For roughly its first half, the film of Isaac Stern's 1979 voyage of discovery to China seems mainly another variation, particularly lively and genial, on the theme of East meets West. Then it changes gears startlingly, into some riveting reminders of the destructive force of the Cultural Revolution.

"From Mao to Mozart: Isaac Stern in China," which opens today at the Key, is told with humor and sensitivity as Stern is shown rehearsing with Peking's Central Philharmonic, giving recitals, teaching master classes, listening to Chinese instruments being played and watching some dazzling martial arts workouts. The recurring theme is the immensity of the gap between Western arts' complexity and the Eastern perception of it -- and vice versa.

Stern is typecast for this sort of film. He plays the expansive master musician with grace and without a touch of condescension. And it is clear in this footage that the novice Chinese players are not only at ease with him, but, more importantly, are actually learning from him some of the subtler lessons of the art. He observes early on that the musicians he is encountering are "not accustomed to playing with passion and variety of color." And it is these matters of the mind, more than tricks of the fingers, that he concentrates on.

The high point of the film's first half is a marvelous scene in which a young girl, maybe 10, is having trouble phrasing her Mozart. It is rigid and labored. Stern finally catches her by surprise and asks her, in front of her colleagues, to sing the same theme. Then he asks her to phrase it the same way on the instrument. When she stops, he embraces her and says, "You've got it. You see, when you sing it you breathe and that's the way the music is meant to go. Just remember to breathe when you are phrasing and you've got it." The young girl beams.

The change of focus in the film -- which won the 1981 Academy Award for a documentary -- from the innocent smiles and burgeoning talents of these youngsters, to the brutal truths of the Cultural Revolution comes, at this point.

It then occurs to Stern to remark to their teachers, "These young children are just remarkable. But what happened to the older ones, at 20 and 21." The teachers turn sober-faced and Stern is told by one of them, "They couldn't play in the Cultural Revolution." That period of Chinese history removed virtually an entire generation of Chinese from exposure to Western music in its most important formative years.

Then comes the most chilling moment. Sitting against a bare background that could be a prison cell, the then-head of the Shanghai Conservatory, a Professor Tan, describes briefly his life early in that period. "It was like a bad dream. I was confined to a small dark room just under the stairs. I was fed in the dark, and I was let out only once a day for a very few minutes. The refuse pipe in the plumbing passed through there and some of it dripped on me. I had to stay there for 14 months. They treated us as criminals because they wanted to get rid of us. Ten of the teachers committed suicide. One hopes it would never happen again soon because people got wiser than before."

The artistic director of the movie is Allan Miller, who won a previous Oscar for his film "Bole'ro" with Zubin Mehta and the Los Angeles Philharmonic. It was produced and directed by Murray Lerner.