Notice

Almost all the time when people say "in medias res", what they should be saying is "in mediis rebus".  The locus classicus of the phrase is in Horace, "in medias res / non secus ac notas auditorem rapit", which this guy renders as "[he] snatches the reader / Into the midst of the action, as if all were known".  "medias res" is accusative here, and the preposition quite properly translated as "into", because the listener is being taken somewhere; there's definite motion towards.  But generally people use the phrase with verbs like "begin" or "start" (a characteristic of epics is that they start in medias res), and such are clearly ablative contexts.  One does not begin into anything.  Consequently: "in mediis rebus". 

Adam Kotsko, with whom I was debating this issue, was skeptical and asked if google attested my favored form.  While I maintained that I'm making a normative claim about English and that, therefore, google could fuck itself in that regard (even though, in fact, there are examples of it, only one of which is due to me), since I was making a descriptive claim about Latin I was forced to find some support for my contention that things so stand with that language.  Fortunately an example, albeit of uncertain date of composition, presented itself.  Thus my claim stands on the firm footing of science, and henceforth anyone who says "in medias res" when "in mediis rebus" would be more accurate has, officially, no excuse whatsoever.

Comments

[David Wharton]

When people say in medias res, they're not speaking Latin, they're using a borrowed Latin phrase which now has a fixed meaning in English that's different from its Latin origin.

I don't think they're under any compulsion to follow Latin grammar, just as legal scholars can use stare decisis and habeas corpus as nouns.

Sed si quis vere Latine loqui velit, sine dubio illi necesse sit "in mediis rebus" dicere.

[John Harvey]

We can stake out a middle and, in my opinion, better position here. Neither demand "rebus" (which deprives us of the fun of quoting Horace verbatim) nor tolerate "begin in medias"; rather in an English context we can use an appropriate verb. So Byron says

"Most epic poets plunge ‘in medias res’" (Don Juan)