I JUST saw a remarkable wee film, and I'm buzzing. They Also Serve, made in 1940, a black-and-white documentary, just nine minutes long, was intended as second world war soft propaganda. Its end title says: Housewives Of Britain - Thank You For Your Courage And Help. Some of the docs from those days are like Harry Enfield spoofs - scratchy, posh-spoken educational films. Others, by the legendary Stirling-born mastermind of the British documentary movement, John Grierson, or by Paul Rotha or Humphrey Jennings, are poetic. Many are about factories or farms, trains or shipyards. Most are quite male.

What was so exciting about They Also Serve was that it was not only about women, but was a successful attempt get inside the head of one of those housewives. We hear her thoughts, as she lies in bed beside her husband or washes the dishes. This was radically new.

Who made this wee movie? There's no name on the credits, but looking up the director, I discover that it's Ruby Grierson, John's younger sister. What do I know about her? Born in 1904, she trained as a teacher and died, tragically, in 1940, when a boat she was filming on for a documentary about children being evacuated to Canada was torpedoed. I've always thought of her as a woman in the shadow of a legendary brother. According to the famously comprehensive website for film buffs, the Internet Movie Database, she made just two films; yet she really directed 13. She's in very few film history books. Not only was Ruby Grierson's life curtailed and penumbral, it is also only patchily remembered.

One of the shocks of watching British documentaries of the 1930s and 1940s is that moment when - perish the thought - working-class people actually talk on camera for the first time rather than being discussed by a posh commentary. This happens in Housing Problems, directed by Arthur Elton and Edgar Anstey in 1935. But look back at the memoirs and recollections of the crew of the film, and you discover that it was one RI Grierson - our Ruby - who, uncredited as production assistant, put the Stepney people living in the slums at their ease in the movie. And Ruby asked the questions in the film too.

Then jump two years later, to 1937's short doc Today We Live, which tells two stories about people setting up community centres. The one about the women, directed by RI Grierson (the name Ruby still doesn't appear), is by far the better. Not only is the lighting less harsh and the focus shallower, the women are far more relaxed and having more fun than the stilted men in the other segment.

There's a pattern emerging here. While the boys of British documentary's golden age brought scale and verve to non-fiction film, while they convinced politicians that regional and class cohesion could be created through film, it was a lass from Stirling with major social skills and a head full of ideals who did the fine tuning. Ruby it was who was interested in that moment when you take your big bulky camera into someone's life. She knew that if the camera was accompanied by big lights and Oxbridge boys in suits and ties, as it often was, this might not only be scary for real people, it would knock the realness out of them; the intrusion would make them stiff and fake. If you were most interested in messages, in instruction, in state-of-the-nation themes, then a bit of stiffness didn't bother you. But if you cared about rendering individuals with a bit of an inner life on screen, then it bothered you a lot. Judging by Ruby Grierson's movies, she was bothered a lot by the detachment of British documentaries, and did something about it.

It would be good to be able to say that, if only she had lived, she would have made even greater films. But look at the career of one of the few other female directors of the 1940s - Jill Craigie, who later married Michael Foot. Her 1940s and 1950s films were very good too, but her credits contain no films between 1951 and 1995's Two Hours From London (which I programmed when I worked at the Edinburgh International Film Festival). Maybe Craigie had other fish to fry in those 44 years, or maybe the documentary world was just that bit too male. I suspect the latter. And I suspect Ruby Grierson might have had something to say about that.

*Land Of Promise: The British Documentary Film Movement 1930-1950 DVD box set is distributed by the British Film Institute, £34.99, from tomorrow