When a narrative begins in the middle of the action, it is said to have started in medias res. Occasionally, authors (and filmmakers) use this literary device to introduce a scene involving the protagonist already at the latter part of the narrative, when the stakes are higher and much of the plot points in the storyline has been established.
When a story jumps right into the action without any preamble, the reader must rely on the author’s description of the situation to fill in the blanks. Using this literary device, an important plot point can be introduced right away, immersing the reader right into a pivotal moment of the narrative that is integral to the unfolding plot.
It requires more than simply omitting the prologue to avoid a cliched opening sequence. As the story progresses, there are sporadic flashbacks that fill up the plot holes, but the narrative generally advances in chronological order. Right at the beginning of an action-packed thriller, for instance, the investigator may already be on the trail of the murderer. Hence, a tense and frightening ambiance is established, hinting at the impending doom.
Beginning a story in the middle has its roots in oral tradition and has become a stylistic pattern of epic poetry, with Homer’s The Iliad and The Odyssey serving as prime examples, both of which employ the in medias res technique. For instance, the argument between Achilles and Agamemnon before the Trojan War serves as the dramatic opening scene of Homer’s Iliad.
Originally from Latin, the phrase in medias res translates to “in the midst of things.” It was first used by the Roman poet Horace, who wrote in “Ars Poetica” (The Art of Poetry) that the best epic poet “hurries to the action, and snatches the listener in medias res.”
In Medias Res by Julianne Day Ignacio, Verge of Verse
Writing In Medias Res by James Scott Bell, Kill Zone Blog
Clearing Up Some Misconceptions About In Medias Res by K.M. Weiland, Helping Writers Become Authors
Ars Poetica (Horace) by Wikipedia