The film was shot in England, and many European actors, led by Robert Donat, portrayed the Chinese characters.
Warning: Wikipedia contains spoilers.
The story begins with Aylword being rejected as a potential missionary to China because of her lack of education. Mr. Robinson, the senior missionary, feels sorry for her and secures her a position in the home of a veteran explorer with contacts in China. Over the next few months, Aylword saves her money to purchase a ticket on the Trans-Siberian Railroad, choosing the more dangerous overland route to the East because it is less expensive.
Once in China, Aylword settles in the town of Yang Cheng, where she secures a post as assistant to a veteran missionary, Jeannie Lawson (played by Athene Seyler), who has set up an inn for traveling merchants, where they can get a hot meal and hear stories from the Bible. The film follows Aylword's acculturation, culminating in her taking over the inn when Lawson dies.
A stubborn but endearing woman, the local mandarin appoints Aylword as his Foot Inspector to ensure that the ancient practice of foot binding is eradicated in the region he governs. She succeeds in this, and manages to put down a prison revolt as well, winning her the esteem of the local population as well as of the mandarin. Meanwhile, however, China is being invaded by Japan, and Aylword is encouraged by Lin to leave. She refuses, and as the town of Yang Cheng comes under attack, finds that she has fifty orphans in her care.
As the population prepares to evacuate the town, the mandarin announces that he is converting to Christianity as a final tribute to Aylword. Aylword is now left alone with the children, aided by Lee, the former leader of the prison revolt that she helped to resolve. Lin tells her that the only hope for safety is to take the children to the next province, where trucks will drive them to safety, but they must get there within three weeks, or else the trucks will leave without them.
Just as they are preparing to leave, another fifty orphans appear from a neighboring town, so Aylword and Lee have to lead one hundred children on a trek across the countryside. Although it should only have taken them a week, the roads are infested with Japanese patrols, and the group has no choice but to cut across the mountains. After a long, difficult journey, they all arrive safely (except for Lee, who gave his life to save them from a Japanese patrol) on the day the trucks are to leave. Aylword is greeted by Robinson, whom she reminds how he rejected her as a missionary years before.