Percy has been spending a lot of time with Rachel over the summer to remind himself the mortal world is still there. Percy is only a week away from turning sixteen, so his stepdad, Paul, lets him take his car down the beach. Rachel asks about Percy going on vacation with her family because they need to talk. Percy is unsure because the war with Kronos could become critical at any second. She asks if he is going to kiss her, but Blackjack lands on the hood of the car and Beckendorf is there to tell Percy that the war is starting. Blackjack takes Percy and Beckendorf out to sea where the Princess Andromeda is heading towards New York. The pegasus drops them off and Percy tells him to take off. The two half-bloods make their way to the engine room and take out the operator, a telkhine. They start duct taping jars of Greek fire to different parts of the engine. It isn’t long before someone starts heading their way. Percy goes off to distract them while Beckendorf finishes the job. He starts running around and killing telkhines, letting one escape so he can tell the others where Percy is. He rushes around and finds a giant crab demon. He get under it to kill it and then heads for the pool. Kronos (in Luke’s body) appears to tell Percy they have been expecting him. Percy decides to fight him right them and attacks. He gets a slight cut on his arm from Kronos’ scythe and his entire right side bursts into pain. Kronos has Ethan Nakamura bring out the captured Beckendorf. After they discover he has already set the explosives (and was carrying cans of food in his bag to trick them into thinking they weren’t set), Beckendorf tells Percy to leave him behind. Percy refuses until Beckendorf uses the detonator on his watch to blow up the ship. Percy jumps into the ocean and has the current take him away. He blacks out and sinks to the bottom of the sea.
Percy has a dream that takes place on top of Mount Othrys (Mt. Tamalpais) with three titans. One of them, Krios, is upset he has to be left behind to babysit Atlas while the others go to throw more tests at the gods. Percy then sees Nico di Angelo eavesdropping on the Titans. He looks right at Percy and tells him he needs Nico’s plan. Percy wakes up and sees his brother Tyson, who explains they are in their dad’s house. Percy asks about Beckendorf, and Tyson says they haven’t found him. Tyson leads them to where Poseidon is directing his side of the war. Percy is shocked to see that he looks aged 40 years. Poseidon explains his appearance reflects the state of his domain. Percy wants to stay and help fight, but his dad tells him it is time for him to return to camp and hear the rest of the prophecy from Chiron. He asks Percy about the sand dollar he gave him for his last birthday and Percy shows him he still has it. Oceanus approaches, so Poseidon sends Percy away. Percy gets to the surface and can just barely see the flashes of light that are the war below.
When Percy washes up on the shore by Camp Half Blood, the other demigods rush to great him. Selina is given the bad news that her boyfriend, Beckendorf, was killed and Clarisse goes to comfort her. After the others disperse, Chiron asks Percy to tell him what happened. Percy tells him everything except Nico’s plan, which he doesn’t want to think of himself. Chiron sends Percy to the attic with Annabeth to retrieve the prophecy. She takes a slip of paper from the pouch on a necklace around the oracle’s neck. They take it down to the war council where Clarisse is arguing with the Apollo councilor , Michael Yew. Clarisse refuses to fight until they see it her way and leaves. Percy then reads the entire prophecy and turns out a cursed blade will reap his soul. Everyone starts speculating about who is Kronos’ spy, but Selina stops them. Chiron goes on to tell Percy they have more bad news. Typhon escaped two days before and is making his way across the midwest towards Olympus. Percy thinks it and the Princess Andromeda are both distractions and that they need to protect Olympus. Percy and Annabeth seem to be the only two who don’t believe Typhon is the worst of it.
Percy dreams of Rachel in her room throwing darts at a portrait she painted of him. Her father comes it to ask if her friend is leaving with them in the morning. She tells him she is not going to the school he wants her to and he leaves the room saying they will have fun at the beach. She uncovers a portrait to show Luke as a child. She uncovers another to show the Empire State Building with lightning around the top and an army at the base. Percy wants to call Rachel when he wakes up, but he has no way of contacting her. He helps Annabeth inspect the campers’ cabins. The Ares and Apollo cabins spend the morning fighting over a flying chariot (Apollo kids claimed it in a battle that Ares kids lead). Annabeth says they haven’t heard from Grover in months. Percy thinks about his empathy link with the satyr. He thinks Grover is still alive because the link has the potential to kill him if Grover does die. Percy decides to tell Annabeth about his dream. She admits if she were Kronos, she would attack Olympus while all the gods are off fighting Typhon. She thinks they should be ready for that. Percy makes a comment about his soul being reaped and Annabeth goes off on him for being a coward. She storms away. That night, they burn Beckendorf’s shroud. Percy goes up to Selina and tells her she made Beckendorf happy for the last year. Selina appreciates the words and goes off to be alone. Percy turns to Clarisse and tells her he owes her a favor for saving her in the Cyclops’ cave in the Sea of Monsters. He wants her and her cabin to fight with them. When she refuses, he calls her a baby and she charges after him. Her boyfriend, Chris (former Kronos’ supporter), stops her and says Percy has a point. She storms off and Chris follows.
Percy goes to the arena to visit Mrs. O’Leary. He starts tossing around the shield Beckendorf made for her as a toy, but she soon starts barking to get out. Percy lets her into the woods and follows after. She leads him to Juniper meeting with a cloven elder, Leneus, and Nico. Leneus is scared of Mrs. O’Leary, so Nico takes her for a walk. The fat satyr tells Juniper there is nothing he can or would do to find Grover. Percy gets involved and tells Leneus to help, pointing out that Kronos isn’t going to spare his kind. Leneus leaves and Juniper thanks Percy. Nico and Mrs. O’Leary come back and she leaves them to talk. Nico points out that they don’t have much time before the Titans will be at their door. Percy agrees to his plan. Nico has him climb on Mrs. O’Leary to shadow travel. They are going to the home of Luke’s mom, May Castellan, to retrace his steps. Mrs. O’Leary charges into a tree and Percy finds they are in the shadows.
Nico knocks on the door and May Castellan answers. She hugs Nico and calls him Luke. When she sees Percy, she does the same. She invites them in and shows them to the kitchen where she has been making peanut butter sandwiches and chocolate chip cookies waiting for Luke’s return. They ask when she last saw Luke and she says he came by and asked for her blessing. Percy asks if he had gold eyes, and she tells him Luke’s eyes are blue. Suddenly, her eyes glow green and she is yelling about her son’s fate. When the glowing stops, she is back to her goofy self. Percy and Nico leave and find Hestia sitting with Mrs. O’Leary. She tells them how she gave Dionysus her spot on Olympus to avoid civil war. Now she is the last Olympian left on Olympus. She tells Percy the hardest decision can be to yield and he will understand when the time comes. She sends them to his own hearth. Sally and Paul are surprised when Mrs. O’Leary is suddenly taking up their entire living room. Paul is excited because this was the proof he needed to really believe what they have been telling him. Percy explains he is going after Kronos and needs his mother’s blessing. Sally is hesitant, but Paul eventually convinces her. Percy and Nico prepare to go to the underworld.
Nico explains most souls go through the Las Angeles entrance, but they will be using the Door of Orpheus. They need music to open the door set into a pile of rocks. Percy calls for Grover, but has some trouble getting through because Grover is sleeping. Percy nags at him until he wakes up. Driads toss him from one side of the park to where Percy and Nico are waiting. He explains he was following a man that humans would fall asleep when he passed. When Grover confronted him, he said he was checking out the battle ground and put Grover to sleep. He is surprised that he was out for two months. Nico explains he is lucky he ever woke up after the confrontation with Morpheus, god of dreams. They tell Grover that Kronos will be attacking soon and have him play his pipes to open the doorway to the underworld. They wake up Mrs. O’Leary and take her down the stairs with them. Nico leads them past the gate and to the waiting furies. Mrs. Dodd, whose real name is Alecto, takes Percy into the air. Nico explains he promised his father he would bring Percy to talk before they went down the river. The furies drop them off in Hades’ house and the god appears with his wife, Persephone, and mother-in-law, Demeter. Hades tells Nico his mother was the daughter of an Italian diplomat in Washington DC. He put Bianca and Nico in the Lotus because some of his other children were running around during the war. He sent Alecto as a lawyer to get the kids out when Percy was revealed. Now that he has Percy, Hades intends to lock him in the dungeon until Nico is old enough to fulfill the prophecy. He believes even if Olympus is taken, it would take Kronos much longer to get to him.
Nico comes to apologize and breaks Percy out of the dungeon. He has put the guards to sleep, but the power to do that required most of Nico’s energy. Percy starts to carry him out, but once they reach the fields, the alarm sounds. Percy carries Nico as best he can as they run. Mrs. O’Leary finds them and they get on her back to head towards the River Styx. Before Percy goes into the river, Achilles tries to warn him against it. Percy knows he doesn’t have a chance without doing it, so he chooses a spot on the small of his back as his weak point and goes in. The pain is unbearable and Percy almost doesn’t make it. He hears Annabeth’s voice and it keeps him tethered to the mortal world. Percy emerges from the water just as Hades shows up with his army of the dead. Percy takes out all the soldiers and then grabs Hades by the robe and says he will not hurt him (knowing he can’t kill him) if he gives him more information. Hades dissolves leaving his robe behind. Percy puts it in the river and frees the souls trapped in it. He tells Nico to try to convince his father to tell him what he knows. Percy takes off to go find Luke.
On his way to the Empire State Building, Percy calls Annabeth to tell her to meet him there with as many campers as possible. Chiron drops off the campers (Ares’ cabin is missing) and then heads out to find them more allies. Percy tells the other kids they have to convince Zeus a trap is coming. They go inside the lobby and Percy tells the security guard if he doesn’t let them up to the 600th floor, the lobby will soon be filled with monsters. They take the elevators and go all the way up. As they make their way to the throne room, Hestia appears. She shows Percy how Luke and Thalia found Annabeth when she was seven trying to fight off monsters with a hammer and immediately adopted her into their family. Next, she sees them running for their lives and stopping at Luke’s mom’s house for supplies because they don’t have any other options. Percy comes out of the vision and no time has passed. Hermes appears and Hestia takes off. Even after Percy begs him to bring back a few gods, Hermes refuses. He says Typhon is their biggest threat and the demigods will have to defend Olympus with the minor gods. The current efforts are preventing the Titans from entering using the air; their army will have to go through New York and take the elevator. He tells Annabeth her mom said to try “plan 23” and Percy to remember the rivers. Annabeth gives her sympathies about Luke, but it only infuriates Hermes. Percy steps in and tells Hermes that Luke is his fault. Hermes takes off. Annabeth opens up her laptop to check her plans. They find the others looking down on NYC. Percy is shocked he can’t hear anything from the world below. He looks down to see everyone has been put to sleep by Morpheus.
Since the spell over the city is so big, it does not affect the demigods when they go down to the streets of Manhattan. Percy splits up the campers so each group covers an entrance onto the island. He realizes he is one group short with Clarisse missing, but Thalia and the Hunters show up to fill the gap. Annabeth goes with Percy to take care of any enemies entering from the water. The steal a man’s scooter and get going. Annabeth has him stop along the way. She goes and activates a statue (which is one of Daedalus’ automatons) and tells it to defend Manhattan. She hopes it follows Daedalus’ programing and activates the other hundreds of statues around the city. Next, Percy goes into the spot where the East and Hudson rivers meet. He calls them out with insults and they try to kill him with debris. When that doesn’t work, they realize not only is he a son of Poseidon, but he also has Achilles’ curse. Percy pulls out the sand dollar his dad gave him and offers them each half if they take out any of Kronos’ army that tries to access Manhattan from the water. They each agree as the sand dollar is already working to clean up their pollution. Percy emerges from the water, and Annabeth tells him Michael Yew and the Apollo cabin need help because the Minotaur is leading the attack down their bridge.
Percy calls Blackjack and Porkpie so they can get to the Williamsburg Bridge to help Apollo’s cabin. Percy and Annabeth arrive to see the monsters greatly outnumbering the demigods. The pegasi land them close to Michael Yew and they all start fighting. He says he tried to get Charisse to come fight with them and even left her the flying chariot, but was unable to convince her. Percy takes the Minotaur down and then keeps the attention of the rest of the monsters while Micheal and his archers pick them off. He tells Annabeth to stay with the archers, but she stays by Percy’s side. Once they are pushed back to Brooklyn, Percy sees Kronos is at the other end of the bridge. Kronos’ army attacks and Annabeth saves Percy from being stabbed in the small of his back by Ethan Nakamura. Percy calls Blackjack and has him take away the injured Annabeth before Kronos can get to her. The Titan is surprised how quickly Percy made the decision to bathe in the River Styx. Michael Yew sets off his last sonic blast arrow and it splits the bridge. Kronos says he will see Percy that night before making his way back to Brooklyn. Percy searches for Michael, but can’t find him. Silena calls him on the phone his mother gave him to tell him Annabeth needs a healer from the Apollo cabin.
Percy takes a motorcycle and Will Solace from the Apollo cabin to the Plaza Hotel. A statue of Pompona (Roman goddess of plenty) is guarding outside and upset when Percy doesn’t know who she is. She makes a remark about how respect for the minor gods could have prevented the war. Percy and Will go inside and find the demigods on the top floors. Annabeth is on the terrace and Will starts to take care of her arm. He washes out the poison from the cut and uses what he can to heal her. He sends Connor and Travis Stoll to get more supplies from a drug store. Silena convinces Percy to let her go back to Camp Half-Blood to try to get Charisse to join them. Mrs. O’Leary has brought Grover and he tells Percy their losses were devastating guarding the north end. Thalia has also returned to report the Titans are surrounding the city, but waiting until sundown to continue the fight. Jake Mason points out the Titan army changed their attack plan after Percy gave out orders, indicating the traitor is among them. Percy tells them they can’t worry about that or it will tear them all apart. He goes to get some sleep before the next attack. Percy dreams about Nico trying to summon his dead mother. His sister, Bianca shows up to tell him he can’t see her. Nico sends his sister away and sees a smokey image of Hades begging Nico’s mother to hide the children from Zeus. She refuses to take the children to the underworld or be away from them. She starts to walk away and tells Hades to watch the children for a moment. Zeus makes the hotel explode and Hades only has time to save the children. He tells Alecto to put them in the Lotus Hotel. The Oracle of Delphi arrives and Hades blames her for the death of the woman he loves. He isn’t allowed to kill her, so he curses her. He makes it so the Oracle can’t move out of the body until his children aren’t outcasted. The dream changes and Percy sees Rachel. She rushes to her dad to tell him she has to get back to New York because Percy is in Danger. He refuses to leave his vacation until Rachel offers to go the girl’s academy he has been trying to get her to agree to. Thalia wakes Percy up to tell him a Titan is there with a message from Kronos.
Prometheus (god of forethought, crafty council) comes with an empousa, Hyperborean, and Ethan Nakamura. He tells Percy he is there to give them a chance to side with Kronos. Percy is reminded of all the times the gods have been just as bad as the Titans. Prometheus shows Luke begging Hermes for help, but Hermes denying him any because Luke has to find his own path. Grover warns Percy that Prometheus is trying to make him mad so that he will join them. Prometheus leaves Percy a jar (Pandora’s box) that only contains Elpis (Hope). He tells Percy to release Hope if he surrenders and Kronos will spare the surviving demigods. Percy offers to let the Titans return to Tartarus or else he will kill them. The parlay party leaves.
Percy tells Thalia what Prometheus showed him in the vision. She remembers that day and tells him Luke became reckless after it. She warns him that Annabeth has a soft spot for Luke and not to make the same mistake because of her. Percy tries to get more sleep, but has dreams instead. He sees his father’s palace burning with greek fire and Tyson charging off to fight. He sees Kronos asking Ethan where he was aiming to stab Percy when Annabeth got in the way. Kronos has a moment where he doesn’t seem in control. Ethan speculates that Luke is still in his body trying to fight the Titan off. Lastly, he sees Hermes fail to convince May Castellan not to try to take the Oracle herself. Percy knows it will not take, but can’t do anything about it. None of them knew about the curse but on the Oracle. Percy wakes up to find Annabeth standing over him. He tells her about his dreams, but leaves out Luke’s possible struggle with Kronos and May’s tragic events. He talks to Thalia and all the cabin leaders to strategize. He, the Athena cabin, Thalia and her hunters, and Grover’s nature spirits will hold off the part of the Titan army coming from Central Park. The rest of the campers each pick a bridge or tunnel to defend against the rest of Kronos’ army. Percy ends up facing off against Hyperion with his own personal hurricane. He gets the Titan of the East (light) over to Grover and the other satyrs who encase him in a maple tree. Just then, the Clazmonian Sow (giant winged pig) soars in and starts tearing about the park. Percy lassos a wing with a grappling hook and is pulled into the air. He ties the rope around a statue of Hermes, but the statue is ripped out. Blackjack comes and Percy rides him instead. He activates the Hermes statue and tells it to kill the flying pig. He does the same with two stone lion statues at the library. They take the pig out easily. Percy tells them to defend Manhattan, but thinks they go off to find more flying pigs. Blackjack takes Percy back to the battle against the army.
Percy goes to Annabeth and immediately starts fighting. They are able to hold back the Titan army, but just enough to keep them from getting to Olympus. Percy knows they are surrounded. Chiron arrives with the centaurs he has gathered. They are able to drive Kronos’ and his army away temporarily. The sun comes up and they have a while to rest and recoup. Percy tells Annabeth about his visions and how he thinks Luke is still inside his body. Mr. D pulls Percy to a far away bar where he has been invoked at a party to tell Percy that Kronos will soon burn away Luke’s body and be much more powerful in his full form. When he finds himself instantly back in NYC like no time had passed for the conversation. He sees a Prius down the street and realize it is Paul’s. He runs down the street and finds his parents in the front seat and Pandora’s jar in the back. Chiron explains the jar will follow him and tempt him when he is at his weakest. they move the car to a side street. They hear a helicopter in the sky and see “Dare Enterprises” logo on the side. Suddenly, the sleep spell hits the pilot and it starts to fall. Annabeth calls to a pegasus, Guido, to go save Rachel.
Guido takes Annabeth and Percy up to the helicopter. Rachel is inside and trying to keep the machine under control while the pilot is asleep. Annabeth jumps off Guido into the chopper, but Guido hits his wing and Percy has to talk him through landing. Suddenly, the helicopter is under control and lands. Annabeth comes out with Rachel safe. Rachel tells Percy and Chiron about her visions, including that Percy isn’t the hero. Percy goes to sleep and dreams about Nico begging his father, Hades, to help. The dream changes and Percy sees Kronos order the release of a dragon immediately while the campers and their friends are unprepared. Prometheus warns Kronos not to be too hasty, but Kronos insists. The Lydian dragon wake up Rachel says it will be killed by a child of Ares. Percy, Annabeth and Mrs. O’Leary start fighting the drakon. Campers and centaurs are pushed back to the Empire State Building. Clarisse arrives with the Ares Cabin, but they soon find out it is really just Selina in Clarisse’s armor when she is covered in poison. The real Clarisse arrives with Chris Rodriguez. She sees Silena dying and quickly kills the drakon.
Silena admits she was the spy and shows the necklace with Kronos’ scythe. Luke convinced her it would save more people. She dies and everyone agrees she was a hero. Percy sends Mrs. O’Leary on an errand for reinforcements. Leneus the satyr dies and becomes a laurel plant. Grover takes the plant to Olympus where Percy and Annabeth are headed. They cheer up the wounded in the make shift hospital then head for the palace. Rachel has Pandora’s jar. She tells Percy their destinies are not intertwined. She gives him the jar. Percy goes over to the hearth and gives the jar to Hestia because hope is strongest by the hearth. Percy gets on his dad’s throne to get his attention. Percy tells his father his plan and Poseidon eventually agrees to come to Olympus. Thalia arrives to tell Percy the Titans are advancing and Kronos is leading them.
On the street, Chiron faces Kronos. the centaur ends up being flung against a wall and buried in rubble. Annabeth attacks, but just hurts herself. Mrs. O’Leary arrives with Nico, who summons zombies. Hades arrives with Demeter and Persephone. Kronos breaks the spell over NYC so the heroes will be distracted with keeping the mortals safe. Percy’s mom (Sally) and stepdad (Paul) wake up and join the fight when they see Percy. Paul uses a sword surprisingly well from his Shakespeare acting in college. They tell Percy to go after Kronos who is heading to Olympus. Nico promises to watch over them. Percy tells Mrs. O’Leary to find Chiron. He and Annabeth go after Kronos with Thalia and Grover.
When they get to the 600th floor and exit the elevator, the ground is crumbling beneath them. Annabeth falls and Percy jumps to save her without thinking. Grover and Thalia pull him and Annabeth back up. They continue on and a statue of Hera starts to fall. Thalia pushes Percy and Annabeth out of the way, and her legs are pinned underneath it. She tells the others to go on without her. When they get to the palace, Kronos and Ethan are in the throne room. Percy sees them watching Typhon being taken down in the Hudson River by Poseidon and his army, including cyclopses lead by Tyson. Percy challenges Kronos to fight with his sword. The Titan’s scythe becomes Luke’s sword, Backbiter. Annabeth reminds him that the cursed blade is supposed to reap the hero’s soul. Percy fights Kronos while Annabeth fights Ethan. Percy convinces Ethan his mom, Nemesis, would want balance, but Kronos can only destroy. Ethan attacks Kronos, who drops him into the sky. Annabeth fights Kronos with her dagger and brings Luke out. He asks Percy to give him the dagger so that he can kill Kronos. He says if either of them try, Kronos will be brought back. Percy hesitates, but ends up giving him the dagger. They say their goodbyes and Luke asks Percy to make sure the god’s don’t forget about their children. He stabs himself in his weak spot and dies. The gods return and Percy asks for a shroud for a son of Hermes.
Apollo heals Annabeth and she sleeps. Tyson arrives with the other cyclops and frees Thalia on their way to the palace. Mr. D is grateful to Percy because his sentence from Zeus is cut in half. Percy asks for the Empire State Building to be lit up blue so his family will know he is alright. The gods’ council convenes and includes Hades. Zeus and Artemis thank Thalia. Mr. D appoints Grover to the Council of Cloven Elders and a lord of the Wild. Annabeth is told she will design the new Olympus. Percy is offered the chance to become a minor god, but asks for a different gift. He asks that all demigods are claimed by age 13 and properly trained and cabins are added so no one is forced in the Hermes cabin. The gods agree to the request.
Percy stops on his way out of Olympus to talk to Hermes, who is watching the news. He tells the god his son, Luke, loved him. He tells him to honor Luke by acknowledging the rest of his children. He tells Percy where several of his other children are. Hermes points out gods break their oaths all the time. Percy thinks they will change. On his way out, Athena stops Percy to give an angry apology knowing he stayed human for Annabeth. Percy accepts and then goes down to the lobby of the Empire State Building. He is greeted by his parents and Annabeth. Nico runs in to tell them Rachel is being taken to camp by Blackjack.
Percy worries what will happen to Rachel if she ties to get into camp since she is not a half-blood. Percy calls for help and a few hippocampi (including Rainbow) come to take them to camp. When they arrive, Rachel is already approaching the mummy oracle. Apollo comes to take her oath. Rachel accepts the dangers and gives her oaths. She becomes the new Oracle of Delphi. She gives a great prophecy about seven half-bloods and the world falling to storm or fire. Apollo goes to make arrangements with Rachel for her to stay in the Big House. Demigods are patched up or mourned when everyone returns. Percy tells Annabeth she kept him tied to the mortal world when he was in the River Styx and when he was offered to become a god. She kisses him. Other campers come along and throw them in the lake. Percy keeps himself and Annabeth in a bubble under the water.
More demigods arrive after being claimed by their parents. The campers start building cabins for the gods that are not already represented. Percy finds Poseidon on the beach. His dad thanks Percy for everything he has done. Percy gets his fourth camp bead that has the Empire State Building and the names of all the fallen campers. Everyone leaves for the mortal world. Rachel heads to New Hampshire. Annabeth will be going to a boarding school in NYC so she can oversee the construction of Olympus. They discuss the great prophecy Rachel gave before racing down Half Blood Hill.