Christmas Eve in Roubaix, Paris. Police captain Théod is on patrol. Witnessing burnt-out cars, violent fights... facing poverty, fraud, and calls for help every day, Théod knows who is right and who is wrong at a glance. Louis, fresh out of the police academy, joins Théod's team. Young and inexperienced, Louis is easily misled and often misunderstands the people and situations he encounters. Théod and Louis are called to investigate the murder of an old woman. Two young women who were neighbors of the old woman, Cotillard and Marie, are arrested. They are drug addicts, alcoholics, and lovers...