After an Earthquake gives The People under the ground quite a scare, Captain Holly Sharp is sent to investigate. What she finds will change the underground as they find out that Fairies were not the only ones hiding from the public eye.
Basically, the world is ending and Artemis, the faries, and the demigods have to defend Camp Half-Blood and defeat Gaea. First few chapters suck. It gets better as it goes... Rated T for violence.