Hayate is a combat butler for the Sanzenin family with a debt of about 150 Million Yen that he has yet to repay.
Hayate is a hard worker who does his best to try and make money for himself. However, because he is underage, he often has to lie about his age to his employers. Not only does his last employer fire him, but his parents take his paycheck for themselves and gamble it away, racking up a debt of several million Yen and then selling Hayate out to the Yakuza in order to pay it. Getting desperate, Hayate decides to kidnap a girl and hold her for ransom.
One night at the park, he ends up helping a girl named Nagi Sanzenin. Though he half-heartedly plans to kidnap her, she mistakes his words as a confession of love, and he ends up chickening out. Suddenly, when Nagi gets kidnapped for real by the Yakuza, Hayate ends up rescuing her. Not only does Nagi turn out to be from an incredibly wealthy family, but to show her gratitude, Nagi decides to hire Hayate as her butler.
Hayate shows incredible speed and skill for one who's supposed to be a high-school aged boy. He is able to do anything from running as fast as a bullet train to kicking a rogue robot in the face when the situation calls for it. Over the course of the series, Hayate is somehow involved with several different girls who each harbor a kind of crush on him, though he either doesn't notice or prefers platonic relationships.