In medias res

In medias res is the plot device of starting a story "in the chronological middle of the plot or conflict, rather than at the chronological beginning." The Latin means "into the middle of things." Horace prescribed it in the Ars Poetica (c. 13 BC), praising the epic poet who snatches the listener into the middle of things instead of beginning ab ovo, "from the egg," at the story's earliest cause; the Iliad, which opens nine years into the Trojan War, is the classical model, and the technique has likely origins in oral tradition.

The trade it makes is explicit: exposition is skipped at the opening and filled in later through dialogue, flashbacks or description. For serial writers that is the whole decision: an in medias res opening hooks readers in chapter one, but the withheld backstory then has to arrive without stalling the rising action.