What does guilt look like? Or innocence? What is the correct way to act when accused of a hideous crime? This smart little true-story-inspired courtroom drama from writer-director Stéphane Demoustier wants you to wonder, encouraging us to fill in the blanks when the girl in question doesn't conform to our expectations. The bracelet she's wearing is an electronic tag and the mystery suggested by that somewhat enigmatic title is the murder of her best friend.

Lise Bataille (Melissa Guers) seems an inexplicable culprit for the crime: a frenzied knife attack, supposedly carried out when she was just 16, on someone she has known since infant school, who she, initially, seems to have no motive to harm. But Lise surrenders herself to the police all too willingly, clams up in court, and can be oddly uncooperative and very matter-of-fact. Even when the evidence leans in her favour, she can't help but make things worse again. The lack of any other suspects inevitably makes you think.

We follow the twists and turns of the fascinating trial along with Lise's helpless father Bruno (Roschdy Zem), who wears an expression of permanent concern; her mother Céline (Chiara Mastroiannni) is absent, insisting she has to work. As events unfold, some things are easily explained away, others seem to firmly implicate Lise. The prosecutor (Anaïs Demoustier, the director's sister) is bad-tempered and unusually bent on conviction. The social lives of the teens become unlikely courtroom fodder, to be dramatically unpicked, with Lise's sex life judged as especially shocking. The sexist nature of such institutions means Céline's mothering, too, will be placed under the microscope.

Despite the inscrutable individual at its centre, the film is not always terrifically subtle, particularly in a handful of the courtroom scenes. However, the characterisation is believably complex and the performances suitably nuanced, drawing you in as we try in vain to read the characters' thoughts, while the accompaniment of sporadic bursts of Hitchcockian strings lays on some classic thriller tension. As it asks us to play detective and explores how well we can ever know those we hold dear, it's all desperately intriguing.

Available to watch on Curzon Home Cinema from Fri 26 Jun.