In 1933 at the height of the Great Depression, Dr. Robert Clayborne, a general practitioner in his sixties, cares for farm families and the city's poor that other doctors won't treat because they too often pay with produce.  His patients fondly call him Big Doc.  His oldest child, Mary, a 17-year-old high school senior, dreams of attending a music conservatory in the fall to become a piano teacher like her mother, Myrtle.  A 16-year-old son, called Little Doc, plans to follow in his father's footsteps.  Two younger sisters, 6 and 8 years old, round out the family.  

Big Doc struggles with a malaria epidemic that has been the leading cause of death in the county for decades.  He discovers that an over-flow from lakes in a new residential development is filling in areas of the swamp that were previously drained.  He takes on the developer, Hudson Kaley, a powerful state highway commissioner.  Big Doc wants federal relief funds to build a drainage canal that Kaley opposes because spending the money for a canal will mean fewer paved roads his development.  Big Doc persuades the city council to help lobby the state senate to appropriate funds from Kaley's development to the canal. 

In the meantime, Mary has a marriage proposal from Bill, a charming young doctor finishing up his residency in St. Louis with plans to go into practice with Big Doc.  She tells Bill that he'll have to wait until she finishes music school.  Then her mother, Myrtle, is diagnosed with tuberculosis and must be sent away to a sanatorium for a rest cure.  Mary must postpone her dreams of attending the conservatory in order to take her mother's place in the home and to care for her younger sisters.

On top of treating his patients night and day, Big Doc has a full schedule trying  to outmaneuver Kaley and get the senate to pass a bill to provide necessary funds for the canal.  Mary and Little Doc help by gathering statistics about the on-going malaria epidemic.

Big Doc is counting on Bill to help him with his patient load, but Bill returns from St. Louis and lets him down.  Bill doesn't want to "cut off toes for a bunch of carrots." His interest is in making a lot of money by going into practice with a successful plastic surgeon in St. Louis. 

The night the state senate votes on his bill, Big Doc returns home exhausted.  As Mary takes the call saying that his bill has passed by one vote, Big Doc collapses with malaria and dies the same night.

Mary's world is shattered.  Most upsetting is the fact that the governor, a golf buddy of Hudson Kaley, won't sign Big Doc's bill.  Mary appeals to Bill to ask Kaley not to oppose enactment of the bill into law, but Bill won't become involved.  Mary then turns to John MacCreighton, a young new minister in the backcountry whom she met earlier at a dance.  

John organizes a caravan of hundreds of country people in jalopies and wagons.  Mary and Little Doc go with them and the caravan descends on the state capitol.  The farmers intimidate the governor into signing Big Doc's bill over Kaley's vigorous objections.

Big Doc's victory makes front-page headlines in the state newspaper.  An excited Mary reads the article aloud at Big Doc's grave.  Unseen by Mary, Big Doc appears behind her and sits on his tombstone to hear the story, a pleased look on his more rested face.

Mary's mother, Myrtle, comes home from the sanatorium and Mary is asked by the country people to teach their children how to play a new church organ.  She is excited about an opportunity to start teaching -- her ultimate dream -- but she panics: How will she know what to do since she has had no conservatory training?  Myrtle reassures her that she hasn't watched her teach all these years for nothing.

Mary's first night in the backcountry, people crowd into the church to hear "teacher" play their new organ.  Not having expected to perform, she is nervous, then gains confidence by teaching a funny song that Big Doc liked to sing.  Unseen in a back corner of the church, Big Doc watches Mary and the country people sing his ditty, his foot tapping to the rhythm.

Afterwards, John walks Mary back to the farmhouse where she's staying.  Along the way he tells her that one day their paths may come together to make one.  Mary nods "yes."