past. He does remember his girlfriend, Annabeth, and waking up in Wolf House about two months ago. But that is all.
When he awoke in Wolf House, he'd been given a magical pen that turns into a bronze sword. The sword is named Riptide. Then Lupa, the leader of the wolves, gave him instructions. As strange as a talking wolf had been, Percy knew that this weird reality of monsters and demigods, of which he was one, was true.
As Percy escapes from the gorgons for the umpteenth time, he spies a maintenance tunnel guarded by two Roman soldiers. Something tells him that this is where he's supposed to go, but it means crossing a busy divided highway to get there. An old woman appears, looking like a deranged hippie, and tells him that his suspicions are correct. He needs to go to the tunnel because it's actually the entrance to a camp. The old woman, called June, gives Percy a choice. He can leave her here, at the mercy of the gorgons, and run to the ocean where he'll be safe for eternity; or he can carry her with him into the camp.
If he leaves June, then the gods will die, everyone he knows will die, and the earth will be destroyed. Percy opts to carry June to the camp. The closer he gets, the heavier June becomes, but he struggles to complete the task. The Roman guards, who are teenagers like Percy, are named Hazel and Frank. They help him evade the gorgons who have followed him into the tunnel. When they get to a river, June informs Percy that if he crosses the river, he will lose his invincibility. If he doesn't cross, the world will end.
Thinking of Annabeth, Percy carries June across the river to the Roman camp just beyond it. The gorgons capture Frank. Percy then commands the river water to fight the gorgons so they are forced to release Frank. Once they reach the other side, June transforms into the goddess Juno and informs everyone at the camp of Percy's identity. He is the son of Neptune. She also warns that if the god of death is not released by the Feast of Fortuna, then they will not be able to defeat their enemies.
Octavian, the camp's auger, looks for messages in the stuffing of toy animals. Hazel brings Percy to meet him in hopes that he will divine whether the gods will allow Percy to join the camp. While there, Percy reads part of a prophecy written on the temple floor. He vaguely recognizes it. It is the Prophecy of the Seven. It states that seven demigods must battle to the Doors of Death.
Octavian informs Percy that bad things happen to anyone who's tried to discover more about the prophecy. Octavian sacrifices a stuffed panda bear on Jupiter's altar and states that Percy may indeed join the legion. He then tries to blackmail Hazel into voting for him for praetor, one of the leaders of the camp. Hazel argues that the old praetor Jason may still be alive, even though he disappeared several months earlier.
On the way to dinner, Hazel and Percy meet up with her brother Nico, another child of Pluto, as is Hazel. Percy is pretty sure he's met Nico before but can't remember where or when. Hazel and Nico explain the real danger in all the weird things that have been happening lately: Monsters they kill, like the gorgons, don't stay dead.
Before they can discuss things further, Frank arrives to help Percy settle into the camp. At dinner, Percy is accepted into the Fifth Cohort, the same division of soldiers as Frank and Hazel. At battle games that evening, Percy helps Frank and Hazel lead their army to their first victory in several years. Before they can celebrate their success, their leader, Gwen, is carried out of the battle on a stretcher — dead. She's been stabbed through the back. But, miraculously, she returns to life. She tells of seeing a river and a ferry man asking her for a coin, but she simply turned around, exited through an open door and returned to the camp.
A voice rumbles through the camp declaring that death has lost its hold. The god Mars appears on the training field. He explains that Thanatos, the god who guards the gates of death, has been chained. Gaea, a kind of mother earth god, is allowing the dead to come back to life. She is also raising an army to destroy the Roman camp and the world. Mars orders a quest be undertaken to free Thanatos who, unfortunately, is being kept in Alaska, a land where the gods have no power. Thanatos is chained and guarded by a giant — Alcyoneus, Gaea's most powerful child. The quest must succeed by the Feast of Fortuna in four days' time.
Mars openly declares Frank his son, much to Frank's dismay, and orders him and Percy to head the quest. Frank picks Hazel as the third member of their group. Octavian, who is threatened by their new positions of power, insists that they need to start the quest immediately and without any aid, as the camp itself will be under attack soon.
Reyna, the camp's remaining praetor, brings Percy in for a private talk before he leaves. She and her sister, Hylla, met and fought Percy four years earlier. She warns Percy that the Roman demigods don't trust anyone Greek. There have been rumors of Greek and Roman demigods battling in the past, but she has no proof of it. Reyna offers Percy the other praetor position, but he turns it down. She asks him to reconsider if he survives the quest. Reyna knows that a powerful army, led by another giant, is going to attack the Roman camp on the Feast of Fortuna. She needs another praetor at her side to help her lead their troops into battle. Reyna asks that Percy find Hylla, who now leads the Amazons, and ask her to join in the fight. Reyna gives him her ring to show Hylla, as proof of his friendship with her.
Percy, Frank and Hazel take all that is left of the Roman navy, a little dinghy, and head toward Alaska. Hazel experiences one of her blackouts. She once was dead, but Nico snuck her through Death's Gates so she could live again. She periodically slips back into memories of her first life. In it, her mother, Marie, made a rash wish for all the wealth under the earth. Now, gold, silver and precious stones appear where Hazel walks. Anyone who takes the gems however, experiences bad luck. Marie became a servant of Gaea as a way to punish Pluto for his "gift," and fled to Alaska. Gaea used Hazel's powers to make a body for her son, Alcyoneus. At the last moment, Hazel betrayed Gaea, killing her mother and herself, rather than allow Alcyoneus to come to life. When they can't wake Hazel, Frank and Percy take her ashore.
After she wakes, Percy returns to the boat to get her a drink. Frank admits that he's figured it out; she was once dead. Hazel is relieved that he doesn't reject her for it. When she is alone, a field of grass creatures attacks her. Hazel uses her powers to call up a boulder from the ground to stop her kidnappers. The servants of Gaea, grain spirits, cannot move over the rock. Percy and Frank arrive in time to save her. From atop the rock, they spy an army of Earthborn, Cyclops and centaurs being led by Polybotes, a giant moving toward the Roman camp. Percy knows this is the army being sent to destroy it.
The friends receive help from Isis (the goddess of rainbows) at a health food store, but before they can continue their journey, Frank must battle three basilisks. He strikes the spear his father, Mars, gave him on the ground. A strange skeletal creature appears and defeats the basilisks. Mars warned Frank that he could only call on the creature three times. Then the weapon would fail.
Percy and the others head to Portland, Ore., to seek the help of a seer named Phineas. They gamble with Phineas in order to get the information they need, using the two vials of gorgon blood that Frank secured back at the Roman camp. One is instant death, and one has healing powers. If Phineas wins, he'll regain his sight, and Percy will die. But if Phineas loses, the harpies he's tormenting go free, and he dies. He'll also give them the location of Alcyoneus so Percy and his friends can find him. Percy wins the gamble. He, Frank, Hazel and their new harpy friend Ella set off for the Hubbard Glacier in Alaska.
Along the way, they stop in Seattle to speak to Hylla, the queen of the Amazons. In the Amazon warehouse, Hazel spies a fantastic horse from her childhood. The Amazons tell her that the horse named Arion is not for sale and can only be ridden by the greatest warrior. Then Percy and Frank are taken prisoner. After seeing Percy's ring, Hylla agrees to speak with Hazel. Hylla reveals that Otrera, a deceased queen who has returned from the dead, is challenging her position. Hylla must battle Otrera to the death every night, but as death's gates are open, Otrera returns to life after her defeat. After seeing that Arion has accepted Hazel as his rider, Hylla gives Hazel an escape plan and promises to help the Roman camp in their upcoming battle, if she can survive the threat to her throne. Hazel frees her friends and they ride away on Arion, the fastest horse in the world.
Arion takes the friends to Frank's grandmother's house in Canada. It is under siege by cannibalistic giants. Frank uses Mars' spear to call the skeletal warrior to help them gain entrance to the house. Frank finds Grandmother dying, and his father is attending her bedside. Mars explains that Frank is one of the demigods mentioned in the prophecy. Mars also hopes that Frank has discovered how to use his family's gift, but Frank has no clue what that could be. Mars warns Frank that his grandmother will die soon, but Frank must remember that the sweetest lives are ones that overcome difficulties. When Frank wakes in the morning, Grandmother is alert enough for one final conversation. She helps Frank to realize his family's gift is the ability to shape shift into different animals.
Frank helps Hazel and Percy set up battle stations in the attic. He holds off the giants by shooting potatoes at them, which causes their disintegration, while Percy and Hazel go to the garage and drive Grandmother's Cadillac outside. The giants destroy the house, but Percy and his friends escape to an airfield where a plane has been chartered to take them to Alaska. The harpy Ella won't ride on the plane, so she takes refuge in the forest.
On the plane, Frank reveals to his friends some of what Mars told him. He also tells them about the stick of wood he gave Hazel to carry. When that wood burns completely, Frank will die. He explains that he and Percy are distantly related, as one of Frank's ancestors was a son of Neptune as well. Percy tells the others that since drinking the gorgon blood, his memory has been returning. He knows he was part of a Greek demigod camp and that Hera sent him to bridge the animosity between the Greeks and Romans. If the Prophecy of the Seven is to be successful, they must work together.
Once in Alaska, the three demigods take a train to the Hubbard Glacier but are attacked by gryphons along the way. Frank uses the last charge of his spear to raise the skeletal warrior so it can save the other passengers while he and his friends escape. Percy then falls into a bog, but Hazel jumps in to save him. She is tempted by Gaea to betray her friends, but resists Gaea's offer. She learns that Gaea has taken her brother, Nico, prisoner.
Hazel saves Percy, and the three continue their quest. When gryphons attack them again, Hazel makes what she thinks is a futile attempt to call Arion, whom they'd left in Canada. Arion arrives in time to save the demigods, and then carries them across the water to the Hubbard Glacier. They find Thanatos, the god of death, chained and guarded by a ghostly army. The only way to free him is to use the flames from the stick of wood that is tied to Frank's life.
Percy takes on the ghostly army while Frank works to free Thanatos. Hazel must battle the giant Alcyoneus. Only an ember of his life stick remains when Thanatos is finally freed. In the heat of the moment, Frank is able to turn into an eagle and peck at Alcyoneus' eyes. He then becomes a bear to take down the giant. Turning back into a human, Frank ties one end of a rope around Alcyoneus' leg and the other end to Arion. Meanwhile, Percy has used his powers to create a tidal wave to destroy the ghostly army. Unfortunately, it takes him under as well.
Thanatos tells Hazel that he has no orders to take her back to the Underworld, so she is free to continue living. Frank and Hazel drag Alcyoneus to Canada, where he is no longer invincible. Together, they are able to defeat the giant. They return to the glacier to find Percy still alive.
The friends have succeeded in their quest. They return to the Roman camp in time to help Reyna and the rest of the demigods defeat Gaea's army. The following day, a flying ship arrives from Camp Half-blood, the home of the Greek demigods. Knowing that Annabeth is on board, Percy convinces the Romans not to attack it, but to listen what the crew has to say.