Though the Greek and Roman crew members of the Argo II have made progress in their many quests, they still seem no closer to defeating the earth mother, Gaea. Her giants have risen - all of them - and they're stronger than ever. The gods, still suffering from multiple personality disorder, are useless. How can a handful of young demigods hope to persevere against Gaea's army of powerful giants?

The penultimate book in the 'Heroes of Olympus' series. The stakes are higher than ever in this adventure that dives into the depths of Tartarus. At the conclusion of 'The Mark of Athena', Annabeth and Percy tumble into a pit leading straight to the Underworld. The other five demigods have to put aside their grief and follow Percy's instructions to find the mortal side of the Doors of Death. If they can fight their way through the Gaea's forces, and Percy and Annabeth can survive the House of Hades, then the Seven will be able to seal the Doors both sides and prevent the giants from raising...

Can Percy Jackson and the Half-blood Heroes succeed on their quest to find The Doors of Death or will the Greek Gods of chaos win their battle to stop them? Percy and his fellow demi-gods face the most important quest of all - the Prophecy of Seva.

Percy Jackson - the son of Poseidon - has had an unnervingly quiet school year. But then he discovers that the magical borders protecting Half Blood Hill, a summer camp dedicated to training young demi-gods, are failing. Unless something is done, the camp will be attacked by demons and monsters.