in medias res

US /ɪn ˌmiːdiəs ˈreɪs/
UK /ɪn ˌmiːdiəs ˈreɪs/
"in medias res" picture
1.

into the middle of a narrative or plot without a preamble

:
The novel begins in medias res, with the protagonist already in a desperate situation.
Many epic poems use the technique of starting in medias res.