As personal as it is, this new documentary is likely to be a revelation for many. It is a record of the quest by a young Australian opera singer to discover more about his grandfather, a charismatic and outspoken Yorta Yorta and Wiradjuri man, who was a leading Indigenous activist long before the well-documented social movements of the 1960s.

William 'Bill' Onus began to advocate for equal rights for his people in the 1930s, after his work took him up north and out west as a boomerang thrower in the travelling shows touring the country at the time. His story is told by his grandson, academic and bass baritone Tiriki Onus, whose voiceover guides us through the many chapters of an extraordinary life.

It was the chance discovery inside a vault at the National Film and Sound Archive of an untitled 75-year-old silent film with Bill Onus and his brother Eric in the frame that really got things under way. The 11-minute film contains rare moving images of Indigenous cultural performance on the Melbourne stage, and it also suggests that Bill Onus was Australia's first Aboriginal filmmaker.

Bill was born early last century at Cummeragunja station mission on a bend in the Murray. As an adult touring the country, he witnessed the humiliations of segregation and the brutalities against Aboriginals, such as "chaining". He bought a Box Brownie camera to bear witness to what he saw.

As he grew more aware of the plight of his people, he quit the travelling shows in the mid-1930s to learn how to make movies. He was developing an understanding of the power of art to contribute to social change.

He began to learn about filmmaking on the set of Charles Chauvel's film Uncivilised, a travesty of Indigenous representation if ever there was one. But in 1946 his advice to the British director Harry Watt on his epic droving feature, The Overlanders, is reflected in the accuracy of representations of the Indigenous drovers and their culture.

Tiriki's quest to know more about Bill takes shape as a detective story. There is a trip, torch in hand, to the basement of the family home which unearths a trove of Bill's photographs and other material, and more trips to the archive. There are interviews with "aunties" who remember Bill, and chats with experts in military history. Piece by piece, Tiriki begins to build a picture of the grandfather he never met.

The sleuthing trope brings a light touch, leavening confronting material that can be difficult to watch and comprehend whatever your background. Tiriki has co-directed with filmmaker Alec Morgan (Lousy Little Sixpence), who used a similar style in his documentary, Hunt Angels of 2006, a lively combination of archival footage and photographs, weaving animations with acted sequences.

A new phase of Bill's remarkable life unfolded in the Melbourne suburb of Fitzroy to which he and many members of his family relocated after a mass walk-off from Cummerajunga. Conditions for the inner-city Indigenous community were dire in the days before the referendum of 1967. Indigenous children were raised on such an impoverished diet that an orange in a Christmas stocking was a special thrill, and racism in the police force was rife.

The film footage from the NFSA also contains interesting, though incomplete, details about distinguished Aboriginal veterans who had had to quit their postwar jobs because of racism. And they were banned from marching with non-Indigenous veterans on Anzac Day, barred from the RSL clubs, and more. No wonder Indigenous activists like Bill Onus forged links with prominent activist Black American entertainers Harry Belafonte and Paul Robeson.

Tiriki is the child of Bill's marriage to Miss Communist Party 1947, Mary Kelly, the daughter of a wealthy family who opposed the union. Bill Onus would eventually earn Commonwealth investigation and ASIO files all his very own, as he connected with unionists and overseas entertainers in his lifelong quest for Indigenous equality.

In the final moments of Ablaze, there is a photo of Tiriki standing proudly in the magnificent possum-skin cloak that tells his life story. It's a simple gesture, a match with the opening scenes of Tiriki wrapping his baby daughter in a possum skin rug, and makes a profound point about retaining cultural expression, language and community.