This is the extraordinary story of an extraordinary woman who, despite 6 1/2 long years of imprisonment and torment in Communist China, not only survived but endured and even prevailed. It is a story that began more than 20 years ago but has special relevance today. That is so partly because many of those who benefited during a decade of madness not only have gone unpunished but are trying to make a comeback, and partly because a story that so vividly documents the triumph of the human spirit over inhumanity is always relevant. Nien Cheng, 72, born into a wealthy landowning family, met her husband, Kang-chi Cheng, in 1935 in England, where both were studying at the London School of Economics. The husband, a diplomat in the Kuomintang regime, was enough of an optimist to decide to remain in Shanghai with his wife and young daughter after the Communists overthrew Chiang Kai-shek in 1949 and gained power in China. He went on to serve as general manager for Shell, the only multinational oil company to stay on after Mao Tse-tung's triumph. When he died of cancer in 1957, Shell brought in a Briton as its new manager and hired Nien Cheng as his special adviser. In 1966, the year in which Mao launched the frenzied upheaval known as the Great Proletarian Cultural Revolution, the company decided to pull out. Cheng was still remarkably prosperous. ''In this city of 10 million,'' she writes, ''only a dozen or so families managed to preserve their old life- style.'' She and her daughter Meiping were one of those families, living in a three-story house filled with antique furniture, books, works of art. There was a piano. There were three servants. Nien Cheng, in other words, was highly vulnerable to the two men who came knocking on her door at 6:30 on a hot July morning. One of them was a man she knew, Qi, a union official at Shell.