Hotaru Tachibana is a girl disguised as a boy. Through strange circumstances she finds herself drawn into the world of survival games by a host, Masamune Matsuoka. The two form a team with ero-manga artist Tohru Yukimura and aim to be the be the best in Japan.
Haruhi is a poor tomboyish student at a school for the ultra-wealthy, able to attend because of a scholarship, and unable to even afford a uniform. One day, she stumbles across the decidedly peculiar but very popular Ouran Host Club. She tries to leave, but accidentally breaks a vase, and unable to pay for it, she's told that she'll have to stay and do odd jobs. That is, until they decide she would be more valuable as a club member. Not realizing she's female due to her appearance, they fix her up and give her a male uniform. She's an instant hit, so they decide to have her keep up the charade even after they find out the truth. Haruhi, being average almost to a fault, doesn't know what to make of the unconventional activities of the Host Club (or of the even more unconventional members), but having little choice, she plays along. Before long, real bonds are formed, friends made, and Haruhi finds herself accepted in a way she could never have been otherwise in this affluent school.
In the "Great Disaster," Japan has been thrown into chaos by onmyouji (a kind of magician). Harutora Tsuchimikado was born into an onmyouji family, but has no power whatsoever. His childhood friend Natsume (another member of the Tsuchimikado family) appears to him one day to take him up the promise he made long ago to become her shikigami (familiar). A battle between onmyouji is about to begin.