There are special students called Butei who are authorized to carry weapons, investigate crimes and solve various incidents. One of the best of these students is an S-rank second year at Tokyo Butei High School named Kanzaki H Aria. On the opposite end of the scale is the E-ranked first year named Mamiya Akari.

Akari absolutely idolizes Aria and wants to become an elite Butei just like her but sadly she just is not very good and constantly fails her rank exams. One day she gets the bright idea to submit a request to become Aria's Amica (assistant), despite her friends telling her that she should not even bother due to the disparity in their skills. Surprisingly though, Aria decides to give Akari a test, which she not only accepts, but passes!

It will not be easy for Akari though, as before she can officially become Aria's Amica she will have to work hard until she proves her value. As that may be, Aria senses a dark mystery regarding Akari's past...

Source: MAL

Note: The first episode received an early screening at a special event on 27.09.2015. The regular TV broadcast started on October 6, 2015.

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Alternative version: Hidan no Aria