Documentary filmmaker Daniel H. Birman had directed just one feature, about “Miracle on the Hudson” pilot Chesley Sullenberger, when he decided to tell the story of convicted murderer Cyntoia Brown.

The result was Me Facing Life: Cyntoia’s Story, which aired on PBS’s Independent Lens in 2011. At the time, Brown was 23 years old and had been incarcerated since 2004, after she shot and killed 43-year-old Johnny Allen. He had picked her up and paid her for sex. She was 16 when the crime occurred, but was tried as an adult in Tennessee.

Clearly, Birman never forgot the case. His newest feature, Murder to Mercy: The Cyntoia Brown Story, takes Netflix viewers from the murder on Aug. 6, 2004, through the trial, various appeals, and a last-ditch plea for clemency from outgoing Governor Bill Haslam.

If you’re aware from news accounts of how it all ends, then the film’s slightly tweaked sense of suspense and slow-piano accompaniment will be wasted on you. But regardless, this is a fascinating look at how a criminal and a legal system can change over the course of 15 years.

Much about a personality can be shaped in editing – just watch Tiger King – but there’s no denying the fact that Brown is no longer the same person

On the latter count, Tennessee’s judicial system would no longer treat a 16-year-old the way it did Brown in 2004. Among other things, she would no longer be classified as a prostitute at that age, but rather as a victim of sexual predators. And lawyers admit that fetal alcohol syndrome, a mitigating factor brought forward at an appeal hearing in 2012, was not something they considered when the case originally went to trial.

Perhaps less surprising is the fact that Brown grew up. The 16-year-old in early footage of her incarceration is angry, confused and, in her own words, “old, tired, weary.” A psychological assessment involved her looking at simple, ambivalent pictures and making up stories about them; hers are violent and without resolution.

But as the years pass, she becomes a thoughtful and intelligent woman. She earned her GED in prison and then went on to earn two degrees from Lipscomb University. Much about a personality can be shaped in editing – just watch Tiger King – but there’s no denying the fact that Brown is no longer the same person as she was in 2004.

That said, Bermin’s portrayal is remarkably one-sided. The only time we hear from anyone on the murdered man’s side is a brief impact statement in that final clemency hearing. Prosecutors and the police are shown in the most unflattering light.

Interestingly, one of the original prosecutors later met Brown in a class he taught in the prison, and was so struck by her maturity that he spoke in her favour at the final hearing. So while I’m not suggesting that Bermin is trying to pull one over on us, he might have offered a little more balance in the picture.

By 2017, Brown’s case had become a literal cause célèbre, with the likes of Rihanna and Kim Kardashian tweeting for her to be released. By the end of the film, she scarcely needs Birman’s well-meaning, one-sided support any more.

Murder to Mercy is available April 29 on Netflix.

3.5 stars out of 5