Watching “West of Memphis” is a harrowing experience. Know that from the get-go.

The documentary tells the story of the West Memphis Three, and the fight that ultimately led to their release from prison some 18 years after being convicted of a grisly murder involving three 8-year-old boys. It is ultimately, at least partly, a tale of good eventually conquering evil, of justice prevailing over corruption. But before that point in the story, the viewer must endure footage of three murdered children and of parents confronted with their worst nightmare. It’s a deeply disturbing experience.

Damien Echols, Jesse Misskelley and Jason Baldwin were imprisoned in 1993 for what was portrayed to the media and jurors in the case alike as a ritual slaying of boys who were captured while riding their bikes after school. The boys were found hog-tied and thrown in a stream, their bodies disfigured in a manner that suggested sexual abuse. Arkansas justice was delivered hard and fast. Echols was sentenced to death; his two companions received life sentences.

As three HBO documentaries – the “Paradise Lost” series, directed by Joe Berlinger and Bruce Sinofsky, and released over the course of 15 years – made clear, the case involved a rush to judgment, a confession made under duress by a mentally disabled suspect, the machinations of career politicians, defense evidence that was at first ignored and later suppressed, and a general attempt to create the illusion of an airtight case where none existed. “West of Memphis” avoids being redundant, despite the existence of these previous documentaries, by introducing new evidence based on the findings of an investigative team funded by acclaimed director Peter Jackson (“The Lord of the Rings”) and his wife, producer Fran Walsh.

This team of legal and forensic experts assembled by Jackson and Co. went back through the case in painstaking detail and, essentially, did the job the way it should have been done in the first place. The result is the uncovering of a plot to ease the public’s understandable angst regarding the case, and quite possibly grease a few political wheels for members of the prosecution, by painting the crime as a ritual murder and pinning it on three low-class youths with an interest in gothic imagery and heavy metal music.

Many high-profile musicians and actors – among them Eddie Vedder of Pearl Jam, Johnny Depp, Henry Rollins and Patti Smith – became involved in the case over the years, and they worked tirelessly on the behalf of the three imprisoned youths. But it was Lorri Davis – a New Yorker who started writing to Echols in prison after reading about the case and eventually married him in 1999 – who ultimately spearheaded the campaign that resulted in the trio’s release.

Berg’s documentary has the aspect of a love story, then, and the portions of the film where Davis is filmed listening to recordings of letters and phone calls between herself and Echols are deeply moving.

Despite the fact that Echols, Misskelley and Baldwin were ultimately released, “West of Memphis” does not have a happy ending.

The murderer of the three boys still roams free.

And the fact that three of the poorest of the rural poor – folks who, as Echols says during the film, “wouldn’t necessarily be missed by anyone” – could be railroaded into prison for 18 years after a dog-and-pony show of a trial does not speak highly of small-town justice in the American South.

Echols, speaking while still incarcerated, delivers what may be the most disturbing line in “West of Memphis”: “This is not unusual,” he says, in reference to the miscarriage of justice carried out against these “white trash” teens.

“This kind of thing happens all the time.”