The Lightning Thief

After descending into the Underworld with Annabeth and Grover, Percy confronts Hades, who denies stealing Zeus's master bolt and instead accuses Percy of stealing his own Helm of Darkness. Hades threatens to kill Percy's mother, Sally—whom he is holding captive—and unleash the dead upon the world unless the Helm is returned. Searching the backpack given to them earlier by Ares, Percy discovers the master bolt hidden inside, revealing that Ares has been manipulating the situation all along.

The group narrowly escapes the Underworld, and Percy confronts Ares on a beach, challenging him to a duel. After a hard-fought fight, Percy defeats Ares, who angrily vows this isn't over between them. Percy hands the recovered Helm of Darkness to the Furies to return to Hades, who realizes Percy was telling the truth and releases Sally unharmed. Before leaving, Percy uses Medusa's severed head to turn his abusive stepfather, Gabe, to stone, freeing his mother from him for good; she later sells the statue as a curiosity and uses the money to start a new life, including taking art classes.

Percy travels to Mount Olympus and returns the master bolt to Zeus, narrowly averting a war among the gods. He also finally meets his father, Poseidon, face to face. Hailed as a hero, Percy returns to Camp Half-Blood for the rest of the summer. On the last night, Luke—one of the older campers Percy had trusted—leads him into the woods and confesses that he was the true thief who stole both the master bolt and the Helm of Darkness, acting on the orders of the Titan lord Kronos, who is stirring from the depths of Tartarus and plotting to overthrow the Olympian gods. Luke argues that the gods are negligent, unworthy rulers and invites Percy to join his cause; when Percy refuses, Luke sets a poisonous scorpion on him. Percy is stung but manages to kill the scorpion before collapsing.

Percy wakes up safe, having survived the poison, and Luke has fled camp, exposed as a traitor. In the book's final scene, Percy is given the choice to stay at Camp Half-Blood year-round or return home for the school year; he chooses to go home to live with his mother, who is finally free of Gabe and building a new, happier life. Grover, having fulfilled his duty of protecting Percy, earns his searcher's license from the camp council and sets off to search for the lost wild god Pan, while Annabeth also departs camp for the year, leaving Percy to face the school year knowing that Kronos's rebellion is only beginning.

Percy Jackson, age twelve, has just discovered he is a demigod—the son of the sea god Poseidon and a mortal woman, Sally Jackson. He spent the summer at Camp Half-Blood on Long Island, where young demigods train and where he bonded with two close friends: Grover, a satyr assigned to protect him, and Annabeth Chase, a sharp, strategic daughter of Athena. Percy also briefly trained and traveled alongside Luke Castellan, a charismatic older camper and son of Hermes, before Luke was unmasked as a traitor.

Over the course of the book, Percy was sent on a quest to recover Zeus's stolen master bolt, suspected to be hidden in the Underworld by Hades. Along the way, the trio battled Medusa, the Chimera, Echidna, and other monsters, and were manipulated by the war god Ares into retrieving his shield in exchange for a backpack of supplies—supplies that turned out to secretly contain the stolen master bolt. After confronting Hades (who was innocent of the theft but was missing his own Helm of Darkness) and narrowly escaping the Underworld, Percy defeated Ares in a duel on a beach, returned the Helm to Hades via the Furies, and delivered the master bolt to Zeus on Olympus, averting a godly war. He also met his father, Poseidon, in person for the first time.

The true mastermind behind the theft was revealed to be Kronos, the imprisoned Titan lord, who had manipulated both Ares and Luke as his agents in a scheme to destabilize the Olympian gods. On the last night of camp, Luke openly declared his allegiance to Kronos, denounced the gods as unfit rulers, and tried to kill Percy with a poisoned scorpion before fleeing camp as an exposed traitor. Percy survived the poisoning.

By the book's end, Percy has been publicly claimed as Poseidon's son and is a minor hero at camp, but the throne room has learned that the ancient oath among Zeus, Poseidon, and Hades never to father more demigod children was broken by Percy's birth, making him unusually powerful and unusually watched by the gods. Percy's home life has improved: his mother, Sally, is free of her abusive husband Gabe (whom Percy turned to stone using Medusa's head) and is starting a new, better life. Grover has earned his official searcher's license and has set off alone to hunt for the lost god Pan, fulfilling his life's ambition. Annabeth has also left camp for the year. Percy chose to spend the school year at home with his mother rather than stay at camp. As the book closes, the central unresolved threats are Luke's defection to Kronos's cause and the looming, larger war Kronos is plotting against Olympus, along with Ares's open hostility toward Percy after his defeat.