In 1873, Irish immigrant Mary Rafferty lives with her wheelchair-bound father Pat and her widowed sister, Kate Shannon, in a small shack in an area of Pittsburgh known as "The Flats." One day, Mary tells her father that she has accepted a job as a live-in maid at the home of steel mill owner William Scott. Patrick, who lost his legs in an accident at Scott's mill and is embittered toward the family, angrily disapproves of Mary's decision. Soon after starting her job, Mary endears herself to William's wife Clarissa, his daughter Constance, and his three sons, Ted, William, Jr., and Paul. Mary falls instantly in love with Paul, who has just returned home from London and has big plans to modernize the furnace at the Scott mill. A romance soon flourishes between Paul and Mary, much to the consternation of Louise Kane, a cunning snob who is determined to marry Paul for his money. One year passes, and Paul proposes to Mary, but Mary rejects the idea because it would be improper for her, a servant, to marry her master's son. Constance, meanwhile, secretly marries Giles, the Earl of Moulton. After announcing the marriage to her parents, Constance asks her mother for permission to take Mary with them to England. Clarissa consents to the arrangement, and Mary leaves Pittsburgh without bidding farewell to Paul. Two years pass, and when William learns that Paul is in love with Mary, he recalls Mary to Pittsburgh and gives his blessing to their marriage. Paul and Mary resume their romance, but Mary's happiness is soon dampened by the discovery that her father has been leading a crippling and violent strike against William's mill. When she overhears William's plan to send in strikebreakers, Mary negotiates a temporary truce and arranges a meeting between the strikers and William. Ted is sent by his father to meet the arriving strikebreakers at the train station and send them home, but he gets drunk and fails to deliver his father's message. As a result, the strikebreakers arrive at the meeting, and a bloody riot ensues. Patrick and William are killed in the fight, and their deaths fill Mary with so much grief that she concludes that she and Paul can never marry. The passage of ten years finds Paul in a loveless marriage with Louise, and the father of a young boy. One day, the aged and ailing Clarissa sends for Mary, her truest friend, and tells her that she is willing her share of the mill to her. Following Clarissa's death, Ted, William, Jr. and Constance decide to sell the Scott mill, despite Paul's impassioned pleas to keep it in the family. Paul nearly loses the family mill until Mary sides with him and persuades Constance to change her allegiances. Paul then leaves Louise, who has only shown an interest in his money, and goes to Mary.