The conflict was that Zeus' Master Bolt was stolen. Since Zeus and Poseidon are in a brotherly rivalry, Zeus blames Poseidon. Percy's mother gets sent to the underworld in a fight with the minotaur. Then later gets claimed. His son Percy, has to go all the way across the United States to L.A. because Hades wants the Lightning Bolt, in return for his mother. Ares the god of war stops them along the way to retrieve something. When the return he gives them a bag.
After, the Lotus casino thing. They have to get to L.A. fast. Once, they reach L.A. and go to the studio they are supposed to the bag starts feeling heavy. Once, they are in the throne room the bag is super heavy. They manage to escape the underworld but without his mom. Then, they check and sure enough, the Lightning Bolt was in the bag that Ares gave them. Then Ares appears and Percy and Ares battle. Percy ends up winning, Hades found out and sent Alecto the Fury (Percy didn't know that yet) with his Mother.
They finally reach Olympus and turn the lightning bolt in right before the solstice. When they return to camp, Luke has a new sword, named backbiter. He comes to Percy and distracts him, then the apparent drinks were poisoned. While Percy was too weak to fight back, Luke stabbed him, then left. That was the cliffhanger for the next book.