When Danny Williams mysteriously disappeared in 1966, he had already made substantial contributions to some of the most vividly defining American pop culture of the century, despite only being in his 20s. In “A Walk Into the Sea,” a new documentary opening today at Cinema Village, Williams’s niece, the filmmaker Esther Robinson, lays claim to her uncle’s forgotten legacy using the on-camera testimony of those who knew and worked with Williams.

Cinema vérité pioneer Albert Maysles recalls Williams’s editorial assistance in shaping the Maysles brothers’ films “Showman” and “What’s Happening! The Beatles in the U.S.A.” into watershed moments in film history. Photojournalist Nat Finkelstein describes how Williams’s lighting design for Andy Warhol’s touring “Exploding Plastic Inevitable” showcase accrued as much press attention as the Paul Morrissey-directed films, the Velvet Underground’s musical performances, and the onstage gyrations of Warhol’s Factory “superstars,” which Williams’s effects accompanied. The filmmaker Ronald Nameth attests to Williams’s little-known behind-the-camera success in taking the Factory cast in a very different cinematic direction than the one Mr. Morrissey and others pursued under Warhol’s aegis.

Ms. Robinson also attempts to unravel the tangle of petty cruelties and capricious rejections surrounding Williams’s ouster from Warhol’s inner circle. The brilliance that earned Williams a seat near the head of Warhol’s creative table and for a time a corresponding place in the artist’s bed led the sensitive Harvard dropout into direct conflict with the volatile egos that made up the Factory scene. A vintage, Williams-shot black-and-white, slow-motion glide down Billy Name’s theatrically lit face cuts to Mr. Name in the present, barely recognizable behind wraparound shades and 40 years of aging, describing the Factory’s social snake pit and his own romance with the man to whom nearly everyone interviewed refers simply as “Andy.”

“Chelsea Girls” star Brigid Berlin’s fond recollections of Williams climax with an on-camera phone call to another Factory crony who had declined to participate in the documentary. “Hello? Genvieve? It’s Brige,” Ms. Berlin enthuses into the phone as if calling for a recipe. “Were Danny and Andy really lovers? They were!?” Music business wunderkind Danny Fields dishes with similar exuberances. “Thank god!” Mr. Fields says on the subject of Warhol and Williams’s tryst, allegedly one of the few physical relationships the nearly asexual Pop art icon had. “Andy was sooo weird!” The fact that Williams acquired a debilitating drug habit while living in “Andy’s” brownstone and then in the Factory building did not help him to navigate the minefield of professional and personal jealousies fomenting within Warhol’s retinue. Strung out and dumped by Warhol, in the summer of 1966 Williams returned home to his family in Massachusetts. One evening he followed a late meal with a drive to the seaside and he was never seen again. Though presumed to have drowned, his body was never recovered and a death certificate remains unissued to this day.

At the same time that Ms. Robinson uses her late uncle’s footage and the memories of his contemporaries to restore Williams’s place in counterculture history, she also explores her own family’s reaction to its loss. In the process, “A Walk Into the Sea” lays bare the guilt and denial that Williams’s mother and siblings (Ms. Robinson was not yet born when Williams vanished) maintain to this day. Though Williams’s mother, Nadia Williams, implicates Warhol’s “domination” of her son as the root cause of his unhappiness, during the course of Ms. Robinson’s stately paced film, Mrs. Williams’s remembered relationship with her son becomes its own four-decade-old smoking gun.

Paging through old photos of Williams at work and play with Warhol, Mrs. Williams’s comments evoke a history of control and domination of her own. A simple, offhand observation about Danny’s weight gain brings his mother’s and his lover’s undeclared rivalry to the surface. Some 40 years after Danny’s life apparently ended, it appears that his mother is still trying to shape her son into the person she wanted — wants — him to be.

Those who made up the Warhol crowd, says the Velvet Underground’s John Cale, were mostly “incomplete people who found a certain completion” in the sybaritic, narcissistic, and competitive Factory scene. Though it is a marvelous oral history of a time and place rarely explored with such unromantic honesty, the haunting thing about “A Walk Into the Sea” is that it gently exposes the ultimately fatal psychic wounds that Danny Williams received at the Factory and those he brought with him.