Hm. So, your intention is to say "into the middles" (accusativus pluralis) so as to mirror the original expression, correct? That's the only way I see this thing as being correct, rules-wise, presuming that the original expression is.
Let's see. a, ae, ae, am, a - ae, arum, is, as, is es, ei, ei, em, e - es, erum, ebus, es, ebus (Not one hundred percent on those.) Original phrase: in medias res, into the middle things, accusative plural. My change: in media Quiet American, in the middle [of] QA, ablative singular? I suppose an 'of' could have been included but I unconsciously elided it for comprehension's sake. Worth noting: there seems to be some confusion online as to the literal translation of in medias res. Even wiktionary etc have it as 'in the middle of things' but unless I'm missing a subtlety, adding an 'of' changes the case to genitive (rerum). Only by translating the phrase "into the middle things" is this problem avoided.