As moving and lovely as the Oscar-winning film biography “Walk the Line” was, we could tell, just watching it, that there was one part of the story that nobody involved was getting right.

And we could tell the filmmakers knew it. Casting sweet and earnest Ginnifer Goodwin as the wife Johnny Cash left for June Carter gave that away.

Vivian Liberto Cash Distin (she remarried) was “very private” her daughters all agree in “My Darling Vivian,” the touching and revealing new documentary about their mother.

Conflicting memories, half-forgotten family lore makes the accounts by Roseanne Cash and her sisters Tara, Cindy and Kathy not sync up, here and there. But this much is clear. This was a love affair and a story straight out of a country song. It’s just that the song would have been one by Tammy Wynette and George Jones, not the Man in Black.

Built on extensive interviews with the siblings — leaning most heavily on the famous daughter, Roseanne — home movies, decades of TV appearances by their father and the movie biographies that “hurt” their mother (“Walk the Line,” “Ring of Fire” and even the parody, “Walk Hard”), filmmaker Matt Riddlehoover finally gives the press-shy Vivian, who died in 2005, her voice — her chance to tell this story.

“Walk the Line” painted a portrait of a provincial wife who needed to be escaped so that Johnny– drug addicted, overworked and troubled — could be redeemed by June. Vivian was all but written out of the Cash saga.

“Honestly,” Cindy Cash says, “I don’t think anyone had an impression of her before that,” and so it stuck.

Riddlehoover, whose prior films were all gay romances and melodramas, sets out to tell the forgotten story of this invisible woman, someone pretty much erased from Cash’s “hero’s journey.”

Vivian Liberto was a Catholic San Antonio girl swept off her feet by handsome Airman Cash when he was briefly stationed there in the service. A thousand letters and tapes he sent her from his station in Germany underscore how “besotted” they were, as Roseanne puts it.

We hear the earliest recordings of Johnny Cash singing in those reel-to-reel letters.

The sisters, interviewed separately, give slightly differing versions of this or that bit of their history. But they all agree that there were several versions of their mother, that she kept it together and kept them going early on, and seemed crushed and broken when their father strayed.

“The happiest years” were in Memphis, right after their marriage and the quick births of Roseanne her sisters. Then Cash moved them west to California to pursue an elusive film career, TV appearances and the like. Pills, even more touring and the eventual split in the family followed.

Vivian Liberto comes off as a tragic and romantic stiff-upper-lip figure who endured repeated humiliations, such as Cash’s El Paso pills arrest, which led to the whole “Johnny Cash is married to a Negress” headlines throughout the racist South thanks to her “exotic” dark looks in court photos. It’s just that she took these blows less stoically than her public silence would have had us believe. Her daughters worried about her health and her sanity even though they were too young to process what they were living through with her back then.

“Vivian” makes for a fascinating account of the psychological scars of a divorce, borne mainly by their reserved, internalizing mother but rippling through to the daughters.

And boy, do we see June Carter Cash in a different light — from family friend to betrayer, to mouthy talk show guest joke-complaining about “how tired” raising “our kids” made her — even those that weren’t hers and that she didn’t raise.

Roseanne, the eldest sister, is 65 now. There’s little trace of bitterness or even edge in her voice, no sense that old scores are being settled, with Nashville lore and Country Music History of that Carter and Cash “epic love story” being rewritten. But man, you can see the satisfaction in Roseanne’s eyes as she and her sisters finally get the chance to give their mother the voice and the platform she lacked over all those decades.

MPAA Rating: unrated, profanity

Cast: Roseanne Cash, Kathy Cash, Cindy Cash, Tara Cash.

Credits: Directed by Matt Riddlehoover. A Film Collaborative release

Running time: 1:30