in medias res

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in me·di·as res

 (ĭn mē′dē-əs rās′)
In or into the middle of a sequence of events, as in a literary narrative.

[Latin in mediās rēs : in, into + mediās : accusative pl. feminine of medius, in the middle of + rēs, accusative pl. of rēs, thing.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

in medias res

(ɪn ˈmiːdɪˌæs ˈreɪs)
(Literary & Literary Critical Terms) in or into the middle of events or a narrative
[literally: into the midst of things, taken from a passage in Horace's Ars Poetica]
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014

in me•di•as res

(ɪn ˈmɛ dɪˌɑs ˈrɛs; Eng. ɪn ˈmi diˌəs ˈriz, ɪn ˈmeɪ diˌɑs ˈreɪs)

adv. Latin.
in the middle of things.
Random House Kernerman Webster's College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.
References in classic literature ?
"I will even do as Horace says, sir," I answered, smiling, "and carry you in medias res." He nodded as if he was well pleased, and indeed his scrap of Latin had been set to test me.
"Well," said Colia, plunging in medias res, as he always did, "here's a go!