Question: what is it called when a story starts in the middle of the action?
Answer: A narrative work beginning in medias res (Classical Latin: [ɪn ˈmɛdɪ. aːs ˈreːs], lit. "into the middle of things") opens in the midst of the plot (cf. ab ovo, ab initio). Often, exposition is bypassed and filled in gradually, through dialogue, flashbacks or description of past events.