Filmmaker "Liz Garbus paints a brave and provocative picture of Nina Simone – artist, civil rights activist and heroine in a film that we’ve loved from day one and are proud to bring to viewers,” says Lisa Nishimura of Netflix and if there's any questioning the film's authenticity then ask the film's Executive Producer, Lisa Simone Kelly, Nina Simone's daughter.

"What Happened Miss Simone?" takes us on a journey to better understand the slow unraveling of an artistic genius at a time steeped in America's racism and the impact on one of its greatest talents.

Nina shares of her friendship with Lorraine Hansberry, the celebrated playwright, who converted Simone to political activism before dying too young in 1965.  Simone’s album Black Gold, recorded at New York’s Philharmonic Hall on 26 October 1969, concluded with a deeply moving performance of To Be Young, Gifted and Black a tribute to her beloved friend.  The song took its name from a hit play about Lorraine and as she speaks about Hansberry, Simone’s voice creaks with emotion. “I think that very soon now, maybe four or five weeks, I won’t be able to sing it anymore because each time I do it she comes a little bit closer and I miss her a little bit more.”

We see rows of black students enraptured by the song she called “the Black national anthem”, her biggest hit since I Loves You, Porgy a decade earlier. It had the pride and optimism of the cover versions she had alchemised into civil rights anthems: Feeling Good, I Wish I Knew How It Would Feel to Be Free, Ain’t Got No/I Got Life. But her Philharmonic performance revealed the undercurrents of pain and loss that were slowly dragging her under.

The tremendous power of Simone’s music has entranced the likes of Kanye West, Antony, Laura Mvula and Lauryn Hill. In 2008, President Obama named her version of Sinnerman as one of his 10 favourite songs. She sang in an almost androgynous baritone that she said ranged from “gravel” to “coffee and cream”, played piano like a virtuoso and interpreted material with power and imagination. When she covered a song it stayed covered, although it’s ironic that her career was revived in 1987 thanks to a perfume commercial that featured 1958’s My Baby Just Cares for Me because that perky jazz standard (which she once called “a piece of shit”) told listeners nothing about who she really was.

There was always something regal about the “High Priestess of Soul” who, at one point, claimed to be a reincarnated Egyptian queen: Nadine Cohodas’s excellent biography is called Princess Noire. “She is loved or feared, adored or disliked, but few who have met her music or glimpsed her soul react with moderation,” Maya Angelou wrote in the 1970 magazine article that gives Garbus’s film its name. “She is an extremist, extremely realised.”

What Happened, Miss Simone? makes the case that Simone was not only one of the most talented musicians of the 20th century but one of the most troubled and unlucky. It shows how she always felt she had been denied her true calling; how she never achieved the success that prettier, more biddable singers enjoyed; how she invested so much of herself in the civil rights movement that she was shattered when it faltered; how she suffered physical abuse from her husband and manager Andrew Stroud and inflicted it on her daughter Lisa; how her bipolar condition was only diagnosed in the 1980s, long after her volatility had inflicted irreparable damage. She was an outcast who only briefly found safe harbour — first as a wife and mother, then as an activist – before it was snatched away. Not fitting in made her great, but it also made her angry and very lonely.

Nina Simone was born in Tryon, North Carolina, in 1933. As young Eunice Waymon, playing piano in church, she felt that “I was a black girl and I knew about it.” After high school, the Curtis Institute of Music in Philadelphia rejected her application because, she believed with good reason, she was black. “I never really got over that jolt of racism,” she said.

With her dream of becoming America’s first black, female concert pianist crushed, Simone turned to performing in bars to earn a living, a job she regarded as such a humiliating defeat that one reason she assumed a stage name was so that her mother wouldn’t find out what she was up to. Neither jazz nor blues nor folk nor soul, she became a musical anomaly, perhaps because all along she would rather have been playing Bach. Decades later she said: “I think I would have been happier. I’m not happy now.” She defined her indefinable style as “African-rooted classical music”.

I Loves You, Porgy launched her career. She shared bills with Miles Davis and Bill Cosby and fulfilled a long-held dream of playing Carnegie Hall, although she always had a reputation for being temperamental. In 1961, she settled in Mount Vernon, New York, with Andrew Stroud and gave birth to Lisa. “The good, sweet days,” she called them. But she needed more and she found it when Lorraine Hansberry, who lived nearby, encouraged her to join the civil rights movement. Hansberry’s words were nagging at her on 15 September 1963 when a bomb planted by a white supremacist killed four black schoolgirls in Birmingham, Alabama. Her first instinct, she recalled in her memoir I Put a Spell on You, was to build a gun and shoot the first white person she saw. Stroud told her to write a song instead.The furious, darkly playful Mississippi Goddam sprang from her like a bullet, expressing black anger in a way that never had been heard in American music before. “We all wanted to say it,” comedian and activist Dick Gregory says in the film. “She said it.”

Two years later, she performed it during Martin Luther King’s famous march from Selma to Montgomery. Simone befriended such towering figures as King, Malcolm X, James Baldwin, Stokely Carmichael and Langston Hughes. Singing for the movement filled the hole in her heart and playing protest songs like Hughes’s Backlash Blues and her own controversial Four Women took her to “a state of grace” after a decade spent in the “nothing world” of pop. “I was needed,” she said later. “I could sing to help my people and that became the mainstay of my life.” In the film, Lisa Kelly Simone says, “I think my mom’s anger is what sustained her.”

Stroud, who thought politics was bad for business, thought she was becoming a “barking dog” but as a girl she had been taught to be silent for self-preservation and she wasn’t going back there. “All my life I’ve wanted to shout out my feeling of being imprisoned,” she said in 1967. “I’ve known about the silence that makes that prison, as any Negro does.” At a time when black artists weren’t supposed to mention racism, she was as militant as Public Enemy. She described American society as “nothing but a cancer” and treated one black audience to a poem by David Nelson of the Last Poets: “Are you ready to smash things and burn buildings?” This surging radicalism coincided with the first signs of serious mental illness, including violent furies, blackouts and hallucinations. “I had visions of laser beams and heaven, with skin – always skin – involved in there somewhere,” she wrote.

One of Simone’s most powerful concerts, captured on the ’Nuff Said! album, took place at Westbury Music Fair on 7 April 1968, three days after Dr King was shot dead. She grieved on a brand-new song called Why? (The King of Love Is Dead), then raged on a white-knuckle version of Mississippi Goddam. “The king of love is dead,” she said. “I ain’t ‘bout to be nonviolent, honey!”

Simone was now less a musician who cared about politics than an activist who sang. Her music was by, about and for black people. She would scan the crowd for black faces and tell them, “I’m singing only to you. I don’t care about the others.” White fans, she said, were “accidental and incidental”. She could not ignore the fact “that I was a black-skinned woman in a country where you could be killed because of that one fact.” No wonder there has been controversy over the casting of beautiful, light-skinned Zoe Saldana in the long-delayed biopic Nina.

Although she was outwardly still engaged, Simone claimed she lost faith in activism in 1970, when the movement fractured and the revolution didn’t come. “Optimists talked about the advances we had made, but all I saw were lost opportunities,” she wrote. Some of that energy was redirected into a new kind of music. Simone lived on stage and didn’t care much for albums. Leftover songs from the 1971 RCA sessions that produced the Here Comes the Sun album were later cut-and-shut with live recordings, often overdubbed with applause to conceal the joins. No longer writing, she expressed a potent personal vision entirely through diverse cover versions. There is nothing to connect the slow-burning soul of Steppenwolf’s The Pusher, the polyrythmic frenzy of Ike and Tina Turner’s Funkier Than a Mosquito Tweeter and the desolate, barely-there jazz-folk of Fairport Convention’s Who Knows Where the Time Goes? except Simone’s formidable conviction.

During an antiwar show at Fort Dix, New Jersey in November 1971, she fused George Harrison’s My Sweet Lord and David Nelson’s poem Today Is a Killer into a jawdropping gospel rollercoaster that concluded with a blasphemous cry of: “Who are you, Lord? You are a killer!” Included on Emergency Ward! (1972), My Sweet Lord made good on her promise “to shake people up so bad that when they leave a nightclub where I performed I just want them to be in pieces.” Sick of America, on the ominously titled It Is Finished (1974) she explored African percussion, sitars and the strange, chilling visions of the maverick Bahamian musician Exuma.

These albums sold poorly and didn’t even rate a mention in her memoir, but they leave you salivating at the thought of what she might have done next had she not abandoned activism, the music industry (“the dirtiest and most immoral business in the world”) and the “United Snakes of America.”

When Simone returned to the stage, all the solidarity and hope of the movement had been burned away, leaving only rage and resentment that was intensified by her illness. Garbus’s film shows her hostile, imperious comeback show at the 1976 Montreux Jazz festival but not the time she told an audience at the music industry conference Midem, “You are all crooks!”, nor the calamity in Pamplona where she told the crowd: “I don’t sing for bastards. I don’t like white people.” She didn’t just rock the boat; she tried to sink it.

By the decade’s end, her finances were such a wreck that when she returned to the US to stand trial for tax evasion she couldn’t even pay her hotel bill. At another bad show, in New York, the audience cried: “We love you Nina!” She snapped back: “Loving me is not enough. I want my money.” Her 1978 album Baltimore was solid but a long way from the thrilling ferment of It Is Finished. When interviewers asked her about politics, she sounded bitter and bereft. “There’s no civil rights movement. Everyone has gone.” She reflected that “artists who don’t get involved in preaching messages are probably happier”.

In the 25 years before her death in 2003, at the age of 70, Simone was relatively stable and prolific. She didn’t experience the triumphant comeback that she deserved, but this was as happy an ending as could be expected from someone who had once appeared to be waging a one-woman war against everything.

As she left the stage after a disastrous show at the Royal Albert Hall in 1978, Simone declared: “I am not of this planet. I do not come from you. I am not like you.” She was in a terrible state, but on that point she was absolutely right. For good and for ill, Nina Simone stood alone.

What Happened, Miss Simone? is available on Netflix from 26 June.

Excerpts taken from an article written for the Guardian by Dorian Lynskey