A quarter century of cataloguing and restoring the priceless early film images began at the newly launched Scottish Film Archive, with a garden shed and a rusty key. Janet McBain and her co-conspirators were given the key to a shed in which resided some 400 stour-encrusted cans of film, handed for safe keeping into the arms of the Scottish Film Council.

That was 1976. This week Janet McBain and the rest of the archive staff celebrate their silver jubilee and a journey of discovery through thousands of disinterred treasures. A sort of This is Your Life Scotland, encompassing a documentary on Clydeside shipbuilding which won an Oscar, to the most wonderfully toe-curling examples of US-funded tartan kitsch.

The shed proved to have been the guardian of some eclectic film-making adventures; the the jubilee of Glasgow trams in 1922, priceless footage of the St Kilda families before the evacuation. Inspired by having mined so much cinematic gold from such a narrow seam, Janet and co, rather rashly, launched an appeal for more of the same. "And almost instantly we were deluged with some 4000 more cans. Some arrived in brown paper bags, some had no labelling at all, and at that stage there was still just the three of us. In some cases I suspect we were springing material from folk's lofts and cupboards, which they had been reluctant to send to London and the National Film Archive."

A quarter of a century on, the archive now has some 22,000 films, the vast bulk of them documentaries. In the mid-nineties the Heritage Lottery Fund came through with a £500,000 award for eight new archivists to tackle a backlog which then still stood at some 13,000. It's clear that McBain is not just the curator of the endeavours of this country's film pioneers, but of priceless insights into our social history.

One of the two earliest films, made in 1896, records Queen Victoria in pony and trap at Balmoral, surrounded by Europe's regal glitterati to most of whom she was related. The other contender for oldest footage, bizarrely, features Dr John McIntyre in Glasgow Royal Infirmary performing experimental X-rays on frogs' legs.

We had to wait till 1911 for the first fiction film, a stirring tale of Rob Roy filmed in Rouken Glen, near Giffnock, with a glamorous location trip to Aberfoyle.

The Rouken Glen studio had begun life as a tram depot and its power supply still came from the tramway power station. One tram passing meant the light faded, two trams passing, and the game was a bogey. Little wonder that the post-war production unit at Rouken Glen, The Ace Film Company, upgraded giving them "lighting installation equal to 800,000 candle power". Even Harry Lauder was enthused enough by new media to launch a film company in 1920 and to make two films, I Love A Lassie, and All for the Sake of Mary - torrid romances starring the great man himself, which did the nation a favour by never being released.

There's little doubt, though, that the archival gems are in the documentary field, from the Gordon Highlanders marching through Aberdeen in 1899 to the George Square riots in 1919, and the return of prisoners of war to Leith in the same year. The fashion for industrial giants to record their processes give us the life and times of companies like Colvilles and Singers, while MacBrayne commissioned travelogues of their routes. Cinema owners, with a shrewd eye to the box office, often made newsreels of local gala days. "You'd see lots of panning shots with as many faces as possible on the assumption that they'd come along next week and pay to see themselves on screen," says McBain.

Glasgow Education Department became the first authority to put projectors into schools. Thus it was that denizens of Maryhill and Pollokshaws, strangers to sheep, were treated to the life and times of the white rhino and the aardvark. These were shot by Elder Dalrymple Films, an Ayrshire outfit which spent 18 months travelling from Cape Town to Cairo with a camera platform on the roof of their jalopy.

The documentary luminaries are predominantly male, but Jenny Gilbertson's untutored work in her native Shetland shines brilliantly through, as do her films made in the Arctic when she was in her seventies.

Gilbertson it was who hand-edited her highly-combustible nitrate film in front of an open peat fire, and whose own archive was unearthed in a henhouse. Now known, for all the obvious reasons, as the guano collection.

Ask Janet McBain for the archive's proudest achievement and she'll point to the new print they made of Seawards the Great Ships, the 1960 Oscar-winning film of Clydebuilt craftsmen. Damaged and fading, it was renovated at a cost of £14,000, twice the price of the original commission. "When we watched the original again, we found the voice of Bryden Murdoch had been replaced by Kenneth Kendall," says McBain. "Apparently the British Council thought Bryden sounded too Scottish."

But Bryden is back in the new version, one of many of the archive films you can now buy on video.

The celebrations began at the Glasgow Film Theatre at 6pm on Tuesday with a programme of McBain's favourites, and move on to Edinburgh Filmhouse on December 3 at 6.30pm and DCA, Dundee, on the 4th at 8.30pm; both nights, The World in 1900. Finally, the Belmont Cinema in Aberdeen is the venue on the 6th for The Granite City on Screen.