What part of the plot is in medias res?

The Latin Word Res means "thing" or "things." It's where we get our Word "Republic." The republic is the res publica. The "public thing." "In medias res" means "In the middle of things." It is not so much a "part of the plot" as the question asks, but a style of writing. To start a book or story in medias res, means to start in the middle of the action, where the characters are already embroiled in a situation.