A pioneering efficiency expert tests his theories on his large family.

Ann Gilbreth, the oldest of the twelve children of Frank and Lillian Gilbreth, begins to relate some episodes from her family's history. In 1921, when Ann is sixteen, the family lives in Providence, Rhode Island, where her father, an industrial engineer, is also an efficiency expert utilizing time and motion studies. Frank informs the family that they are moving to Montclair, New Jersey. Once the family is established there, Frank chairs one of his regular family council meetings to assign the children various chores that will assist Lillian and the two servants, Mrs. Monahan and Jim Bracken. When Frank goes to enroll five of the children in school, he tells the principal that he would like to meet the teachers, explaining that he wants the children placed in higher grades as their mental ages exceed their physical ages thanks to the complete home training program he has devised for them. Teenagers Ann and Ernestine, who will be attending a different school, have accompanied their father and are mortified when, though fully clothed, he demonstrates his time efficient method for taking a bath in the time it takes to play a phonograph record. Some time later, a doctor's car is parked in front of the Gilbreths' house, which normally signals the arrival of a new baby, but this time means that several of the children have whooping cough. While he is there, Dr. Burton examines everyone's tonsils and decides they should all come out, including Frank's. Always looking for opportunities to use time more efficiently, Frank decides to make a filmed record of the surgeries in order to help physicians eliminate wasted motions. The stalwart Frank thinks there is nothing to a tonsillectomy, but when it comes his turn to visit the operating room that has been set up in the house, he emerges from the procedure weak and shaken, a situation that is exacerbated by the revelation that the cameraman, Mr. Higgins, forgot to put film in the camera. Some time later, while several of the children are receiving a music lesson and are mangling their rendition of "Love's Old Sweet Song," Frank tells Lillian that he may be invited to speak on motion study at an international management conference in Prague, Czechoslovakia. Lillian then reminds him that she is on her way to the hospital to have another child. They name the child Robert, and he rounds out the "even" dozen children they planned of six boys and six girls. At another family council meeting, young William moves that they buy a dog, a motion that is carried, even though the chairman, Frank, objects. The dog, whom the family names "Mr. Chairman," develops a great affection for Frank. One day, a neighbor suggests that Lillian, who also had a career as a psychologist and industrial consultant, might serve as president of a new local chapter of Planned Parenthood, and is chagrined to find out how many children the Gilbreths have. Every summer, the family spends their holiday on Nantucket Island. One summer, Ann is attracted to young Tom Black, who attends her school, and, along with Ernestine, Ann is upset that Frank forbids them to wear bathing suits that reveal their knees. Back home, when Ann is invited to the senior prom, Frank insists on going along as a chaperone, and because the family car will not start, Frank has to go into the rumble seat of Tom's jalopy. Despite Ann's fears, Frank ends up being the hit of the dance, with many of Ann's friends wanting to dance with him. Later, Frank prepares for his two-month-long trip to Europe. After saying goodbye to the family, Frank suddenly and totally unexpectedly, collapses at the Montclair station and dies. Lillian calls a family council meeting of the older children and tells them that the money Frank left has had to go back into the business. She has talked with their grandmother in California who wants them to live with her. Their only other choice would be for Lillian to take over father's business, but that would mean that the family would have to live very simply and the children would have to help more around the house. Lillian then decides to go to Prague and London to give Frank's speeches. The children all agree that they should try to stay together and that they will manage things well. Ann finishes her account of the family history by saying that her mother did carry on Frank's work, became the foremost woman industrial engineer in the world and, in 1948, was named America's "Woman of the Year."