Without giving too many details away (because I do not like spoilers and am assuming you don't either), Percy returns to Camp Half Blood after a deadly game of dodgeball and finds that this safe haven for demi-gods (children of Gods) is losing the protection of it's magical borders due to a bit of sabotage. Of course, Percy and his friends embark on a quest to save the camp and have a whole lot of fun (fun for me, not so much for them) on the way.
The book, like its predacessor, is a fast and easy read. The action is fast paced, includes lots of humor, and leaves you wanting to read more. However, what I love about this series and the reason that I have kept interest in the series, is the mythological aspect of the stories. Granted Riordan takes liberties with the Greek characters, but they are based on solid "facts"and have renewed an interest in Greek mythology and literature. Seriously, any book that makes me want to put it down and pick up Homer's The Odessey is pretty awesome! LOL.
I also quite enjoy the modern slant that Riordan has taken with the Greek mythology--explaining why Mt. Olympus is now situated above the Empire State Building, giving a modern take on the gods and goddesses (really, Hermes the messenger God as a jogger and the head of a postal service is exactly how I would have imagined him), enlightening us as to what really goes on in the Bermuda Triangle....pure imagination and lots of fun.