The citizens of a small Welsh mining village await the arrival of L.C. Moffat, who is to inherit a large house in their midst. They believe Moffat to be a retired soldier, and are surprised to discover that she is a spinster and a schoolmarm. As Miss Lily Moffat settles in, she is moved by the ignorance of the children around her, indeed of the general townfolk, and decides to start a school. She immediately faces resistance from the villagers, who are set in their ways, and the town Squire, who will not rent her an ideal place as he sees no point in providing education to miners.
Miss Lily is determined, however, and soon changes her large house into a school. It is open to not just the children, but anyone interested in learning. Before long, her efforts win her some aides and allies. She is also rewarded with the finding a real diamond in the rough, a young orphaned miner named Morgan Evans. Morgan's essay about life in the mines under the fields where "the corn is green" shows a talent for words and language, and as Miss Moffat takes him under her wings, his native brilliance is unearthed. Within two years, Morgan is so promising that Miss Moffat wants to send him to Oxford and eventually coaxes the crusty old Squire to become Morgan's sponsor. Tragically, Morgan's chances are thrown into jeopardy when a brief affair he had with the tawdry daughter of Miss Moffat's housekeeper results in a child. Now Miss Moffat must weigh the future of her prize pupil against his undeniable moral obligations. What lesson must she teach him now?