Percy had been claimed as the son of Poseidon. Which means, he would be sent on the quest to retrieve the Master Bolt and return it back too Zues. But customarily you need three people too go on a quest. So Percy has too choose two people too join him.

Obviously, he chose Grover since they are best friends. But what's shocking is that he also chose you.

Now you, Percy, and Grover are on a train, beginning the quest.