There is a special fascination about lost documentary film that turns up decades after the events it depicts have passed into history.

Last night one of the 20 episodes of "The Unknown War" was screened at the National Archives. It told the saga of the siege of Leningrad, a high point in Soviet Russia's epochal stand against the Nazis in World War II.

The series will be shown at the Archives Sept. 14 through Dec. 8, Thursday nights at 7:30 and Fridays at noon and 2:30.Later this fall it will be screened on TV in nine U.S. cities and 15 foreign countries including West Germany.

For those who remember, Russia's war certainly was not "unknown": Stalingrad was usually considered the war's pivotal battle, the invasion of Europe coming only after the basic issue was settled. But even for these old-timers, the film should be a revelation.

It is part of an astonishing cache of 3 million feet of war footage made by 250 Soviet photographers, 50 of whom died in action. After a small amount was cut out for contemporary newsreels, the miles of outtakes were stored, mostly unseen even by the Russians.

Some captured German film is woven into the documentary to give a nice balance. Burt Lancaster, as narrator, shows us scenes of Leningrad today, in color, contrasting almost too neatly with the black-and-white shots of devasation and death.

The series, put together by Air Time International, a small but ambitious media firm, was produced and directed by Isaac Kleinerman, editor for the "Victory at Sea" TV series, and was blocked out by Harrison E. Salisbury, the much-honored New York Times reporter, one of the first correspondents to enter Leningrad three days after the siege lifted in 1944 and later an authority on Russian affairs.

The film can't help but be moving, after haunting. We see straving people boiling wallpaper to get the paste off it. We see people hauling chunks of telephone pole, grinning, for wood was almost as valuable as life itself during the 900-day ordeal. We see people falling dead in the street. And the people who carry off the bodies fall dead themselves.

After the terrible first winter, some ships get through on Lake Ladoga, and we see small children being evacuated, running on small, stockinged legs up the gangplank, smiling for the camera. And then we see the ship strafed, bombed and sunk.

It is believed a million people died in the 900 days.

For those of us never visited by war in our cities, the film brings the experience alive as vividly as the great British documentaries of the Battle of London. Perhaps the most affecting sequence is the reenactment of the Leningrad premiere of Shostakovitch's 7th Symphony, written while the composer lived in the encircled city, and played in the fall of 1943 with musicians who commuted from the front (the streetcars ran to within a few blocks of the trenches.)

We are taken to a memorial concert a generation later, with the original musicians and the original audience. While the ominous invasion motif beats on our ears, the camera pans through the audience - there can't be more than two dozen of them left - and down the rows of empty chairs on the stage, each with an instrument lying on it.

(Music coordinator for the film was poet Rod McKuen, and executive producer on the Soviet side was the late Roman Karmen, dean of Russian documentary film producers.)

Salisbury told of roaming through the city just after the siege and learning of the casual heroism and the familiarity with death. In his later years in Russia, he said, he could always spot a Leningrad survivor in any group: It was something about the person's bearing.

Nicholas Laskovsky of Arlington, one of those few, attended the screening. The reality was worse, he said.