Boy oh boy, we didn't see that coming. And by "that," we mean Luke. Just when Percy completes his quest and returns to a life of bliss at Camp Half-Blood, things go horribly wrong. Luke lures Percy into the Camp Half-Blood woods and conjures a deadly scorpion out of the ground. After he tells Percy that he was the lightning thief and the helm of darkness thief, Luke also confesses that he is working for Kronos and that he's leaving camp to serve the old Titan ruler full-time. As he leaves, the scorpion stings Percy, and Percy nearly dies. Later that day, once Chiron's medicine has worked its magic, Percy decides that he will go home and live with his mom for his seventh grade year rather than staying at Camp Half-Blood.
Poseidon warns Percy on Mount Olympus that, when Percy gets home, he will have a choice to make: "When you return home, Percy, you must make an important choice. You will find a package in your room" (21.99). This package happens to be Medusa's severed head – you know, the one Percy lopped off? What exactly is the choice Poseidon refers to here? The very last paragraph of The Lightning Thief makes us think that Poseidon refers to the choice between going home for the year and staying at Camp Half-Blood for the year.
When Percy finally makes his decision, he tells us, "I wondered, if Poseidon were watching, would he approve of my choice?" And then Percy speaks to Poseidon directly: "'I'll be back next summer,' I promised him. 'I'll survive until then. After all, I am your son'" (22.214). In promising Poseidon this, Percy almost admits that he knows that his dad is watching. He seems to know that his dad would prefer him to stay at Camp Half-Blood for good. Percy knows that Poseidon wants to protect him. Perhaps we could go so far as to say that Percy understands that his dad loves him in his own way.
And that's not the only thing Percy realizes at the end of the book. Luke's betrayal and attempt at murdering Percy is a big surprise. We totally thought he was a good guy. We have a feeling that this brush with death helps Percy fully realize that he can't trust just anyone, and that the life of a demi-god is a hard and lonely one. Interestingly, Percy is almost energized by this last encounter with Luke. He wants to get out there and fight against Kronos and his minions. He wants to kick some Titan booty.