The book opens with Percy and Beckendorf trying to destroy Kronos ship and defeat him before he reaches Olympus. Not one to be outsmarted, Kronos is in possession of Luke's body, the son of Hermes. Percy learns of the prophecy while at his father's underwater palace. to prepare for the battle, Percy dips himself in the famed River Styx. When he returns, he finds that all the residents of Manhattan have been put to sleep by Kronos, so that they do not get in the way of the battle.
Amidst all this, Percy and Annabeth, daughter of Athena, also realize their feelings toward each other. The centaurs, hunters, nature spirits and other members of Camp Half-Blood unite to join Percy as they take on Kronos and his army. Percy and Kronos go head on in the throne room at Olympus. Percy also realizes that someone is betraying them and passing on secret information to the Titans. to top it all, the Titans have a monster that they plan to unleash on Percy's side. Will Luke be able to get rid of Kronos possession and will Percy manage to save the day?