May 2002. For several weeks, the documentary filmmaker Mosco Boucault has been filming the daily lives of police officers at the Roubaix police station, when two young women appear in front of his camera, suspected of having strangled a woman in her seventies. They gradually admit to having killed the old lady to cover up a burglary at her home.
Arnaud Desplechin describes his horror when, six years later, he discovered Roubaix, commissariat central, the result of that exploration of police mysteries, released after the conviction of the two lovers. At first he was gripped by the horror of the evil act committed by the young women, then by their humanity, which seeps through their confessions and abruptly blurs their image as cold criminals.
"For the first time, inside two criminals, I discovered sisters. These were wonderful roles to give to two actresses," says Arnaud Desplechin, who explores the boundaries of humanity in Roubaix, une lumière (Oh Mercy!) , his thirteenth feature film. The filmmaker explains that he reworked the raw material provided by the documentary to build a fictional account which stays close to reality and depicts the "soul's worst torments".