This movie about a lynching that galvanised the 1950s American civil rights movement opens on a family moment between a single mother and her teenage son. They are close and easy with each other, like good friends. Some gentle banter between them allows us to imagine that 14-year-old Emmett Till was confident and comfortable around adults.

As the adored only child of his widowed mother, he had been staunchly encouraged to believe in himself, as every child should. But this was a confidence that would do him no good in parts of the country where he would soon be going, where black Americans knew their place.

Young Till (Jalyn Hall) had good reason to feel confidence in the future. His mother, Mamie Till-Mobley (Mrs Bradley at the time) had managed very well since her husband died in service in World War II. She was the only black woman at her office, she owned her own home and car and she always appeared in public impeccably dressed. With the camera on her most of the time, Danielle Deadwyler is wonderful in this demanding central role.

In a day or two, Emmett would be off on a holiday, visiting relatives in Mississippi. Mamie is apprehensive about her boy's safety, as he has had no experience of the vicious racist oppression that his own people were experiencing there. It's clear she is afraid of how her son will manage in the South, where violent resistance to desegregation was being waged. It was worlds apart from his life experience in Chicago.

Once in Mississippi, Emmett helped his cousins' family with the cotton harvest. Then, on a visit with his cousins to a store in the tiny town of Money, he flirted with an attractive 21-year-old, the married proprietor, Carolyn Bryan (Haley Bennett), while they were alone inside. Accounts differed, but outside he whistled at the woman for all the world to see. As soon as that happened, he was bundled into a car as he and his cousins fled the scene. There could be deadly retribution for that sort of thing.

Several days later a knock came at the door of the place where Emmett was staying and he was dragged away, lynched by the woman's husband and several others and his body thrown in the Tallahatchie River.

It's a terrible chapter in the history of race-relations in the US, but the violence at the heart of this film from director Chinonye Chukwu takes place out-of-frame. The camera sits in the night-time gloom, observing a distant farm building from which heart-rending cries are emerging and the effect is still intense and distressing.

Although we are spared the murder, the sight of Emmett's broken body brings it all home as the horror is channelled through Mamie's response, often shot in close-up. Then, as she takes control of the situation, demanding that the casket remain open for the world to see what has been done to her son, she ensures that tens of thousands would share her story. What a woman.

Chukwu has chosen to tell this story of Emmett and Mamie Till with lush visuals, interpolated with uplifting jazz music. As a result, Till is a celebration of the upsurge of black American pride and resistance that was brought about by an atrocity. At the same time, it overturns a few expectations. It is told from the perspective of enfranchised and relatively empowered black Americans, whose white fellow countrymen in the Deep South look every bit the grim, ignorant lynch mob.

Even though his father's fate had no bearing on what happened to Emmett, it would have been better to mention in the epilogue what actually happened to Louis Till. Screenwriters Chokwu, Michael Reilly and Keith Beauchamp have omitted a fact that was apparently used to bolster the defence of the men accused of Emmett's murder.

On the other hand it is heartening and uplifting to see how Mamie realised in her grief that she had to continue her campaigning, even after the trial that had allowed the perpetrators to go free. She was an excellent public speaker and she turned to education and activism to further the cause.

However, it is sobering and also shocking to read that the Emmett Till Anti-Lynching Act, making lynching a federal hate crime, has only just been signed into law. Last year.