Percy has surprisingly made it to his last day of school without being expelled. However, he has a dream in the eve of the last day of school where he sees his friend satyr, Grover, stuck in a bridal boutique in Florida. The next day in school, when Percy tries to defend his friend Tyson from the bully Matt Sloan, the latter threatens him to "wait until P.E". Subsequently, Gym starts and Percy is stuck playing dodgeball against rather large seventh graders who turn out to be Laistrygonians (cannibal giants) who summon explosive fireballs, and attempt to kill Percy. Percy is protected by Tyson, who is somehow unharmed by the monsters' fireballs. Annabeth saves Percy by stabbing the last Laistrygonian from behind. She leads Percy and Tyson from the school, and gets a ride from the Gray Sisters (also known as the Fates) .
After their arrival at Camp Half-Blood Percy, Annabeth, and Tyson find the camp under attack. When danger passes, Percy finds out Thalia's tree, which protects the camp, has been poisoned by an unknown intruder, so consequently the camp is constantly being attacked by monsters. Chiron, the centaur assistant director, has been fired because he is suspected of poisoning the tree, and is replaced by Tantalus, a spirit from the Fields of Punishment,which is in the Underworld.Tantalus is not allowed to eat or drink for all of eternity. During dinner, it Percy finds out that Tyson is a cyclopes, and Poseidon claims Tyson , therefore becoming Percy's half-brother and upsetting Percy, because this only damages his reputation even more. Later, Annabeth and Percy ask Tantalus to send somebody on a quest to find the Golden Fleece. Under pressure from Tantalus, Clarisse accepts.
Percy, upset for not getting the quest, sneaks out to the beach at night. He meets up with a jogger, who turns out to be Hermes, god of thieves and travelers. Hermes, who is Lukes father, asks Percy to try to convince Luke that he has always loved him, despite neglecting him, and gives Percy gifts to help him along the way. Annabeth and Tyson arrive right after Hermes leaves, but Harpies tried to eat them, so Percy gets some help from some sea hippocampi who are summoned by his father Poseidon, and they sneak onto a passing cruise ship, the Princess Andromeda, which is soon revealed to be owned by Luke. Percy, Tyson, and Annabeth are captured, and they learn that Luke is trying to revive Kronos, an evil titan who is also Zeus's father. They manage to escape. They are saved by Clarisse, who has a boat of her own, given to her by her father, but the boat is destroyed on entering the Sea of Monsters and the adventurers are separated.
They reach an island resort, belonging to Circe, an evil sorceress that temporarily turns Percy into a guinea pig, but Annabeth turns Percy and some pirates back. Percy and Annabeth then steal Blackbeard's ship. On the way to the island where the Golden Fleece awaits, Annabeth is tempted by the Sirens. She nearly dies, and afterward Annabeth tells Percy that her fatal flaw is her hubris, or pride, and that every hero has a fatal flaw. They soon reach the island of the Cyclops, and save Grover, with the help of Tyson and Clarisse. They recover the Fleece and make their way to Florida. Percy sends Clarisse to camp, with the Fleece, on an airplane.
Soon after, Percy, Annabeth, and Tyson are captured by Luke. Percy manages to contact camp with an Iris-message, tricking Luke into admitting he poisoned Thalia's tree. In a fight with Luke, Percy is nearly killed, only saved by Chiron and his family of wild centaurs. Chiron is re-hired after being proven not guilty he tells Percy that the reason he was suspected of poisoning Thalia's was that he was Kronos' son .The Fleece cures Thalia's tree of its poison, but they find out that the Golden Fleece also brought Thalia (Zeus's daughter) back to life, thus giving Kronos another chance to control the prophecy of the very confusing fate of Olympus.
Blackjack is mentioned as "the black pegasi" who was Luke's steed. Interestingly enough, before Blackjack is named in the next book, Riordan said that it was a female. In the other books Blackjack is male.
source: Wikipedia
