From DGT:
When Asano Arisu was little she was badly bullied by the local boys, now a high school student she’s famous for her vehement hatred of men and boards an all girls school.
When stunning Fujimi Kira enters and becomes her dorm-mate, proclaiming that she also hates men, Arisu thinks that it must be a fated meeting. However, they both have secrets that they need to keep from the Dorm Head, strict Kamitani-Sensei: Arisu keeping a pet ferret in the dorm, and Kira the fact that she’s actually a BOY--much to Arisu’s horror. It appears that he’s at the school to uncover the mystery of his older half-sister’s supposed suicide 2 years before.
Will Arisu be able to cope with having to share a room with a boy, let alone help him with his investigations? And just how dangerous will it be to solve the mystery?