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 from both sides and prevent the giants from raising

...

Stacey McGill is moving back to New York, and her friends must face the sadness of saying good-bye, hunt down the perfect farewell gift, and take on the difficult task of filling Stacey's place in the Baby-sitters Club.