The Son of Neptune opens with Percy Jackson being chased by two gorgons (sisters of Medusa), Stheno and Euryale, who hate him for killing their sister Medusa. They reveal that they're working for Gaea. While making his escape, he runs into an old woman named June. The old woman reveals herself to be Juno (Hera) and gives him a choice: provide her with a favor or live safely under the sea and everybody and everything destroyed. As a favor, Percy carries her across the Little Tiber, which is a powerful river. In doing so, he loses his invincibility (Achilles Curse) from the River Styx. When he reaches his destination he meets Hazel Levesque, a daughter of Pluto (Hades), and Frank Zhang, a son of Mars (Ares, Greek god of war). Percy saves Frank from the Gorgons by drowning them. Percy is then taken into the camp after Juno introduces him to Camp Jupiter (training camp for Roman demigods). Reyna seems to recognize him, but refuses to acknowledge him. She sends Percy and Hazel to Octavian, the camp augur, to see whether or not he's fit to join the legion. Along the way, Lares, the camp's ancestral spirits, call him Greggus, or Greek, which makes Hazel act worried. When they reach Jupiter's temple, Octavian is cutting teddy bears open and reading the stuffing. Octavian threatens Hazel by telling her that if she doesn't vote for him, he'll reveal her secret. He declares good omens, and Hazel and Percy go to the temples, where they meet Nico di Angelo (brother of Bianca di Angelo), a main character from the Percy Jackson series. He acts suspiciously and claims not to know Percy, even when he recognises him. Meanwhile, Frank is organizing weaponry when a Lare appears and tells him he knows about his stick and his birthday. Frank then runs off to find Hazel and Percy. He takes Percy to the baths while Hazel and Nico talk. Hazel reveals that she's from the 40's and has a blackout. She realizes that she's late for the war games, so she and Nico rush to line up.
During war games, a Camp Jupiter activity, Percy impresses everyone with his battle skills and instincts, even though they are not Roman tactics and are more Greek-style. After the games, Octavian hurts Gwen, a centurion of the Fifth Cohort, and she gets severely injured and dies, only to come back to life a few moments later. Mars, the god of war, then appears and informs the campers that "Death has been chained." (Thanatos). He also claims Frank Zhang as his son and gifts him a spear with a dragon' tooth. Mars insists that Percy and Frank must go on a quest to free Thanatos, the god of Death. He has been captured in Alaska and is no longer able to keep mortals dead. Frank requests that Hazel also come on the quest, and they set off towards Alaska. While traveling on their quest, Percy struggles to regain his memory. Time and time again, he recalls things but never where from. The only clear memory to him is a girl named Annabeth (his girlfriend). Hazel reveals that she had died long ago, but that Nico di Angelo, son of Hades and her half-brother, saved her from the Underworld. Frank informs the other two that he was given a piece of wood that is tied to his life; if the wood burns up completely, he will die. Percy regains most of his memory after he gambles with gorgons blood and Gaea lets him win saying she still needs him as a pawn to free herself. After many events and monsters, such as basilisks, giants, and an unexpected meeting with the Amazons (an all woman tribe), they finally get to Alaska, where they find Thanatos in chains. His chains can only be broken by the fire of life i.e. the piece of wood tied to Frank's life. Percy defends Frank while he melts Thanatos chains with his burning stick, losing more and more of his life. Hazel attacks Alcyoneus, the giant born to oppose Pluto. Thanatos is eventually freed and with the help of Hazel's horse, Arion, they drag the giant out of its land (Alaska) to Canada where it is able to be killed (Alcynoneus couldn't be killed in his birthland).
Percy regains his memory as they race back to Camp Jupiter. Knowing that the camp is under attack, Percy takes charge and leads the Romans into battle with the help of some of his old friends from Camp Half-Blood and the Amazons. The morale of the Romans is recharged after discovering that Percy recovered the camp's lost standard, which had been missing for years. Percy then challenges the giant, Polybotes the giant born to oppose Neptune, to a duel and calls to Terminus, the Roman border god, to help him defeat the giant. He has been told that only when gods and demigods work together can the giants be defeated. Percy eventually defeats Polybotes by using the head of Terminus' statue as a weapon. The camp then name him a praetor, a leader of the Roman camp. The Greek demigods from Camp Half-Blood appear in the flying ship Argo II sailing towards the camp. The Romans argue over attacking the ship. As praetor, Percy orders them to hold their fire. If the Greeks attacked, he would be to blame, but he is not worried. With Frank and Hazel, he approaches his "other family".
This book continues on the prophecy from the previous book which states:
“Seven half-bloods shall answer the call
To storm or fire, the world must fall
An oath to keep with a final breath,
And foes bear arms to the Doors of Death”
slowly through a turn out of events, the seven mentioned in the prophecy is revealed. My opinion of this book is of course...High. Cause i LOVE the percy jackson series! DUH! if i didnt would i read all 5 books plus continue on to the spin of series? I THINK NOT! sooo like READ THEM! THERE AMAZING AND YEAH!