Percy arrives home from boarding school and gets to go on a trip with his Mom to the ocean. They are at the ocean when his friend Grover tells him they need to leave because a monster bull is trying to kill them.
"Lets find the bolt."
"The helm is also gone."
The monster bull kills his mother before Percy kills the bull. Percy is introduced to camp half-blood and is shown his cabin. He also learns his father is Poseidon who is the sea god. Percy finds out that Zeus's master bolt is stolen and he is accused of being the thief.
"Fight me, Ares, for the helm and bolt."
"Ok, and I will win!"
Percy, Grover, and Annabeth travel from camp half-blood to Los Angeles to go to the Underworld. They go to the Underworld to get the bolt back from Hades who they believe is the real thief.
"Mom! You're not dead!"
"Percy, you saved the world!"
There will be a fight between the gods if they do not find the bolt. They make it to the Underworld to get the bolt and find out the helm of Hades was also stolen.
Percy fights Ares to get the helm and bolt because Ares was the one who stole them and if he wins the helm and bolt are his to give back to Zeus and Hades.
He won the fight with Ares and gave back the helm to Hades and the bolt to Zeus. His Mom comes back because Hades paid his debt to Percy for finding his helm. Percy kept the gods from fighting and saved the world!