Percy, Thalia, and Annabeth are driven by Ms. Jackson to a military boarding school, Westover Hall, in Main just before winter break. The three heroes make their way inside, but are quickly stopped by two teachers. Thalia uses “mist” to make them think they are students, and Grover comes to take them to the gym to join the other students for the dance. After greetings, Grover tells them he has found two halfblooded - a brother and sister. He points them out when they get the guys, but Dr. Thorn, the vice principal, has followed them, not convinced by the mist. Grover tells them he is the monster that is after them, but luckily the kids don’t know who they are yet. The heroes split into twos and dance while watching the siblings, but not bringing attention to them. Annabeth tells Percy she loves her new school in New York, but her dad wants to move to SanFrancisco. She suddenly realizes the siblings are missing. ANnabeth goes after Thalia, but Percy sees Dr. Thorn and decides to go after him alone. He finds the siblings in the hall and tries to tell them they will be safe with him. Dr. Thorn shoots a poisonous thorn that grazes Percy’s shouldering, pinning him to the wall. He tries to fight back, but is not effective. He fells the pain of the poison as Dr. Thorn says he is taking all three of them.
Percy is lead outside with Bianca (10) and Nico (8). Dr. Thorn calls for their transportation while Percy tries to use his link to Grover to call for help. Percy can feel the sea over the edge of the cliff they are on. Dr. Thorn says he is taking them to the General. The Great Stirring of monsters is underway and they are needed for the army to take down Olympus. He calls for their ride. Annabeth knocks the three halfblooded over using her invisibility cap, which gives Thalia the chance to attack Thorn with her spear and shield. Dr. Thorn reveals he is a manticore. A military style helicopter arrives with some heavy artillery. Percy goes to help Thalia, but they get pinned down. A horn sounds and silver arrows com out of the woods and fly at the manticore. He complains that divine intervention isn’t allowed, but Artemis appears and says she is allowed to hunt him. She grants permission for her girls to kill him. Thorn goes after Thalia and Percy. Annabeth jumps on his back and stabs him with a dagger. The monster leaps off the cliff. Percy tries to go after Annabeth, but the helicopter starts shooting at them. Artemis turns it into a bird with a wave of her hand. Percy wants to go after Annabeth, but Artemis says she is beyond saving. She introduces herself.
Percy wants to go after Annabeth, but Artemis points out that she has been taken away by some sort of magic. They try to explain to Bianca and Nico they are demigods and need to be trained to fight monsters. Artemis takes Bianca aside to talk to her. Thalia blames Percy for trying to defeat Thorn on his own. The Hunters set up their camp and Percy sits by the fire feeling guilty. Nico asks him questions until Zoe comes to get him for Artemis. She asks about the Manticore. She is worried when Percy explains Thorn said the General will explain things to him and there is a monster that will bring the downfall of Olympus. Artemis plans to hunt the monster herself. She wants Percy to take Nico and the Hunters back to camp while she is gone. Zoe isn’t happy about not going with her, but Artemis insists. Bianca gives her oath to Artemis and becomes a Hunter. Artemis calls her brother, Apollo, to give them a ride back to camp.
They wait for the sun to start coming up and Apollo arrives in his sun car in sport car mode. He teases Artemis and flirts with Zoe until Artemis reminds him to stay away from her Hunters. She takes off into the woods. Apollo turns the car into a bus to transport them to camp. Once everyone is on the bus, Apollo tries to explain the car is made up of what people think about the sun. He insists Thalia will be sixteen (aged slower when she was a tree) and should drive. She tries to decline the offer, but Apollo won’t let her. She has trouble keeping a good height and Apollo and Percy try to give her advice. She continues to say she is in control until they land in the canoe lake at Camp Half Blood.
The hunters immediately start heading for their cabin and Grover trips over himself to show them the way. Percy and Thalia take Nico to the Big House to introduce him to Chiron, who is playing a game of cards with Mr. D. Grover arrives to show Nico the welcome video. After Thalia and Percy explain what happened, Mr. D. refuses to let them go on a mission to save Annabeth. Percy’s temper slips out, but Nico comes back and distracts Mr. D by telling him about his game. Chiron announces there will be capture the flag, Hunters versus campers. As they make their way back to their cabins, Thalia tells Percy she found out her mom died in a late night drinking and driving accident. She goes to split up an Ares camper and a Hunter arguing. Percy goes to tell the other campers about capture the flag. When he gets to the Ares cabin, he asks about Clarisse. The boy there says she has ben on a top secret mission from Chiron for about a month. Percy goes to his own cabin to find a new fountain installed (a gift from his dad). He makes an Iris call to Tyson. His cyclops half-brother is working in the forges making swords to arm the mermaids. The titans have been gaining power and they are helping hide Luke’s ship, which is heading towards the Panama Canal. Percy’s feelings of loneliness continue when he has to sit alone through dinner at the Poseidon table. That night, he dreams about Luke asking Annabeth for help. She hesitates, but as the ceiling of the cave starts to crack and fall, she jumps in to help him bear the load. Luke leaves her there as bait. Percy wakes up.
Percy tells Grover about his dream and is surprised to learn Zoe also had a nightmare. Grover followed her to the Big House where she asked Chiron for permission to leave camp because Artemis is lost. Percy wonders if Kronos has reassembled himself enough to have kidnapped a god. Grover shows Percy a brochure aimed at girls who are thinking of joining Artemis’ Hunters. He found it in Annabeth’s things. While everyone is off performing their duties, Percy necks into the Big House attic to talk to the oracle. When nothing happens, he turns to leave. He finds Aphrodite’s scarf he and Annabeth retrieved two years ago. That night after dinner, capture the flag starts. Even though Thalia and Percy agreed to be co-captains, she insists on taking lead. She has Percy run defense and splits up her group into distraction and retrieve teams. Percy climbs up onto the rock that holds the flag and sees both of Thalia’s teams have encountered Hunters. Percy figures the middle ground is clear and takes off running. He is able to pass everyone, grab the flag, and run back towards his own territory, but Zoe beats him over the creek that serves as the boundary line. Thalia and Percy start using their powers against each other in a fight until Percy sees the Oracle of Delphi approach and acknowledge Zoe. When she asks how to save Artemis, the oracle tells her a combination of five Hunters and campers will travel west to free Artemis, who is chained at the top of a mountain. She says one will be lost in the desert and another will be killed by a parent.
While Percy and Grover are returning the Oracle to the attic, Grover admits he wants to go on the quest. The Great Stirring might turn up Pan. All the cabin leaders meet with Chiron, Mr. D, and Zoe, who brings Bianca with her. Once they get Zoe to agree to have campers on the quest, Grover and Thalia volunteer. Percy realizes he is being left out and protests, but Zoe refuses to have a boy along with them. She only agrees to Grover because he is a satyr. Chiron and Grover visit Percy in his cabin later. Chiron admits Thalia wasn’t his first choice, but neither was Percy, as both are very rash. He points out their fight after capture the flag and that it is best they aren’t both going. Percy calls his mom with an Iris message and sees her with a guy. When the strange man goes to the bathroom, Percy gets his mom’s attention. She explains the man is in her writing class. Percy tells her everything that is going on. She tells him to follow his heart and she understands if he decides to go after Annabeth. That night, Percy has another dream about Annabeth. Artemis takes the weight of the boulder from her, but Luke has to convince the General not to kill her. Blackjack (Pegasus from Luke’s ship) comes to tell Percy the Hippocampi need his help. Percy grabs Riptide and Annabeth’s hat.
They follow the van downtown and land on the Chrysler Building. Mr. D starts wrapping them up in vines. Percy asks why he hates heroes. Mr. D explains his wife, Ariadne, was abandoned by Theseus after giving up everything for him. He says Zoe also has a reason to hate heroes. Mr. D Decides to let Percy continue following them in hopes he will be the one to die. Percy and Blackjack follow the van to DC. When they land, Percy sends the exhausted pegasus home. He watches the questers go into the Smithsonian and then sees Dr. Thorn come out of a car that was following them. Percy dons the invisibility cap and follows him into the Museum of Natural History. Dr. Thorn informs the General the heroes are falling into their trap. Luke is there with them. They plant teeth in the ground and pour blood over them. Kittens rise the first time, but the second attempt reaps T-rex/human hybrid soldiers. The General plans to give them the Hunter’s scent to kill them all, but Percy snatches the scarf away. One gets a piece of his shirt and passes the scent to the others. Luke is positive it is Percy causing the issues. Percy is just able to sneak out the doors before they slam shut.
Percy runs across the Mall to the Air and Space Museum. He finds the others and tells them what he saw and that a monster is coming for them. As Thalia and Zoe argue over whether or not Percy should escape with them, the Nemean Lion appears. It’s fur is too tough for any of them to pierce, but Percy realizes it’s mouth is vulnerable. He has Thalia keep it distracted while he goes to the gift shop for space food. He gets the lion’s attention and throws packets of the food into it’s mouth. IT chokes and gags on it giving Zoe and Bianca a chance to shoot it in the mouth and kill it. The monster starts melting away and leaves just it’s fur behind. Zoe insists it is Percy’s spoil of war. He picks it up and it becomes a duster jacket. Grover’s confusion spell on the guards is about to war off. Zoe rushes them all out admitting Percy is now part of their quest.
They aren’t in the van very long before the General’s mercenaries are following them in a helicopter. Bianca tells Zoe to pulling into a parking lot. They find the underground train station to loose them. Bianca says she saw the station when she and Nico passed through DC previously, but the subway wasn’t there when she lived in DC when she was younger. The others wonder how that is possible. At the end of the line, they find a homeless man with a fire. He points out a train hulling cars west and they sneak onto it. Percy sits in a car with Thalia and she tells him Zoe asked her to join the Hunt. She couldn’t leave Luke behind and Zoe said he would’ only let her down. Percy points out she may have to kill Luke. Thalia asks Percy to leave her car and tells him Annabeth was also thinking about joining the Hunt. Apollo appears to Percy. He has helped because of his sister, but doesn’t have much more information, except to seek out Nereus. Percy falls asleep and dreams about running from a monster called Ladon with a girl that turns out to be Zoe. She gives him her hairpin and it turns into Riptide, Percy’s sword. Grover wakes up Percy when the train stops and they get off in the mountains.
The group finds themselves in Cloudcraft, New Mexico, which doesn’t have good transportation schedules. Percy talks to Bianca while the others are off. She is grateful there are people like Percy at Camp Half Blood so she can trust leaving her brother there. They were told their parents are dead and lawyers moved them around until they ended up at Westover. Percy asks about her conversation with Zoe about the General. Bianca realizes he used Annabeth’s hat to hear them. He tries to explain but Zoe and Grover return with food. Thalia comes running up from checking out the stores. Grover starts to collapse as fake and printed animals come to life. They start to leave town, but the skeletal soldiers appear. They fight, but only Bianca is able to kill one with a dagger to the chest. The Erymanthian Boar appears and knocks the soldiers apart. Percy and Thalia lead it out of town and over a cliff. They sled down the mountain on Thalia’s shield. Percy realizes Thalia is afraid of heights. The others join them. Grover points out the giant boar is their ride from the Wild to head west. Zoe explains she felt Pan’s presence when the boar arrived. They all get on it’s back before it frees itself.
The boar stops in Arizona to eat and they sneak away from it while it is distracted. Grover tosses out some acorns an points in the direction of their next challenge. They see a junkyard and decide to camp until morning and search it in daylight. Zoe suggests they head to Las Vegas, but Bianca doesn’t want to because she was there before. Percy asks if she stayed at the Lotus Casino. Bianca confirms and Percy explains the trap. Bianca doesn’t want to believe show s stuck in the casino for seventy years. Percy asks who the lawyer was that got them out, but a white limo pulls up. Ares steps out and says the person inside only wants to talk to Percy. He agrees and gets in to find Aphrodite. He is stunned by her beauty at first, but when Ares laughs at him, his anger clears his mind. She says she has been helping him because his quest is to find Annabeth rather than Artemis. She warns him not to touch anything in her husband’s domain before she leaves. They are left in front of the junk yard and make their way through. At the end, they come to a set of giant metal toes and decide to go around. When they try to leave, they find out the toes are connected to Talos. They try to get away, but it’s size prevents their escape. Percy sees Bianca looking guilty and asks what she took. Eventually, she admits she stole a figurine for Nico from his game. She tries to drop it, but Talos still comes after them. Percy sees a hatch on the bottom of Talos’ foot. Bianca goes for it when Percy gets him to lift his foot. Talks starts hitting himself and crumbles to pieces. They search for Bianca until morning, but Grover points out the prophecy said they would loose one in the land without rain.
They find an operational tow truck and hit the road. When they come to a dead end, they get out of the truck and it breaks down. Grover wants to go down a cliff on a goat path to get to the river, but Percy knows Thalia will have trouble, so he insists they go up stream. They find a closed canoe rental shop. Percy leaves a few gold drachmas and takes two canoes. Thalia asks him to ride with Zoe and he agrees. Once in the boats, Percy asks for a couple of Naiads to help them upstream. Zoe gives Percy a piece of her story. She has been leading the Hunt for two thousand years. She lived in a garden with her four sister that was guarded by the dragon, Ladon. She was exiled for helping a hero by telling him how to trick her father and get the golden fruit. They reach the Hoover Dam and get nostalgic about Annabeth’s love for architecture. They find a path that takes them up and they get to the top of the dam. They are on their way to the snack bar when Percy hears a cow. He sends the others ahead and looks over the side to see Bessie. He says he can’t go with her and she disappears into the water. Percy looks up to see skeleton warriors coming after him. He tries to get to his friends, but ends up in an elevator going 700 feet down to the turbines. The tour guide shows him her grey eyes and tells him there is always a way. Percy gets to the other side as quickly as possible but hears something behind him. He slashes his sword through a girl, but it doesn’t hurt her. Rachel Elizabeth Dare starts asking a bunch of questions. Percy says he is in trouble and in a hurry, and she hides him in the bathroom while she sends the skeletons in the wrong direction. Percy leaves her behind saying it is best if she forgets it all. Percy makes it to his friends in the cafeteria, but they are surrounded by skeletons. Grover starts a food fight to get them out, but they don’t make it very far before they are trapped again. Percy sees the angel statues (people run the toes for luck) dedicated to Zeus and tells Thalia to pray to her father. After several protests, she does and the statues come to life. They defend them and then take off into the sky with them.
The angel statues are happy to take them to San Francisco and start planning to meet up with other statues they have partied with previously. Once they are dropped off, Zoe takes them to a clothes donation bin, and they dress Percy like a homeless person. She says he will know Nereus because he will smell different than the other hobos. Percy makes his way down the pier and finds an old man that smells like rotting fish and seaweed. Percy grabs onto him and Nereus jumps into the water. He changes into several different sea creatures trying to shake the hero, but he eventually gives up and agrees to answer one question. Percy wants to ask about Annabeth but instead asks about monster. Nereus points to Bessie and then disappears. The others are shocked to see the Ophiotaurus. They tell him if one were to throw the bull serpent’s entrails into a fire, that person would be granted the power to overthrow Olympus. Suddenly, Dr. Thorn arrives with his mercenaries. He wants Thalia to kill Bessie and join the Titan’s. Percy tells the Ophiotaurus to get deep into the water and then drags Thalia up the pier to a souvenir stand. HE uses his sword to break apart a water fountain. IT sprays Thalia and gets her out of her daze. Percy uses the water to send an iris message to camp. Mr. D answers because Chiron isn’t there. Dr. Thorn catches up with them and they are surrounded. Percy swallows his pride and asks Mr. D for help. The mercenaries go crazy and Thorn is swallowed by grape leaves. Mr. D tells them Zee knows where to go and they have to make it by sundown.
They start heading to the garden of Zoe’s sisters (the Naiads of sunset), but they are moving too slow. Grover agrees to take Bessie back to camp. Percy offers the Lion coat to his father for their safe passage across the water. He realizes Hercules was the hero that hurt Zoe and he doesn’t want to be like him anymore. Thalia remembers Annabeth’s dad lives in San Francisco and pulls his address out of her bag. They find him working on and distracted with writing a paper about a battle. They tell him about Annabeth and ask to borrow his car. They start heading for Mount Tamalpais, the Mountain of Despair, and see Luke’s ship in the water nearby. Thalia yells for Zoe to stop the car. She slams on the breaks and they slide to a stop. Thalia gets Percy out just before a bolt of lightning destroys the car. Percy tells her he thinks Kronos did it to get her to turn against her father, Zeus. They go into the garden at sunset and meet Zoe’s sisters. They don’t acknowledge her as family and threaten to wake the dragon. Zoe does it herself and distracts Ladon to let Percy and Thalia get past. Zoe joins them knowing the dragon won’t stray too far from the tree it guards. They go up the mountain to find Artemis holding the sky up. Luke has Annabeth. The General, Atlas, is Zoe’s father.
Luke tries to convince Thalia to join them. Percy has to keep himself from thinking about Bessie to keep him from appearing in a nearby pool of water Luke points out. He notices Annabeth’s hair is streaked with gray. Thalia starts fighting Luke; Percy attacks Atlas. Percy quickly realizes he doesn’t have a chance and takes the weight of the sky from Artemis so she can fight him. The goddess fights the Titan and is eventually able to throw Atlas at Percy. Percy gets out of the way and lets the sky fall onto Atlas’ back. Atlas is suck again. Annabeth tries to convince Thalia not to kill Luke, but she pushes him and he falls off the cliff, landing among his army that is storming up the mountain. Artemis is holding Zoe, who is dying from a poisonous scrape from the dragon. Mr. Chase, Annabeth’s dad, comes along in his antique fighter plane and shoots Luke’s army with bullets made of celestial bronze. Artemis gathers them all into a sleigh pulled by deer and they take off into the sky with Mr. Chase following behind.
They land and Percy suggests Artemis try to heal Zoe, but the Hunter is ready to die. Artemis makes her into a constellation and then takes off for Olympus. Annabeth says goodbye to her dad as three Pegasi appear. Blackjack greets Percy and they take off. Thalia sleeps. Annabeth says she knows Luke isn’t dead (like Percy knew she wasn’t dead). He is mad she has any feelings for Luke when he should have died instead of Bianca or Zoe. Thalia wakes up and points out Olympus.
The heroes walk into Olympus together and find Grover already there with Bessie. The gods start to argue over whether or not they can let Thalia and Percy live. Artemis quickly talks to Zeus and then offers Thalia to join the hunt as her new lieutenant. Thalia accepts and now will never turn sixteen. Percy is voted to be allowed to live and Poseidon vouches for him. He convinces them to keep Bessie safe in a tank on Olympus rather than kill him. They celebrate with a party. Poseidon tells Percy he better not be disappointed in him. Percy promises not to let him down. Athena tells Percy she thinks it is unwise they let him live. She doesn’t like that he and Annabeth are friends. She worries his fatal flow (personal loyalty to his friends) will be a danger to them all. Percy asks Annabeth to dance; she accepts.
Percy calls his mom and brother to let them know what happened. HE tells his mom to be happy with Paul. Tyson plans to spend the summer at camp. Clarisse has returned from her quest with bad news. Nico comes to ask about his sister. Percy takes him for a walk and explains everything. Nico hates him. Four skeleton-zombies attack, and Nico opens a crevice in the ground to swallow them. Percy realizes Nico’s father is Hades (fathered them before the pact, but the Lotus Casino kept Bianca and Nico from aging). Grover and Annabeth agree not to tell Chiron because Percy doesn’t want Nico in trouble with the gods. While Annabeth and Percy are saying their goodbyes until summer, Grover comes to tell them Pan told him he is waiting for him.