Rugged entrepreneur Diamond Jim proposes that railroad cars be made of steel instead of wood, and he offers to ride a fully-loaded train of steel cars across a weak wooden tressel to prove that the cars are safe. As the train approaches this crucial test, Diamond Jim contemplates his career. Diamond Jim, gifted with a magnetic personality, meets the beautiful Sarah and falls in love with her.
Diamond Jim's brother Dan is an evil person who leads a low, vulgar life. Diamond Jim's best friend Mark is a dashing figure who takes his place in high society . The train rolls safely across the tressel, and Diamond Jim calls for a celebration at his mansion that includes Lillian, a tempestuous singer. Jim's boon companion, hard-drinking John L. Sullivan, celebrates in his own fashion in a low-life bar across town and watches a ribald fight between two women. In 1906, Mark and Lillian travel to San Francisco and are caught in the earthquake. Mark suffers a broken leg and returns to Jim's mansion to recuperate.
Jim suffers a heart attack, and while he is in the hospital, Mark and Jim's beloved Sarah fall in love. Dan breaks into Jim's house and is shot to death while attempting to steal Jim's diamonds. Jim's doctor tells him that his penchant for overeating could prove fatal, and when Jim is confronted with the love of Mark and Sarah, he orders a huge meal and begins to gorge himself.
