The Oracle at the Camp Half Blood tells Percy to find the stolen master lightning bolt of Zeus, since Zeus blames Poseidon for stealing it. After defeating several mythological monsters, including Medusa, Echidna, and the Chimera, Percy, Annabeth and Grover find Hades, who has taken Sally but had his Helm of Darkness (his symbol of power) stolen too. Hades also accuses Percy of stealing his helm and threatens to kill Percy and his mother and release all the dead back into the real world. Percy and his friends use magical pearls to transport themselves out of Hades' kingdom. After discovering that Ares has all the stolen items, Percy challenges him to a duel and wins, to his own surprise. When Percy presents the Helm to Hades, Hades realizes that Percy has not stolen it and returns Sally safely.
After the bolt is given back to Zeus, Luke is revealed to be the lightning thief and the number one servant of Kronos, the Lord of the Titans. Luke tries to kill Percy by calling on a pit scorpion, but Percy manages to kill it. Percy decides to spend the school year with Sally instead of staying at camp.