Percy and Annabeth wanted to get out of the underworld, but there are many monsters guarding the way out. The way out also needed to be opened from the overworld. So the rest of the team goes to the House of Hades to open them. They let Percy and Annabeth out of the underworld.
I liked this book. One reason was that it had a lot of battles. Another reason was that it had many awesome monsters, like Titans, Giants, and Tartarus.