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 at 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 for 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?