Rick Riordan’s The Last Olympian is the fifth and final installment of the Percy Jackson and the Olympians series. Published in 2009, this fantasy children’s book was a #1 bestseller on the lists of USA Today, the LA Times, and the Wall Street Journal. The novel follows the teenage demigod Percy Jackson, son of Poseidon and of a mortal woman. He and other demigods spend their summers at Camp Half-Blood, located in Long Island, NY.

When the novel opens, two sets of Greek gods are at war: the original race of gods, the Titans, versus the Olympians, their descendants. The demigods fight on the side of the Olympians, whose home, Mount Olympus, is located on the 600th floor of the Empire State Building in Manhattan, NY.

In the first scene of the novel, Percy and his fellow demigod Charles Beckendorf infiltrate Kronos’s ship. The latter is the Titan god of time, and he is leading the fight against the Olympians. Kronos has overtaken the body of Luke Castellan, a demigod who has bathed in the River Styx to become invulnerable. Although Percy and Charles succeed in blowing up the ship, Charles dies before he escapes.

Percy returns to Camp Half-Blood, where he encounters his mentor Chiron (a centaur) and love interest Annabeth Chase (daughter of Athena). The Oracle of Delphi, in the form of a mummified doll, reveals the Great Prophecy—the hero will make a choice to ensure the ultimate success or destruction of Olympus, a choice which will ensure the hero's demise. Although it appears as though Percy is the hero mentioned in the prophecy, the author later reveals that it is Luke. 

Once deciding to accept the Prophecy, Percy goes on a journey with Nico di Angelo, son of Hades, to learn more about Luke’s background in order to better be able to defeat him. They visit May Castellan, Luke’s mother, who has lost touch with reality. She is a mortal but has the gift of prophecy and tried to take on the role of the new Oracle of Delphi but was cursed.

Percy and Nico next journey to Central Park, which offers a passage to the Underworld. While there, they meet up with Grover Underwood, a satyr and Percy’s best friend. Grover becomes instrumental in helping lead the war efforts. Percy and Nico descend into the Underworld, and Percy bathes in the River Styx, rendering him invulnerable save for a patch on his lower back.

Next, Percy meets his fellow demigods at the Empire State Building. Their main objective is to keep the Titans away from Mount Olympus. They devise a plan to block off all of the entrances to Manhattan, and the various teams disperse. Percy and Annabeth stick together.

The demigods fight several battles against various monsters, such as the Clazmonian Sow, the Lydian pig, and the drakon. They force Kronos’s army back, but the fight wages on. During the battle, Rachel Dare, a mortal with the gift of sight, arrives to tell Percy that he is not the hero in the Prophecy. Rachel is another one of Percy’s love interests.

Percy, Annabeth, Grover, and Thalia finally go to Mount Olympus in a last-ditch effort to defend it. The city is crumbling around them, and Kronos has entered the throne room. Percy fights him, and Annabeth speaks to Luke, whose spirit is still present in Kronos. Annabeth and Luke have a history together, and she is able to get him to turn against Kronos and defeat him. Luke uses his own sword to kill Kronos, effectively killing himself. In this way, Luke fulfills the Prophecy.

Everyone returns to Camp Half-Blood to tend to the wounded and give burials to the dead. Rachel becomes the new Oracle of Delphi and tells Percy that they cannot be together. Percy confesses his feelings to Annabeth, and they two become a couple. The book ends as summer draws to a close, and the various demigods leave to go back to school in the normal world.