In medias res*
Grammar lesson time.
In English, there is no strict notion of case. Most (all?) other European languages have different cases for every noun, at least three or four. But English, oh, no-one uses case any more, nor have they for centuries. That’s why English has such strict word order. The subject must always come first. The verb must always come second. If present, the indirect object follows, then the direct object. Incorrectly conveyed or received ideas causes any divergence from this word order. (That is to say: any divergence from this word order causes incorrectly conveyed or received ideas.)
On the other hand, compare “Ich schrieb einen Brief” (I wrote a letter) to “Einen Brief schrieb ich” (I wrote a letter). Their word orders are inverted, but they mean the same thing! That’s because “einen” means that der Brief is the thing being written, wherever it appears in the sentence. If it were doing the writing, it’s be “ein Brief”.
Latin is even freer in terms of word order. It has seven cases (only four are/were commonly used, though) and everything is declined, always, depending on gender, number and case.
In medias res means the exact same thing as medias in res.
The common usage in English context is in medias res. Look at that shining lexical ambiguity! So to reduce that, I swapped a couple words around. The precise meaning of the phrase is untouched.
So make an effort to understand a phrase’s meaning and origin before you decide that you understand it.