- Percy reunites with his friends Annabeth, Grover, and Tyson.
- Chiron is revealed to have gone missing.
- The Oracle, Rachel Elizabeth Dare, gives a prophecy about the "Lost Hero" and "the seven demigods shall answer the call".
* Chapter 2: "Percy Jackson, Missing" - Thalia Grace leads a group of Hunters of Artemis on a search for Percy.
- Rachel is questioned and reveals the prophecy she received.
- Percy, Grover, and Tyson leave Camp Half-Blood to find the seven demigods and stop Kronos.
* Chapter 3: "The Prophecy of Seven" - Percy, Grover, and Tyson arrive in Manhattan and meet Rachel Elizabeth Dare.
- Rachel reveals the complete prophecy given to her by the Oracle.
- The group is attacked by monsters and is saved by Nico di Angelo.
* Chapter 4: "The Mark of Athena" - The group travels to the Underworld and meets with Hades, who confirms Kronos' plans.
- Nico shows Percy and Rachel the Mark of Athena on their palms, which leads to Athena Parthenos, a statue that contains immense power.
- The group learns that finding Athena Parthenos will help them defeat Kronos and save the world.
* Chapter 5: "The House of Hades" - Percy, Annabeth, Piper, Jason, Hazel, Frank, Nico, and Rachel arrive at Epirus, Greece to find Athena Parthenos.
- They face numerous challenges, including the giant Polybotes and the traps of Tartarus.
- Leo, who is flying the Celestial Bronze dragon Festus, comes to their rescue and helps them escape from Tartarus.
* Chapter 6: "The Blood of Olympus" - The group of heroes splits up to gather allies for the impending battle against Kronos.
- Percy and Annabeth return to Camp Half-Blood and unite the demigods for war.
- Octavian, the former praetor of Camp Jupiter, leads a group of Romans against Camp Half-Blood, but they are defeated.
- Kronos' army arrives at Manhattan, and the final battle begins.
* Chapter 7: "The Last Olympian" - The demigods and their allies fight bravely against the forces of Kronos.
- Percy, Annabeth, and Tyson face off against Kronos himself.
- Kronos is defeated and destroyed, and the demigods claim victory.
- The story ends with Percy and Annabeth sharing a kiss, and peace being restored to the world.