The Mark of Athena ends with Annabeth and Percy falling into a chasm that leads directly to the Underworld. In order to discover the mortal side of the Doors of Death, the other five demigods must set aside their sorrow and follow Percy's instructions. The Seven will be able to seal the Doors from both sides and stop the giants from raising Gaea if they can defeat Gaea's armies and Percy and Annabeth can escape the House of Hades. Leo questions how Percy and Annabeth will be able to escape if the Doors are shut. They have no other option. If the demigods fail, Gaea's soldiers won't perish.