Plot

Mount Olympus beckons... and Percy must face his greatest battle yet.

At Camp Half-Blood, before going on the quest for the Master Bolt, Luke trains Percy in sword fighting. Percy has trouble picking it up, and doesn't understand why form would be important in a fight against opponents that don't fight fair. Luke pushes Percy to try harder, and tells him he needs to know the rules of combat so he can use them to his advantage. Percy asks Luke to show him an example.

On the shore of the sea, Percy challenges Ares to single combat in exchange for their lives and Hades' Helm of Darkness, first blood wins. The god of war is incredulous, but humorously accepts the challenge, warning Percy there cannot be any trace of him left to tell of Ares' plan to steal the bolt. Percy tells him he knows it was Kronos' plan, not Ares', asking if the Titan Lord spoke to him in his dreams like he did to Percy. Ares angerly insists that gods do not dream, and the two clash. Percy holds his own at first, narrowly avoiding being impaled on Ares' sword, but the war god's strength proves too much for the young half-blood. When Percy is kicked to the edge of the surf, the sea grows restless. As he rises, Percy summons the water into a massive tidal wave that smashes down upon Ares, washing him away. Before Ares can fully recover, Percy is able to deflect his sword and draw first blood, winning the challenge. Annabeth and Grover celebrate Percy's victory, but Ares warns Percy that he has made a permanent enemy of the god of war, before transforming into his true divine form and disappearing, leaving the helm behind. As they gather it, Percy hears a woman's voice from along the shore, turning to see his and his mother's cabin, realizing they are on the beach at Montauk. They go to the cabin, and find Alecto inside. Percy hands over the Helm of Darkness, understanding it, not the bolt, was what she had been sent to recover. He asks her to remind Hades of their deal, and release his mother. Alecto takes the helm and leaves, wishing Percy luck on Olympus. Grover and Annabeth try to talk Percy out of going to Zeus' palace to return the bolt. They have already missed their deadline, and Zeus will certainly kill him, either because he thinks he is the lightning thief, or merely because he is a forbidden child. Percy tells them he has to try and get Zeus to call off the war, as the last full-scale conflict between the gods began World War 2 on earth. Aside from that, Kronos wants the gods to fight as part of his plan to return from Tartartus. If they are to have any chance of stopping him, Percy must make an appeal to Zeus. Annabeth gives him her camp necklace for good luck, and Percy heads to the Empire State Building. He shows the guard the Master Bolt, and is let onto the elevator to the 600th floor. Percy exits the elevator onto Mount Olympus, a vast and beautiful city in the sky.

Before the quest, Luke continues teaching Percy about the gods and demigods. Percy realizes that the gods are all powerful, but must adhere to rules and laws that govern the use of their powers. Their children however, are not beholden to such rules. Percy asks if that means the half-bloods should deserve as much fear and respect as the gods themselves. Luke is proud of Percy for learning so quickly, but warns him to be careful of such ways of thinking. He likens the situation to Annabeth's intense fear of spiders; despite being so much bigger than them, she still fears them, though she still has the power to easily kill one when she runs into it. He tells Percy not to be both small and scary at the same time. In the present, Percy goes to Zeus' throne on the peak of the mountain, and presents the Master Bolt. He tells the king of the gods that he didn't steal it, and tried to return it on time, but Zeus points out his failure to do so before the deadline. He takes the bolt from Percy, who tells him about Kronos's attempt to rise from the Underworld. Zeus dismisses him, and seems unsurprised to hear his titan father is gathering strength, saying that's simple what immortals do, try to overthrow eachother. Nevertheless, he still intends to pursue his war with Poseidon. For returning the bolt, Percy will be allowed to leave alive. Percy demands he stop the war, but Zeus won't listen and will not negotiate, warning the young demigod to leave while he can. Percy is persistent, telling him Kronos wants the gods to weaken eachother and points out the easy betrayal of Ares, Zeus's own son. He tells him all the other gods follow him out of fear, not loyalty. If they fight, then once Kronos returns, the other gods will serve the more powerful being, and Zeus will cast down by his own family. Angered, Zeus moves to smite Percy where he stands, but is stopped by Poseidon. The sea god formally surrenders to his brother, asking only for his son's life in return. Zeus points out Percy should never have been born, but Poseidon reminds Zeus of his own child, Thalia, and the heroic deeds of her and other Half-Bloods like Percy. Satisfied enough with Poseidon's submission, they discuss the matter of their defeated father. Zeus leaves to call for a council of the gods, but warns Poseidon to make sure Percy never comes into his presence again. After he disappears, Poseidon finally comes face to face with his son. He recognizes Percy's stubborn nature, a trait inherent from the sea itself. Percy asks his dad if gods dream, and Poseidon tells him they do, no matter what Ares might say. Percy then asks if his father ever dreamt of Sally. The sea god does not answer, but becomes emotional at the question. Reluctantly, Poseidon tells Percy he should leave, and sends him back to Half-Blood Hill with another pearl.

Percy enters camp to a hero's welcome, and Annabeth is there waiting for him. He sees that Clarisse is still roaming free at camp, and Annabeth takes him to see Luke and explain. Luke tells him that while they were gone, the entire camp was on a knife's edge under the threat of their parents going to war. He couldn't risk accusing the head of the Ares cabin, Clarisse, without proof, or risk setting off their own war in camp. However, now that Percy has stopped the war, they can let Chiron know to bring her in. Annabeth volunteers to stay behind and watch Clarisse while Luke and Percy meet Chiron away from the main camp, and talk to him without Clarisse's cabin mates finding out. That night, as the celebrations commence, Luke takes Percy deep into the woods to wait for Chiron. Percy confides in Luke that he's still thinking about the prophecy, assuming that him "failing to save what matters most in the end" means Hades will never return his mother. Luke is empathetic, but tells him not to put too much stock in prophecies. Percy tells him that everything the Oracle told him has come true, but then remembers he forgot one line; "You shall be betrayed by one who calls you friend". He realizes that Chiron isn't coming to meet them, Luke never told him about Clarisse, because he knew she didn't steal the Master Bolt. Luke did. He helped Ares get the bolt into Percy's backpack, ready to appear as soon as the flying shoes Luke gave him pulled him into Tartarus, right down to Kronos. Luke tells him he was supposed to be wearing the shoes, not Grover, but says he never wanted to hurt Percy. He wanted to recruit him. He shows him his new sword, Backbiter, made of both Celestial Bronze and Steel, a magic item capable of tearing open secret doors in thin air. He starts to open one, and tells Percy they can escape through it and begin taking down the gods, with Kronos' help. The gods care only for themselves, demanding loyalty and great deeds and giving nothing in return, even to their own children. Percy doesn't want any part of Luke's plan to overthrow their parents, but Luke tells him the gods fear their offspring uniting against them. Kronos will bring back his golden age, without the selfish, petulant gods in charge. Percy says Kronos has gotten to him, but Luke tells him he finally knows the truth. He was able to steal the Helm and the Bolt all by himself, but for the next step, they will need help. Before he can finish cutting open the door, Percy draws his sword and stands in Luke's way. He tells him that despite their parents' flaws, they are trying their best, and still love them. He tells Luke he spoke to Hermes, but at the mention of his father, Luke attacks. Percy tries to hold him off, but Luke finishes opening the portal. He gives Percy one last chance to come with him, but Percy counter attacks, staggering Luke back. Thinking he's hurt his friend, Percy lowers his guard, and Luke takes advantage and disarms and injures Percy. He hesitates striking the killing blow, and suddenly deflects a dagger thrown by Annabeth, who used her invisibility cap and heard Luke's confession. Luke is stunned as she draws her own sword to defend Percy, and he bolts through the portal, which closes behind him.

On the last day of Summer, Percy packs to go home. Chiron comes to say goodbye. Percy reminds him he may be back if Hades hasn't held up his end of the bargain and returned his mother, but he needs to find out regardless. Chiron doesn't want Percy to go alone in case Luke comes looking for him, but Percy doesn't think Luke wants to kill him. Chiron is worried that Luke, and more importantly, Kronos, will want to sway Percy to their side as the son of Poseidon would make for a powerful ally. Percy assures him he won't be so easily persuaded to turn on them, and Chiron is proud of how far his student has come. Dionysus arrives to send away all the campers except those that will be staying year-round. Percy finds Annabeth at Thalia's tree on the border of camp. She is also packed to go home, having decided to give living with her dad and step-family another try. Percy is happy for her, and encourages he to just be a kid while she's gone. Grover meets them, and shows them his newly awarded Searcher's License. He will leave right away to begin his search for Pan, planning to skip the over-examined wildlands and try the seas instead. Percy tells him to let him know if he ever needs help. The three make a pact to meet eachother at Thalia's tree next year, no matter what, before saying goodbye.

Percy goes to the cabin at Montauk and finds Sally inside, and they tearfully reunite. After this, Percy finds himself once again in a dream, where he is greeted by the skeletal figure that's been haunting him; Kronos. He tells the titan lord to accept his defeat, but Kronos says they will meet again one day, in person, so long as the young demigod lives long enough. Percy warns him he won't be killed so easily, but Kronos is pleased. He needs Percy to survive, as he is an essential part of his return. Percy is woken up by his mom in their apartment, and he tells her he had another dream about Kronos, though he doesn't tell her what his titan grandfather said. Sally makes him blue pancakes, and the two leave to take Percy to his first day at a new school.