The story begins at Mount St. Helens in Washington state. The Titan, Kronos (antagonist), unleashed the most fearsome of all the titans, Typhon. The Gods go to meet this monster in combat to halt it march toward Olympus.
After a lot of consideration, Percy realizes he has no choice but to accept Nico's offer to bath in the river Styx. Being the son of Hades, Nico, freely takes Percy to the underworld to bath in the river. This grants him immortality and renders him invulnerable in all but the small of his back.
With the Gods off fighting Typhon, Olympus is left undefended. Kronos uses this as a chance to march on New York to take over then destroy Olympus. The only ones who stand in his way are Percy jackson and the other demigods.
Kronos and his army march on New York. Percy knew that they would be at overwhelming odds, but he fails to realize to what level. Kronos had an army of mythical creatures led by himself and several other titans.
For a while, it was just brutal battle after battle. The Percy and the other kept fighting to no avail. Kronos's army seemed endless. They were relentlessly bombarding the demigods with assaults.
After many brutal battles, Annabeth is injured while fighting kronos on the Hudson Bay Bridge. She threw herself in the way to protect Percy's only weak spot.
Kronos quickly makes his way between Annabeth and Percy. Disarmed and fearing for her life, he backs away.
Percy goes to grab Annabeth's knife, and then realizes something. Luke, the person who's body holds kronos, is still somewhat in control. He asks Percy to give him the knife so he can kill himself and Kronos before he kills him only friend, Annabeth. He then takes the dagger and drives it into his side just under his arm.
The Gods finally defeat Typhon and return. They realize that Percy has defended Olympus and defeated Kronos. They offer him immortality and a seat among them, but he refuses. He then asks for only one thing. He asks that they give more respect to minor gods, and do a better job of claiming the children.