Miss Susie Slagle has run a Baltimore boardinghouse for medical students since her parents' death when she was a young woman, and by 1910, her boardinghouse has been home to two generations of doctors. Among the new students who now move into Susie's are Pug Prentiss, who has worked for three years at hard labor in order to afford medical school, and Elbert Riggs, who grew up in China and plans to return and work as a missionary after obtaining his degree. Spirited second-year student Ben Mead joins the new group for their first dinner with Susie, at which she announces her house rules: No smoking in bed, no women in the house, and none of them may fail their studies. During the first class in anatomy, Professor Metz warns Elijah Howe, Jr., whose father is the medical school chief, that his work will suffer due to his irreverent behavior. That night, Pug guides a drunken Elijah home, and meets Elijah's beautiful younger sister Margaretta. When Pug later becomes unnerved while watching an operation, he runs from the surgery, and confesses to Susie a childhood trauma in which he was terrified by his own helplessness when left alone with his father's dying patient. Susie consoles Pug with the knowledge that all great surgeons experience fear when they first start. Months later, at a Christmas party, Pug becomes smitten by Margaretta, who has blossomed into beautiful womanhood, and she becomes determined to marry him. Homesick Elbert skips the party and meets Nan Rogers, a student nurse, who is also homesick. After Elijah has a confrontation with his father, whose career has always intimidated him, he moves into Susie's boardinghouse, and passes his second-year examinations only after receiving encouragement from Susie and her houseman. Ben's happiness at passing his own examinations is dampened when he learns that his fiancée has married someone else. Nan and Elbert then become engaged. Third-year training begins with work in a free clinic, where interns are allowed to treat patients. When the clinic turns away a little boy who comes in with an injured dog, Pug rallies his friends and secretly operates on the animal. The chief surgeon is impressed by Pug's adept surgery, and Pug, who always thought his large hands would preclude the possibility of becoming a surgeon, is elated by Faber's encouragement and proposes to Margaretta. At Easter, Elbert is stricken by diphtheria, and dies despite the devoted nursing of Nan and his friends. Pug's confidence is destroyed because he was helpless to save Elbert. During the fourth year, the students are assigned to handle maternity cases. Fearing failure, Pug asks to be excused, but instead is put on call. His reputation already in jeopardy, Pug prepares to leave school, but Susie forces him to confront his fear. Nan works with Pug as the attendant nurse when he is sent out on call to the Johnson home and panics when he finds Mrs. Johnson in a weakened condition. However, Pug finds his strength at last, and after emergency surgery, he and Nan leave behind a healthy mother and baby. Afterward, Nan announces her intention to carry on Elbert's work in China. All of Susie's boarders graduate from medical school, and Pug plans to return following his vacation to intern with Dr. Faber. Before he leaves, Pug tells Susie that he owes everything to her, as does every doctor who ever left her house. Margaretta picks Pug up for his ride to the train station, and as they depart, one of Susie's new students knocks on her door.