Rick Riordan's The Last Olympian is the last in a five book series describing the adventures of Percy Jackson, the young half human son of Poseidon. Percy Jackson must lead the other half-blood children of the gods to defend Olympus, which is now at the six hundredth floor of the Empire State Building in Manhattan from the Titan Kronos.

Kronos, who has reincarnated in the body of Percy's old friend, Luke, has sworn an oath of vengeance to destroy Olympus, brick by brick. He has been massing an army of monsters, demigod children, demons, and a few other Titans for a year. If he is successful, the age of the gods will end and a new age of chaos will begin. To distract the gods from their seats of power in Olympus, Kronos simultaneously launches an attack against Poseidon's undersea fortress and unleashes the most fearsome of all monsters, Typhon. Typhon he sends on a mission of destruction from his prison under Mount St. Helens to New York. Typhon is to destroy the gods and Olympus if he is able, but he is just a distraction. Poseidon is embattled in his fortress and will not join the fight. Hades is sulking in the underworld, nurturing a grudge against Zeus for killing the mortal mother of his children. The remaining Olympian gods leave New York to do battle with Typhon, leaving Olympus virtually undefended. The gods believe Typhon to be the main threat, that Kronos would not dare march on Olympus himself. However, Percy has seen in a prophetic dream that Kronos will indeed invade New York, and that only he and his half-blood peers from Camp Half-Blood are left to defend against the invasion. If that isn't enough for the young demigod to worry about, he also has a prophesy telling of a single choice he must make on his sixteenth birthday, which is only a few days away, that will either destroy or save Olympus. On top of that, regardless of his choice, "The hero's soul, cursed blade shall reap."

In order to stand against an overwhelming army and to fight Kronos, Percy accepts the offer of his friend Nico, son of Hades, and bathes in the river Styx. After emerging from the river, his body immortal except for one small point at the small of his back, Percy leads the charge in defense of Manhattan. After a series of brutal battles against overwhelming odds, after slaying several mythical monsters, Percy, Annabeth, and Grover catch up to Kronos in the throne room of the main palace at Olympus.

After fighting Kronos for a while, Annabeth is injured and Percy is disarmed. Kronos, noticing the way Percy is effected by her injury, maneuvers between the two demigods, and threatens to kill her should Percy continue. At this point, Percy gets Annabeth's knife and Luke regains a small bit of control over his body. He begs Percy give him the knife so that he can kill himself. He knows that if Percy tries to kill him, Kronos will be able to stop him. Percy realizes that this is the decision of the prophesy. Can he trust the humanity of his former friend? Percy gives the knife to Luke, who then unbuckles his armor, and stabs himself just below his arm, in the only remaining mortal part of his body. Luke dies, and Kronos, who was not yet strong enough to manifest in his true form, is once again defeated.

Just as Kronos is defeated, the gods march into the palace at Olympus ready to do battle, only to find that the kids have already taken care of it. They set about repairing the damage to their thrones, then the council of the gods meets to discuss aftermath of the war. As reward for their service in the war, Grover is made a Lord of the Wild Council. Annabeth, whose dream is to be an architect, is given the job of redesigning Olympus however she sees fit. Percy, who led the defenders of Olympus and ultimately saved the gods, is granted one wish. The gods offer him immortality and a place as one of them for eternity. He turns this down in order to stay mortal and live his life with Annabeth. Instead of becoming a god, Percy makes the gods swear on the Styx that they will do a better job of claiming their children and getting them to Camp Half-blood, as well as recognizing every god including the minor ones at Olympus.