We've seen Percy grow up the past 5 books and we've finally reached the end of his story (semi?). The prophecy in the first novel (so long ago) spoke about a child of the big three doing bad things at the age of sixteen. Well, Percy is nearly 16 now and he's a child of Poseidon (spoiler?) which is one of the big three Gods. 
 The evil Titan Kronos has taken form (view spoiler)[in Luke's body (hide spoiler)] and war is coming to Camp Half Blood and the demigods. The rest of the prophecy has been heard and Percy and all the other demi gods are in for the battle of their lives versus Kronos and the titans. This book was spent majority of the time tying up loose ends (though there were a few left) and fighting as expected. Every battle scene played it's part and role though so it didn't feel like it just went on and on and on. 
 (view spoiler)[Percys gift/wish after defeating Kronos (which we assumed would happen) was that all demigod children be recognized. He turned down the gift of immortality and to become a God of his own. How kind! & for every child to no longer just be thrown into the Hermes cabin but for every Gods child to have a place of their own. Great wish Percy. 
 Rachel - Percys semi girl-friend - became the new oracle! (hide spoiler)] 
 Really great end to the Percy Jackson and The Olympians series and I can't wait to start on more Rick Riodan series!