Bête noire presents the side effects of an unforgivable act committed by Jérémy, a 16-year-old teenager. With the help of the investigating psychiatrist-coroner, his mother tries to figure out how her son, a seemingly trouble-free boy, came to commit an act of hate. What was the trigger for this nightmare? When did everything change? Together, the two women make it their mission to unravel the mystery of Jérémy.