Given the danger, Artemis leaves her Hunters at Camp Half-Blood, but Artemis is soon captured, and based on the Oracle's prophecy, five heroes are chosen to go on a quest to save her. Originally, this includes three Hunters (Zoe, Artemis's lieutenant; Bianca; and one other) as well as Thalia and Grover. However, one Hunter is out of commission before the quest even starts so Percy secretly follows them so he can help save Annabeth.
This novel revisits many of the monsters and challenges that Hercules faced, and Percy also learns a bit more about the nature of Greek heroes from some of the immortals. The prophecy continues to loom, especially since Thalia's sixteenth birthday is less than a month away, and whatever the prophecy foresees was supposed to occur when the next halfblood child of one of the top three (Zeus, Poseidon, Hades) reached 16.
The series is definitely very fun, especially watching the young demigods explore their powers. However, I don't remember the heroes of ancient Greece being quite so powerful - obviously, they tended to be smarter and stronger than the average human, but were the heroes of mythology able to affect the elements in the same way as in these novels? Percy as the son of Poseidon, for example, can speak to horses, control water and breathe underwater, while Thalia can call up lightning, and not just in a "the elements are nice to them due to their parentage" way, but they actually manipulate their surroundings. It definitely makes the novels more entertaining so I don't have an issue with this, I'm just genuinely curious, and don't feel like googling it right now. Oh, my one other comment regarding these novels is that it's surprising to me how often Percy still comes across beings he doesn't recognize - he remembers some of the Greek myths but considering how his summers go, you'd think he would spend every waking minute of his off time reading up on obscure creatures of Greek mythology. Of course, it also gives the readers more motivation to read the original myths to find out exactly what happened.