It’s about a girl name Hikaru Takabayashi who decides to attend a all girls school boarding school after her mother remarries and moves to France. Oddly enough, her roommate is up and coming idol with a secret. And that secret being Izumi Kido is really a guy.
Hikaru finds out and Izumi turns to blackmail. As the story progressed, their friendship grows and Izumi fall in love with her. Hikaru falls in love with their new Sensei and Thanks to Izumi they end up together. But Ayase Sensei later breaks off their relationship allowing Izumi to move in.
Hikaru and Izumi relationship finally blossom! But ironically, due to various unseen circumstances Izumi disappears without a trace. Hikaru continues down the path of towards her dream all the while searching for him. A year passes before there united once again. Tenshi Ja Nai in a nutshell.