In medias res is a storytelling technique that begins in the middle of the action, with backstory revealed later.
The Roman poet Horace named the device in his 'Ars Poetica' (c. 19 BCE), praising Homer for plunging readers straight into the Trojan War rather than narrating its origins. The technique became a default for Western epic: the 'Odyssey', the 'Aeneid', and 'Paradise Lost' all open mid-event. Its power is twofold — momentum and curiosity. The reader is hooked by action, then doubly hooked by the gradual revelation of how the situation arose. Modern thrillers, almost without exception, use it; Quentin Tarantino's films are extended exercises in non-linear in medias res. The opposite — beginning at the chronological start — is now associated with children's stories and biographies, while serious narrative tends to trust audiences to assemble the past from clues.