The country doctor, Monsieur Benassis, practices in a village called Voreppe at the base of the Grande Chartreuse Mountains. He is a seedy and unkempt, but very kind-hearted, bachelor of 50 who lives with his authoritarian housekeeper. Benassis was brought up in the country, but had lived for many years in Paris where he enjoyed a dissipated life and loved two women. He left the first, only to learn later that she bore him a son and died of heart disease. Later his illegitimate son died.

His second love, Evelina, broke off their engagement when her parents objected to the suitor’s sordid past. Benassis became very depressed and considered suicide. After visiting a monastery in the Grand Chartreuse region, he decided to move to Voreppe and devote his life to serving the poor rural people. He not only practices medicine, but over the years has also initiated a number of economic and community development projects in the area.

Above the village is a hamlet that contains a dozen cretins among the thirty families who live there. Cretinism is common in the region. Dr. Benassis decides that it would be good for the public health to have all the cretins sent to an asylum in Aiguebelle, some distance away. When Benassis becomes mayor, he arranges to have the cretins transported to Aiguebelle, despite opposition from the local people. One cretin remains "to be fed and cared for as the adopted child of the commune."

Benassis later moves the other inhabitants of the hamlet to a new, more fertile, site in the valley and installs an irrigation system for them. At the end of the novel, Benassis has a stroke and dies. He is the first to be buried in the new cemetery.