The chapter starts with Percy having a nightmare that his friend Grover was being chased by a monster, although he can’t see what the monster is. He wakes up and tells his mom about his dream and she tells him that it is probably not a big deal. She also tells him that he can’t go to camp because they are having problems but wont tell him what those problems are and says that she will tell him after school. He goes to meet up with his friend Tyson and sees a shadow with no person, someone is watching him.
He goes to school with Tyson and we learn that Tyson’s only friend is Percy and vice versa, and that Tyson is homeless and he goes to that school because he is a community service project. Tyson’s bully, Matt Sloan, threatens Percy, Tyson is upset and Percy comforts him. Then in PE Matt Sloan's friends turn into monsters and start fighting Percy and trying to eat everyone fighting with the dodgeballs in the gym. He goes to get Riptide (his sword) to fight the monsters and Tyson protects him and Percy doesn't understand how because Tyson is a regular mortal. After that when Percy is vulnerable again and this time Tyson isn't able to help and he thinks that he is going to die but then Annabeth appears (she was invisible) and kills the last monster and Percy realizes that she was the person following him. Then the police get there and they have to leave because they are the suspects for the crime.
Percy Annabeth and Tyson get out of the gym and go into an alleyway where Annabeth tells them that the monsters were Laistrygonians. Then she tells Percy that she has been having dreams about bad things happening at camp, the same thing that his mom said. Next tell Tyson about the Greek gods being real but he doesn't seem surprised and Annabeth says that the reason why this is the first time that monsters have attacked him is because of Tyson. After that they leave the ally because they are the main suspects in a magical taxi with three old ladies driving the car, Anger, Tempest, and Wasp. Next he realizes that out of all of them they only have one eye and tooth to split between them and the ride is very unsafe with all of the ladies fighting and the car swerving a lot. In an attempt to get the eye back from Percy (he wouldn't give it back until they told him what the numbers meant) they told him the numbers 30,31,75,12 but he didn't know what they meant. Then they get to camp and see that it is under attack.
They see that the camp is being attacked by monsters but that shouldn't be possible because there was a magical tree that kept monsters from going in camp borders. So Percy and Annabeth go and help fight the Cochis bulls with another group of campers and Percy helps Clarisse who doesn't like that he is helping her. Then Percy is outnumbered by the bulls and is not winning until Annabeth lets Tyson into camp who is immune to their fire breath and fights the cows with help of Clarisse and wins. Next, Clarisse gets really mad a Percy because he ruined her battle plans. After that Percy is confused because Tyson didn’t die from the fire and Annabeth tells him that he is actually a Cyclops (a one eyed monster, but in this case he is a good monster) who are immune to fire he just couldn't see because of the mist. Things have been changing too, Chiron the activities director was gone and Argus the head security guard was fired then he saw that Thalis tree had been poisoned.
Percy walks around camp showing Tyson and sees that it is different because it is dangerous and everyone is focused on that. He runs into Chiron and finds out that he is being fired because he was being blamed for poisoning the tree because someone had to be blamed but Percy knows that Luke did it he just can’t prove it. Chiron leaves and he meets the new camp director, Tantalus who is cursed to never be able to eat at dinner and finds out that he is being blamed for the gym, so he is a criminal. Then Tantalus announced that they would be doing chariot races and was asking what cabin they should put Tyson in or if they should let him stay, then a symbol of a trident showed over Tyson's head, Poseidon claiming him as his son.
Percy was not happy that Tyson was his brother downplaying it to others when he got the chance. At first, Percy is working on a chariot with Annabeth then Annabeth calls Tyson a monster so he switches to working with Tyson. At night Percy has another dream this time Grover is communicating with him and is being held hostage by a cyclops, Polyphemus in a cave on the sea of monsters. The monster thinks that Grover is a female Cyclops because he can't see very well Grover has on a wedding dress that smells good. Then he woke up and it was the day of the chariot race and he tried to tell Annabeth but she didn’t believe him. The race starts and they are allowed to have two people the driver and the fighter Percy is the driver and they are doing well and even knock out a couple of group's carriages until a giant group of stymphalian birds startes pecking all of the campers and really hurting them. Until Annabeth remembered how Hercules defeated them using noise. So they got some of Chiron's loud music and used that to get rid of the birds. And when they thought that they had been the heroes Tantalus declared Clarisse the winner of the race and that Annabeth and Percy should be punished for ruining it
Annabeth, Tyson, and Percy all got in trouble because Tantalus said that they provoked the birds. During their punishment, Percy talks to Annabeth and now she believes him about his dreams and tells him about a thing called the Golden Fleece that would cure Thalia’s tree and it is in the same place that Grover is so they would be able to rescue him. He figures out that the numbers 30,31,75,12 are sailing coordinates. So they decide to ask Tantalus for a quest. So they ask him and get others to pressure him into saying yes, but then he gives the quest to Clarisse. Tyson is upset so Percy comforts him and then they go to bed. Percy can’t sleep so he goes to the beach and sees a weird guy there that he didn't know then the guy pulls out a phone there are snakes on it. They talk for a little while and Percy figures out that the guy is Hermes. Hermes gives Percy two gifts, a thermos that will speed him up on the water (it is wind), and very powerful vitamins. He gives Percy these gifts because he wants him to save Luke saying that he has not given up on him because they are family.
Annabeth and Tyson find him because they thought he was in danger, he tells them what happened and they decide to go on the quest even though it is against the rules. Hippocampi come to take them to a boat Princess Andromeda that Poseidon sent for them and Tyson bonds with his hippocampi who he named Rainbow. They get on the ship and Tyson says he smells monsters, he has that ability but they still continue and go to bed. When he is sleeping Percy has a dream that he is at the entrance of Tartarus and Kronos is taunting him. The dream changes and it is Grover in the cave running out of time, he only has a week left. They wake up and discover all the guests on the ship are in a trance and that there are monsters on the ship, then they hear Luke (who they didn't know was on the ship) talk about them taking the bait and that he is doing something bad so they decide to follow him and stay on the ship even though that is extremely dangerous.
They got their stuff then went to go see what Luke was doing. They sneak by lots of monsters and discover that a camper Chris Rodriguez joined Luke. They get to where Luke is and Tyson uses his Cyclops abilities to hear and tell them what is going on in the room Luke and another person are talking about the prophecy but they realize that they are being listened to and captured them. The room had a glowing golden casket in it and two of Luke’s assistants Agrius and Oreius (monsters). Luke talks to them asking Annabeth to join them and admitting that he poisoned Thalia's tree and that he has spies at camp but she refuses. Luke talks about the Prophecy more and says that it is about Percy if he gets to 16. Tyson gets mad after Luke calls Percy a fool and attacks Luke. To stop Agrius and Oreius from killing Tyson, Percy tells Luke about what his dad said and all that he did for them but Luke gets very mad saying that he feels like his dad abandoned him and that is why he wants to defeat Olympus. He says that Kronos is in the casket and everyone that joins them more pieces of him come out of Tartarus, and will soon get a physical body and asks Annabeth to join them but again she refuses. So Luke sends Annabeth and Percy to get eaten by an Aethiopian drakon but leaves Tyson behind with Oreius. While they were separated Tyson attacked Orius and they all escaped in a lifeboat.
They left using the wind in the thermos and called Chiron on an Iris message to tell him what was going on. An hour passed and they made it to Chesapeake Bay and discovered that Percy has perfect bearings because he is the son of Poseidon. After they stopped they made it to a hideout that Annabeth Luke and Thalia had made a long time ago and Percy felt jealous because she stayed there with Luke. When they got there they sent Tyson to go look for Donuts in the woods to get him to leave. They talked about what had happened but Tyson interrupted with doughnuts. They were in the woods so they went to look at the shop and Annabeth explained that some stores pop up fast because they are magically connected to monsters when she got interrupted by the Hydra. The Hydra smelled them and started fighting them. Percy cut off one of its heads but the head grew back doubled when it was cut off. Annabeth told him that they can only be defeated by fire and they were losing the fight then a Steam engine, the CSS Birmingham came and shot a fireball at the monster, killing it.
Clarisse was on the ship that her dad gave her with dead soldiers that do whatever she wants them to. She is keeping the three of them there because she wants it to be her quest even though the oracle says that that will not happen. Percy falls asleep and dreams that Polyphemus shows Grover the island and the golden fleece on it and tells Grover that he needs to be done with the wedding dress tomorrow. He woke up and saw Ares threatening Clarisse, telling her she needs to do the quest by herself. Percy goes back to Annabeth and Tyson where Clarisse tells them that they are going to go through the sea of monsters (the Bermuda Triangle) in between Scylla and Charybdis. They have started going into the whirlpool Charybdis makes and get information that the ship has a problem in the boiler room. Tyson goes down to fix it because he is immune to fire but it is still dangerous. They attacked Charybdis but it didn't do anything, and Tyson momentarily fixed the ship. The whirlpool stopped and she spit the water back out again, causing a lot more damage to the ship. A sailor tells them that they need to abandon ship but he is pulled off the ship by Scylla. Below the deck is on fire so everybody tries to make it to a lifeboat to escape Scylla and Percy goes to get Tyson. On his way, he gets picked up by Scylla but he swings his sword at her and she drops him while he is falling the ship explodes killing everyone except Clarisse and Annabeth who were on life boats then he falls into the water feeling terrible about Tyson's death.
Percy woke up on Annabeth's lifeboat without most of his stuff. He was very upset about Tyson's death and Annabeth said that she was wrong about Cyclopes. Percy asks Annabeth about the prophecy and she says that it is about a half-blood child of the big three that lives to sixteen that makes a decision that will either save Olympus or destroy it. They got to an island with a spa and went into a room with a lady named C.C.. Annabeth left to get a makeover and Percy stayed with C.C.. She shows Percy the better version of himself and he drank a potion that she gave him but it turned him into a guinea pig and put him in a cage with other guinea pigs because she hates men. Annabeth came back after her makeover and she figured out that C.C. is actually Circe (a sorceress) and that Percy is a guinea pig. Circe offers Annabeth a life where she can be whatever she wants and she asks to say goodbye to Percy first. Instead, when she left she took a vitamin and when Circe came back fought her. Circe's magic didn't work because of the vitamins, and Annabeth dumped the vitamins in the cage and they all turned back to human. The other guinea pigs turned to Blackbeard and his pirates and Percy and Annabeth left escaping in Blackbeard's ship.
On the ship Annabeth confesses that the reason she doesn't like Cyclopes is because her Luke Thalia and Grover were captured by one and it copied her dad's voice and Thalia would still be alive because it delayed them getting to camp. Percy fell asleep and dreamed that he was in the room with Kronos's coffin and a girl that he didn't know he couldn't move so the girl went to destroy the coffin and opened the lid but she got overtaken by golden light. Annabeth woke him up and told him that they were going to hear the Sirens soon and she wanted to hear them because they tell you things about yourself you don't even know. So he tied her to the ship and when she heard the song she started begging for him to let her go it was hard for him to see so he looked away. When he looked again she had cut herself free so he left the ship and swam after her. When he caught up to her he saw what she was seeing, a life with her parents together and with Luke in a Manhattan that she re-designed. He swam back with her underwater because she couldn't hear under there making an air bubble so she could breathe and she started sobbing. They got back on the ship and Annabeth apologizes, saying that she didn't think it would be that hard to resist and that her fatal flaw was hubris- thinking that you can do things better than everyone when she saw the Cyclopes island.
They got to the island and it was very pretty because of the fleece they see that there is a man-eating sheep guardian it and that there was another lifeboat from the ship. They hid their ship in a discreet spot and climbed up a wall to get onto the island. When they got up they realized there was a cave below them and Grover and Clarisse were in it Clarisse tied up over a pot of boiling water. Polyphemus still thinks that Grover is a girl and he wants to eat Clarisse as their wedding meal. While he is discussing this with Grover Clarisse says that Grover is a Satyr and now he wants to eat them both but Grover convinced him to wait saying that he has a recipe and now Polyphemus is marrying Clarisse. Polyphemus leaves to get ingredients and seals off the cave with a boulder they can't get past or communicate through. The sheep would be let into the cave so they decided Annabeth would go in invisible and Percy would under a sheep's belly. Once they were inside Annabeth pretended to be one of his enemies nobody, and tricked him into moving the boulder. While Annabeth distracted him Percy went to free Clarisse and Grover and there was an explosion.
He snuck out with Grover and Clarisse and saw Polyphemus captured Annabeth and she was hurt. He fought Polyphemus with Clarisse while Grover got Annabeth and they escaped out of the cave one they were out and got across the bridge Percy could have killed Polyphemus but he didn’t because he reminded Percy of Tyson so instead he asked if he could just take the wool and Polyphemus said yes but when given an opportunity he got back up to fight with Percy but as soon as he got up he fell into the chasm because Tyson had hit him on the head. Tyson told Percy that Rainbow saved him and brought him here and Percy was very thankful that he was alive and sent him to get the magic fleece to heal Annabeth. After it healed her she still had broken ribs so they carried her to the ship and on the way there Polyphemus back from the chasm.
Polyphemus started throwing rocks at them and Tyson wanted to fight him but Percy didn't want him to die after he went through all the pain of thinking that he was dead so they both fought. They were fighting and Percy realized Polyphemus was just going off of sound because he was mostly blind. Polyphemus was asking Tyson for help talking about how he was orphaned and lived on his own when Tyson tried to bargain with him Polyphemus hit him and Percy got mad and stabbed him when he swung the tree again Percy grabbed it and landed on Polyphemus's eye, damaging it even more. They ran into the water in an attempt to escape and Clarisse was yelling at him and mocking him making him even more mad so he threw a boulder at the ship making it sink. Percy couldn't save them all but he found out that he could communicate with Tyson underwater because they were both sons of Poseidon and they called Rainbow and his friends to save them while Polyphemus was celebrating, thinking that he drowned them and Percy fell asleep.
Annabeth woke Percy up in Miami because the hippocampi wouldn't take them any further and they went onto land. Once they were there they realized it had been 10 days because time works differently when they go to magical places and Thalia's tree would die if they didn't get there right away. They thought that they didn't have any money but Tyson said that he had money and pulled out around three hundred dollars from Rainbow's saddle bag and Percy realized that it was Clarisse's quest so she should get to finish it and sent her with the fleece to go back to camp. Then as they turned to find a way home Luke was there. Agrius and Oreius were with him and Luke wanted the Fleece thinking that they still had it, but Percy told them otherwise and Luke sent Agrius to go and try to get it. Then Luke captured them and Percy tried to find a way to use Luke's anger to prove that Chiron was innocent, by sending an iris message to camp and having him reveal that he poisoned the tree. After the iris message, Mr. D sent Tantalus back to the underworld and said that Chiron would be back. But after that, Luke sent in his guards to kill Percy.
Percy challenged Luke to a duel and he accepted, not wanting to look weak. They were fighting and Percy was losing, he had been stabbed in the leg and tried to use his water powers but it wasn't enough then, someone shot Oreius with an arrow and centaurs came out of the stairwell with Chiron. The Centaurs all looked unique and like they were going to a party not into battle but they surprised Luke and the people that worked for him they fought each other then Percy, Annabeth, Tyson, and Grover left on Centaurs and stopped at a trailer park to decide what they are going to do next, they are going to stay at camp half-blood because they are not powerful enough to fight them right now. Annabeth and Grover left to watch Tyson and Chiron talked to Percy about the prophecy and Percy realized the string that the fates cut was for the life that will be lost when he is sixteen. Chiron talks to Percy more saying that he believes that Percy can defeat Kronos and tells Percy that Kronos is his dad and that is why people blamed him for the tree being poisoned.
When they got at camp half-blood and Clarice put the fleece on the tree and everything got healthy again. Chiron announced that they would re-do the chariot race and Percy teamed up with Annabeth. Two days later, the night before the race at the stables Hermes was there, Percy told him what happened and Hermes wasn't mad because Percy still talked to Luke and that Hermes can’t do that because their kids have to be independent. Hermes also gave Percy a letter from his dad that said “Brace yourself” leaving Percy confused. In their cabin Tyson gave Percy a watch and said that if he was ever in trouble press the button. Percy didn't understand but accepted and started to apologize but Tyson said that he understood and that he was thankful because Poseidon because he sent him a friend when he needed it most. They started the race and they had a chariot that helped make. Annabeth started fighting with the other chariots and their wheel was damaged, but they got the Apollo chariot out. The Hephaestus chariot used ropes to grab onto their chariot and Annabeth switched spots with Percy. The Hephaestus chariot threw Greek fire in their chariot while Percy was fighting Beckendorf, the leader of their cabin so Percy pressed the button on the watch and it turned into a shield. It broke Bachendorf's sword and Percy threw the fire into the Hephaestus cabin and won the race. Percy thanked Tyson in front of the whole crowd and said that they were brothers and he was proud.
Percy calls his mom and she is mad at him for leaving but happy that he is alive and they decide that he will come home for the school year. Next, he talks to Tyson and he tells Percy that Poseidon gave him an internship at a Cyclops forge and that he is leaving right now. They said goodbye then he left and Percy ate dinner and went to bed. In his dreams Kronos is taunting him, then he woke up because Grover was banging on his door. Saying that something was wrong with Annabeth on the hill. He got up and rode up the hill on Chiron's back but when he got up there Annabeth was fine, and there was a girl lying on the ground unconscious he woke her up and realized why Kronos wanted him to to get the fleece, so that there were two people the Prophecy could be for. The first thing she said was that her name was Thalia and she was the daughter of Zeus.