Asick feeling of dread propels Chinonye Chukwu’s powerful movie: dread at the racist violence about to happen, dread at the racist violence that is then threatened against those standing up against it. This film is about Emmett Till, the black 14-year-old tortured and lynched in 1955 Mississippi for supposedly whistling at a white woman, whose testimony at the subsequent murder trial was disputed. (The closing credits icily remind us that this woman is still alive.) It is also about the boy’s mother Mamie Till, and her courageous campaign for justice, which began with a laceratingly painful decision to have an open casket at her son’s funeral to show everyone the shocking truth. Danielle Deadwyler plays Mamie Till, a calm, determined professional woman and single parent; Whoopi Goldberg has a cameo as her mother, Alma. Jalyn Hall plays Emmett as a smart, extrovert kid with an irrepressible puppyish enthusiasm: cheeky, but no more cheeky than many other teenagers, and mostly just naive. He is sent from his home town of Chicago to stay with family in Mississippi for the summer, with a stern warning from his mother to be careful around white people. Some of the film’s purest nausea resides in the trial itself, with the swaggeringly open racism of the court officials and police. Quite clearly, the fact that any trial was possible at all was down to Mamie Till and the NAACP who took up her cause; it was virtually a private prosecution, the terrible burden for this being placed entirely on Mamie Till herself. Chukwu contrives a stunning coup for the scene in which she arrives at the courthouse, surrounded by NAACP officials and is about to make a quiet, dignified statement to reporters, whose attitude is not obviously different from the jeering crowd behind them; what happens next is best not revealed here, but it is a heartstopping lurch of ironised fear, a bathos of horror. Till is a fierce portrait of courage and a sombre study of the human cost involved in resisting this kind of barbarity. Till is released on 6 January in cinemas.