In 1820, young Noboru Yasumoto (Yuzo Kayama) completes his medical education in Nagasaki and returns to his native Edo hoping both to marry the daughter of a wealthy man and to achieve affluence himself through his medical practice.
He happens to visit the famed Koishikawa clinic for the indigent, which is run by the autocratic Dr. Kyojo Niide (Toshiro Mifune), better known as Red Beard. To his intense displeasure, he soon finds himself assigned to the clinic for his internship.
At first, the young intern is arrogant and rebellious, intent on displaying his knowledge of medical innovations and contemptuous of the older doctor for spending his life among the poor.
But as time passes, he gains an intimate knowledge of the kind of suffering that is endemic to the impoverished, and at length, becomes an acolyte of this seemingly dictatorial physician, who heals his patients with gentleness and humility as much as with his medical skill.