This is a documentary from 1991, just after the collapse of the Soviet Union, which also brought about political changes in Mongolia, which till that time had been for 70 years more or less a vassal state of the USSR.
It recounts the purges which were made in the 30s and 40s against the only ideological enemy and rival it faced, which was Buddhism and its organised expression in the temples and monasteries.
For a few hundred years Mongolia has had a form of Tibetan Buddhism, recognising the Dalai Lama as its spiritual head, but after the communist takeover, tens of thousands of lamas were murdered outright, and others forced to disrobe; and most of the monasteries were destroyed and their treasures looted.
Some of the killing fields are shown in the film, and they appear not to be hard to find, as this was basically a countrywide massacre.
While the film was being made the Dalai Lama returned to the country, which suffered a similar fate as his own Tibet, for the first time in over 50 years, and was able to give teachings there again.
The film is only secondarily concerned with Buddhism in the country, and is really a political report on the country following the collapse of the Union, but it throws light on the fate of the religion, which faced one of its greatest challenges from communism at that time.
Since then Buddhism has in fact made a big revival, and the majority of the country now follow their traditional beliefs, but some of the people have been completely secularised during the communist period.