In the end of The Mark of Athena, there is a big cliff hanger. In this book, it picks up where they left off. Percy and Annabeth fall into Tartarus and have to find the doors of death to get out. But the doors of death also need to be closed from both sides of the underworld and the mortal world. The rest of the demigods on the Argo ll need to get to the doors of death. If they do not close the doors of death, Gaea’s monsters will never die.
Bottom line: It is a bit scarier because they are in the underworld and that is a scary place. I love all of the characters on the Argo ll so much. They are all amazing. It is for 5th grade and older.