Nanoka, the main character, discovers one day that she has a special ability... she can hear other people's true thoughts (or "heart voice"😉! All of her brothers also have different special abilities, so she fits right in, however... one day Nanoka discovers that she is not really her brothers' sister and starts developing romantic feelings over one of them, Takaya. On top of that, somebody is bent on harming Nanoka and one of Takaya's best friends, Ryou. And why do Ryou and Nanoka get along so well (to Takaya's dismay)?

In addition to the main story, Oshiete Nanoka contains a couple of short stories and one epilogue about Nanoka and Takaya.