According to what you are describing, the plot device used is the "in media res"
This technique In Media Res means " in the middle of things", and that is exactly how it works, in media res is to start a story in the middle of the story itself instead of its beginning, that is, start the story of the facts showing the characters already involved in the conflict that structures the story, using this technique allows a more dramatic beginning of narration: the story will begin with the protagonist already in trouble, and with this we will capture the interest of the reader from the first moment.
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