Misaki Ayuzawa is the student council president at a school that was all-boys until just a few years ago. Even now, the student body is 80% male...but if Misaki has anything to say about it, that'll change soon! She took on the job as student council president so she could make the school more female-friendly and attract a bit more equilibrium.
And why does she care so much, rather than enjoying being surrounded by guys? Well, her dad ran out on her mom, leaving Misaki, her younger sister, and her mother more impoverished. And thus began Misaki's distaste for men, particularly the slovenly sort who typify the male student body at her school.
But all of Misaki's "improvements" aren't so well-received by the boys at school, who think she's being unfair (at best) or just get mad at her for having high demands (at worst). So when the most popular boy at school learns Misaki's secret-- that she works part-time at a maid cafe to help support her family --she assumes that she's doomed to humiliation from the student body.
But that boy, the intelligent and talented Takumi Usui, doesn't reveal her secret. Instead he spends the next few days observing Misaki at school, and then becomes a client at the maid cafe. This combined with increasing pressure from her student council duties result in Misaki becoming sick from being overworked-- and on the day that three more of her male classmates come upon her at her workplace and try to harass her.
Usui saves the weakened Misaki from her tormenters (it's clear that at full strength she'd have no problem taking all three on), and says he'd rather keep her secret as his secret as well.