In the film Karma, Himansu Rai and Devika Rani took part in a ground-breaking moment for Indian cinema: one of the first on-screen kisses. It lasted a full four minutes.

Karma was a landmark in other ways. One of the earliest Indian talkies, it showcased the talents of the new Bombay Talkies studio, and was created and directed by Rai and Rani, who were also a real-life couple.

They had returned to India after falling in love in Berlin, where they had both worked with German film studios. Rai had boldly skipped out of London, where his family had sent him to study law. Sorry, this audio has expired Hear the full story on Books and Arts

Rai's first wife, Mary Hainlin, a German dancer, remained in Berlin with their daughter Nilima, who later emigrated to Australia.

That's where Melbourne businessman Peter Dietze comes into the story.

In his 30s, Mr Dietze found an old photo in the attic. Mr Dietze couldn't help but note that he bore an odd resemblance to the dark-skinned man in the photo. He asked his mother who it was. "That's your grandfather," she replied.

Peter Dietze (right) with his brothers Walter and Paul at the opening of the Bombay Talkies exhibition. ( Supplied: ACMI/Charlie Kinross )

Born in Orange and raised in Melbourne, Peter had considered himself a white Australian of German ancestry, so who was this Indian grandfather?

When Nilima grew up, she didn't share the identity of Himansu Rai with her three sons. She was ambivalent about her father, who had deserted her mother for a glamorous Indian movie star, but her silence also reflected the Australia she had migrated to.

Ruled a "half Indian" under the White Australia policy, hiding her heritage allowed her to take up a new life here.

But the discovery of the photo prompted Peter Dietze to embark on a voyage of discovery into his family's Indian background and their role in the birth of Indian cinema.

Himansu Rai's daughter Nilima emigrated to Australia and married Ernest Dietze. ( Supplied: ACMI/Charlie Kinross )

"Once it did come out, it was a very centring moment for myself," Mr Dietze told Books and Arts. "It explained a lot about my personality and character.

"It wasn't just the German and Australian side, all of a sudden there was a mixture that made sense to me. I think I get a lot of influence in the creative side from my grandfather."

On a business trip to New York Peter stumbled across another unexpected treasure. After Himansu Rai's death at the age 44, Devika Rai married the Russian painter Svetoslav Roerich.

She kept all the documents connected with the Bombay Talkies studio, and after her death in 1994 they ended up in a museum dedicated to the Roerich family in New York.

"I visited the gallery and met the curator there, and after listening to my story, he said, 'I think I have something for you.' He brought a box of material," recalled Mr Dietze.

"I had a look at the first photo on the top, and I was already mesmerized by Devika Rani, by this stage, because she was such a beautiful woman."

Much of the Bombay Talkies collection was preserved because it was moved to New York. ( Supplied: ACMI/Charlie Kinross )

All told, the collection contained 3,000 items, including film stills, business letters and telegrams. Together it comprises one of the most comprehensive collections of 1920s and 1930s Indian film studio ephemera in the world.

Fiona Trigg has combed through all the boxes to curate Bombay Talkies, a new exhibition at the Australian Centre for the Moving Image in Melbourne.

"It's really rare and it's really significant. The climate of India and aspects of Indian culture don't work for fantastic archival practice, especially for commercial concerns like a film company," she said.

"Many film companies around the world don't necessarily look after their archives, because that's not their main business, their main business is making films."

Bombay Talkies made films about contemporary India and employed people from all castes. ( Supplied: ACMI/Charlie Kinross )

Bombay Talkies was a unique company in many respects: it made films that reflected contemporary India rather than an exoticised past, it encouraged employees of all castes to mix together and it was co-run by a woman.

"It is still rare for a woman to be running a film studio," said Ms Trigg.

Peter Dietze has visited the Bombay Talkies studios, which have been repurposed into offices and repair shops, and hopes that the archive he has inherited will find a permanent home in India.

After all, the Bombay Talkies story isn't just important to his family. As he puts it, "It's a national treasure for India."