Percy Jackson has had problems since he can remember. He has dyslexia and ADHD, only part of the reason he has been to six schools in as many years. His current boarding school, Yancy Academy, goes on a field trip and he has been warned that he will be expelled if anything bad happens. His best friend, Grover, keeps him as calm as possible. Mr. Brunner (Latin teacher in a wheel chair) and Mrs. Dodds (pre-algebra teacher who favors Nancy, the girl that picks on Percy and Grover constantly) are chaperoning them through the Met. Mr. Brunner quizzes Percy on the story of Kronos eating his children. Percy tells the story, but can’t figure out how it relates to real life. Mr. Brunner tells him he needs to learn that lesson. While eating lunch, Nancy comes to pick on Grover. Percy gets so angry that he blacks out and Nancy ends up in a fountain. Mrs. Dodd leads him away from the others. When they are alone, she transforms into a bat-like monster. Mr. Brunner comes along to toss him a pen that turns into a sword. Percy kills her and goes outside to find everyone acting normal, except no one knows who Mrs. Dodd is, including Mr. Brunner.
Percy spends the rest of the school year confused and angry that no one remembers Mrs. Dodds. His grades fall while his fighting increases, but he does still try in his Latin class. He decides to go to Mr. Brunner for help before the final and overhears Grover talking to him about Kindly Ones and the summer solstice deadline. Mr. Brunner wants to wait as long as possible to tell percy anything. When it is time to leave the school, Percy and Grover take the same bus to Manhattan. Percy asks Grover about the odd topics he eavesdropped on. Grover gives him his card in case Percy needs him over the summer. The bus breaks down, so Percy and Grover get out to wait. Percy sees a trio of old ladies knitting at a fruit stand. The middle one cuts the yarn and Grover rushes him back on the bus. He makes Percy promise to let him walk him home. Percy starts realizing the cutting of the yarn symbolizes death.
As soon as the bus arrives at the station in Manhattan, Percy ditches grover and takes a cab home. His stepfather, Gabe, is there playing poker with his friends and demands Percy’s money as soon as he walks in the door. Percy retreats to his room until his mom comes home to tell him the two of them are going to the beach for the weekend. When they finally escape gabe and make it to the cabin, Percy asks about his father. His mom tells him (as she always has) that he was lost at sea before Percy was born. She mentions a school that he wanted Percy to go to, but his mom refused. In the middle of the night, Percy wakes up from a dream about an eagle fighting a horse. Suddenly, Grover arrives, furious that Percy didn’t tell his mom what he saw. She tells them to get into the car and Percy notices that Grover’s fluffy bottom half ends in hooves.
With his mom driving towards the summer camp, Percy is partly filled in by Grover. All the myths are real. The car is knocked into a ditch. Percy gets them all out of the car and he and his mom drag Grover as far as possible. She tells him how to escape the approaching Minotaur’s rush. When he does, they are separated. She leads it away from the boys and it crushes her into light in its hand. Enraged, Percy goes back for the monster. He manages to rip one of its horns off and stabs it in the side. He grabs Grover and stumbles towards the camp where he is found.
Two days pass with Percy drifting in and out of consciousness. There is a girl that has been feeding and taking care of him. When he finally wakes up, Grover helps him drink something that tastes like his mom’s cookies. He takes Percy to Mr. Brunner (whose real name is Chiron) and Mr. D. They start talking about the Greek gods, and Chiron explains that the immortal beings moved with Western Civilization. Percy realizes that Mr. D is actually Dionysus. They warn him to be careful about using names because they hold power. Chiron stands up out of his wheel chair to reveal that his bottom half is a horse’s body. He takes percy to get him settled in a temporary cabin until he is claimed.
Chiron shows Percy around the camp. They arrive at twelve camper cabins (one for each Olympian and all completely different). Cabin eleven is for the children of Hermes, god of travelers and thieves, who welcome all campers whose Olympian parent is undetermined. Chiron leaves Percy with Annabeth and she introduces him to Luke, the counselor. After he is settled, Annabeth takes him to the volleyball court. She is disappointed because she thought he was “the one.” She explains they train to kill monsters like the Minotaur, but the monsters don’t really die. Eventually they reform. She goes on to tell him why he had so many issues in traditional schools; He is hardwired to read ancient Greek and be a warrior. Clarisse, daughter of Ares, comes along to tell Percy that there is initiation for new campers. She and her friends drag him into the girl’s bathroom and try to put his head in the toilet. He feels a tug in his stomach and the water explodes around him. Clarisse and her friends are shot out of the bathroom. They whole place is flooded, but Percy is completely dry. Annabeth says she wants him on her team for capture the flag.
After Annabeth finishes showing Percy around, he asks about her parents. She is a daughter or Athena and has been at the camp since she was seven, which is longer than most campers. Percy asks if he can leave and she explains that children of lesser gods usually do, but some of them stay year round because they attract too many monsters. The grounds are warded against them and mortals. She has a theory that when they went to Olympus , something was stolen and they have until the summer solstice to return it. Annabeth wants a quest to find whatever it is. Back in Cabin Eleven, Percy asks Luke about it. he tells percy that he ruined quests for everyone a few years ago when he failed his. Chiron visited the Oracle, who prophesied that Annabeth would have to wait for someone special and accompany them on their quest. They all file out for dinner and make burnt offerings to the gods. Afterwards, there is a campfire and then bed.
Through the week, Percy tries a series of different activities, but isn’t particularly skilled at any of them except canoeing. He takes lessons from Luke on how to use a sword, but none seem to work right for him. However, is is able to surprise everyone by disarming Luke. Grover tells him how the big three (Zeus, Poseidon, and Hades) vowed on the River Styx not to have any more demigod children after they started World War II. When Zeus had a daughter, Thalia, she attracted more monsters and made her way to camp with two other halfbloods and a satyr guide. They almost made it, but were attacked. Thalia sent everyone ahead and was able to distract the monster long enough for the others to get to safety. As she lay dying, Zeus took pity on her and turned her into a pine tree (which helps protect the camp). When capture the flag starts, Percy is with Athena’s team and told to guard the border. It isn’t long before Clarisse comes along with five of her cabin mates. They come after Percy with swords and Clarisse with and electric spear. They shove him into the creek and he feels stronger. He takes them all on and ends up breaking Clarisse’s spear. Luke rushes along and gets the flag to their team’s side. Annabeth notices Percy’s wounds healing while he is in the water. A hellhound, summoned by someone inside the camp, appears and attacks Percy. Chiron shoots it down, but Percy suffers from bloody scratches along his chest through his armor. Annabeth instructs people to get him into the water. Not only does he heal, but his father, Poseidon, claims him with his symbol above his head.
Percy is moved into the sea god’s cabin the next day. Grover comes to fetch him for Mr. D. When they arrive at the big house, Mr. D starts making threats before disappearing. Chiron explains that Zeus thinks Poseidon sent a halfblood to steal his master lightning bolt while the demigods were visiting on the winter solstice. Since Poseidon has claimed Percy, Zeus thinks that he has the bolt. Percy decides that the quest is better than being turned into a dolphin by Mr. D. He goes to see the Oracle. She tells him to go west to face the god that has turned and he will complete the quest. However he will be betrayed and loose what matters most. Percy tells Chiron the first half of the prophecy. Chiron points out that Hades would be the one to gain if Zeus and Poseidon went to war, so Percy will have to go to the underworld. Percy agrees to have Grover and Annabeth go with him, secretly hoping that he can save his mom while completing the quest.
There isn’t much for Percy to pack, so he is ready to leave quickly. Luke gives him a pair of winged shoes, but percy can’t go in Zeus’ domain (the sky), so he gives them to Grover. Chiron gives Percy the sword-pen, Riptide, explaining it is a gift from his father. The had of security, Argus, drives them to the bus station. They aren’t on very long before the three Furies arrive. They decide to attack in the Lincoln tunnel, asking where “it” is. Annabeth gives Percy her hat to make him invisible so they may not attack, but they start to whip Annabeth and Grover. Percy crashes the bus in New Jersey and takes off the cap. he uses Riptide to kill two of them. the third calls for reinforcements and the three heroes run into the woods to get away.
The three kids come across a small statue store and smell food. They go inside and meet Auntie Em, who is more than willing to feed them. She starts telling them about her sisters and comments on Annabeth’s grey eyes. Suddenly, his friends want to leave, but Percy wants to stay and agrees to have their picture taken by the old lady. Eventually Percy realizes that Annabeth has been hinting that the old woman is Medusa. Grover uses his flying shoes to hit her with a stick, but is literally flying blind to make sure he isn’t turned to stone. Annabeth disappears with her cap after giving Percy a reflective glass to find her. He is able to cut off her head. Annabeth explains that Athena cursed Medusa because she met with Poseidon in her temple. Percy mails the gorgon’s head to the gods, steals all the money out of the register, and they move on.
They find a spot to camp for the night, and Annabeth falls asleep instantly. Grover makes a comment about the environment which leads to why he wants to get his searcher’s license. He wants to find Pan, god of wilderness, who has been missing for two thousand years. Grover mentions that there is something about their quest, especially since the Furies were looking for an object rather than Percy. Percy admits he took the quest to find his mom. Grover offers to take first watch and Percy falls asleep. He dreams of an ancient voice telling him to betray the gods and give it the bolt in exchange for his mom. Annabeth wakes him up. Grover is talking to a pink poodle that will allow them to return him to his miserable home for the reward money. Annabeth points out the trains tracks nearby as their best way of traveling.
They have enough money to get them to Denver on the train. Annabeth warns Percy that he can’t make a deal with Hades for his mother. She tells him about how she was an inconvenience to her dad and step family, so she left. Athena guided her to help - two other halfbloods and a satyr. They have a layover in St. Louis and Annabeth insists on seeing the arch. They go up and when it is time to leave, the elevator is full before Percy can get in. He insists that he will be fine and that his friends go on ahead. A fat lady, Echidna, has her chihuahua that turns into Chimera (lion head, goat legs, and a snake head for a tail). Percy pulls out his sword and fights it, but it burns a hole in the arch and his sword is flung out. There is another family still up in the arch with him, so he decides that if he is gone, the monster won’t bother with them. He jumps out the hole and into the Mississippi river.
Percy hits the water and lands on the bottom of the river. He realizes that he is dry and can breathe. A woman that looks like his mom appears and tells him not to trust the gifts. When he gets to California, he needs to go to Santa Monica. The woman leaves in a hurry because of the pollution in the river. Percy finds his sword and goes to the surface. Authorities are everywhere. By the time he finds his friends, newscasters are already associating him with the disappearance of his mom, the bus explosion in New Jersey, and the destruction of the arch. They get back to the train as quickly as possible and it takes off for Denver.
Once in Denver, Annabeth finds them a carwash so that they can talk to Chiron through Iris messaging. Luke answers the call and Percy updates him on their adventure before the water runs out and the rainbow disappears. They find a diner and try to think of how to get food with no money. Ares arrives and pays for everything. He asks Percy to retrieve his shield that he left at a water park when he was there with his girlfriend. In return, he offers to get them out west and give information about Percy’s mom. He tries to refuse, but Annabeth warns him against it. Without much choice, they find the water park which has been abandoned. Annabeth gets them fresh clothes and whatever supplies she can find and then they head for the tunnel of love ride. Percy tells Grover to watch out while he and Annabeth go after the shield. She is a bit embarrassed at first. They make their way through the ride and to an empty pool. They slide down and Percy grabs the shield. Hephaestus’ trap is sprung: cameras appear and a net starts to form over the pool. Robotic spiders come after them and Annabeth as a daughter of Athena is terrified of spiders taking their revenge on her. Percy tells Grover to turn the ride on, but the satyr can’t seem to get it working. Percy summons the water himself and they use a boat to flat under the net and towards the rides exit. There is a gate ahead of them. Annabeth uses physics to launch them over the sealed gates and Grover uses his shoes to fly to them and slow down their fall. Hephaestus broadcasted the whole ordeal to Olympus hoping to catch Ares with his wife, Aphrodite.
They are able to return the shield and Ares points out an animal transport truck that will take them west. He gives Percy a bag of supplies and tells him his mom is being held hostage, not dead. Percy is not very appreciative and Ares altering his mood doesn’t help his manners. They get in the truck and it takes off. They try to make the three caged animals less miserable and settle down for the trip. Percy puts together that Annabeth and Luke where the other two demigods from Grover’s first mission to retrieve Thalia. Percy tries to reassure him that he will be the best searcher and will find Pan. Annabeth tells Percy she tried to go home once, but it didn’t work out and she returned to camp. As they continue on, he falls asleep and dreams about the evil voice in the chasm. He hears a familiar voice saying that no one will suspect his deception. The evil master realizes that Percy is there, brought by his own will. He shows the young hero his mother chained in the underworld. When the truck stops in Vegas, a man comes back to torment the animals. Percy can hear the zebra’s plea (as a horse, he is one of Poseidon’s creatures) from the smugglers. Annabeth distracts the men and Grover gives the animals a satyr’s sanctuary which will keep them safe until they find a safe place to live. Percy uses his sword to open the cages and they all run free. Percy, Annabeth, and Grover get lost in Las Vegas and are invited into the Lotus Casino. They are given everything for free, including unlimited gaming. Percy talks to a kid from the 1970’s and realizes they have to get out of there. It is a struggle, but they make it out. Percy finds a newspaper and sees they only have one day to complete their quest.
Annabeth uses the Lotus card to pay for a taxi ride to Santa Monica. On the way, Percy tells them what he remembers about his dream. Annabeth has some doubt that Hades has the bolt, but thinks the underworld is where the answer lies. They make it to the pier and Percy goes into the ocean. A shark takes him out further to talk to the woman he met in the Mississippi. She explains that his father cannot help him directly and gives him three pearls to use when needed. Back on land, he and his friends start looking for the address he found at Medusa’s shop, but DOA Recording Studios doesn’t seem to exist. He sees a newscast of his step-dad, Gabe, saying how Percy ruined his life to Barbara Walters. A troublesome group of the kids chase them into a waterbed store. Grover jumps on one at the salesman’s insistence. The man then pushes Annabeth down, snaps his fingers and says “ergo.” They are tied down. Percy tricks him into a demo and then does the same to him. He cuts off the monster’s head and frees his friends. In the back, he finds a flyer for DOA Recording Studios with directions.
They find the studio and approach Charon, trying to convince him they are dead. Charon sees through their lies and percy pulls out the drachmas Ares left them. Charon puts them on a boat and sends them across the River Styx. They see three lines with metal detectors guarded by Cerberus, the three headed dog. Annabeth pulls a red ball from her back and plays with the huge dog. It gives them time to sneak through the EZ Death line. However, they set of alarms and have to run and hide until things settle down.
Annabeth, Grover, and Percy try to blend into the dead mass of the Fields of Asphodel. They see Elysium and keep moving. Suddenly, Grover’s winged shoes take off on their own. Percy and Annabeth run to catch him as they head towards a chasm chanting with an old voice. One shoe falls off and they are able to catch him. The other comes off and kicks them in the head before taking off. The chasm tries to suck them in, but they are able to get away. They find Hades and percy asks for the lightning bolt. Hades is furious and thinks Percy has it as well as his Helm of Darkness. He shows that he has Percy’s mom. When Percy checks the bag Ares gave him, the bolt container is in it. Percy gives a pearl to each of his friends. He promises he will return the helm and free his mom. They each crush their pearls and are floated to the ocean.
Percy helps his friends get to the shore. Ares arrives and is disappointed that Percy didn’t die to complete his plan. The god of war explains that the bolt would not return to its sheath (transformed to look like the backpack) until they hit the underworld. He also has Hades’ stolen helm. Percy realizes that Ares stumbled onto this plan and it wasn’t his own. He challenges the god for the helm. They fight with swords but their is an audience gathering and they see guns due to the mist. Ares puts up a wall of fire to block the humans. Percy uses the sea to his advantage and is able to stab Ares in the heel. The god disappears and leaves the helm. Percy knows they will have to take a plane to get back to New York (and Olympus on the 600 floor of the Empire State Building) in time.
Suddenly, the news starts reporting that Percy and his friends were kidnapped. They are able to raise airfare for a ride home and Percy goes directly to Mount Olympus. He explains his story to Zeus, who grants him his life before leaving. Poseidon stays to talk and Percy tells him about Kronos in the pit talking to Ares in his dreams. Poseidon doesn’t think his father will rise, but is just stirring like he does every few hundred years. Zeus has already closed the issue. Poseidon tells Percy that he has a decision to make when he gets home. Percy goes to see his mom and Gabe is worse than every. In his room, Percy finds the box he sent to Olympus returned and on his bed. He offers to take care of Gabe for his mom, but she insists she wants to get rid of him on her own. Percy returns to camp.
When Percy returns to camp, everyone celebrates his success. In half-blood tradition, cabin mates had made burial shrouds for the questers. He was glad to burn his because Ares’ cabin had volunteered to make it and did not put much effort into it. Percy’s mom sends him a letter telling him that she sold her first sculpture and that he can come home for the school year. He doesn’t know what to do and spends the rest of the summer training and putting off the decision. On the last day, he goes to practice with his sword to clear his mind. Luke is already there. When the older hero finishes, he offers to take Percy into the woods for one last monster battle. When they get by the creek, Luke summons a scorpion. Percy realizes this is the friend that betrayed him and has been serving Kronos. Luke disappears and Percy battles the scorpion, but is still stung. A couple of nymphs help him to the Big House. When he wakes up, Annabeth is helping him. She tell him that she is going home. His decision is made and he goes to pack his things.