Di, the 13-year-old Hmong girl at the heart of Ha Le Diem’s shattering documentary, has a beautiful, infectious laugh that rings through the misty mountains of northern Vietnam where she lives with her family in a small village. With her big eyes and rosy cheeks, Di has an endearing mischievousness about her, but even in the games that she plays with other youngsters, the shadow of an archaic tradition looms large. At one point, Di and her friends pretend to act out a “bride kidnapping”, a Hmong custom that during the New Year celebration allows a boy to snatch a girl away and force her into marriage.

This practice was how her parents met, and it was also how her older sister was married at the age of 14. Most of the housework and farming labour are performed by the women – even Di has to carry wood logs much bigger than herself – while the patriarch spends most of his day drinking. Still, Di is not defined by her restrictive social environment; her bubbly personality instead dominates the screen. From her desire to complete her studies to her crushes on other boys, Di is determined to make her future her own.

However, this dream is cut short when an innocent flirtation leads to Di’s bride-kidnap by another boy. As she is dragged kicking and screaming from her family home, the scene questions the usually detached, ethnographic approach of film-making about marginalised communities. From behind the camera, Ha Le Diem attempts to protect Di by reasoning with kidnappers, but is pushed away; she admits to the young girl later that she did not anticipate the tradition could be so brutal. The decision to leave in such details is particularly thought-provoking, fracturing the supposed neutrality of documentary film-makers.

• Children of the Mist is released on 16 September at Bertha Doc House, London.