TAMMY TYREE, 17, lives with her GRANDPA in a houseboat on the Mississippi River. Although Tammy lacks "schooling," she has a remarkable, unpretentious wisdom about life, which she imparts by way of an effective, if amusing, use of archaic words and phrases. Tammy adores her "amateur preacher" Grandpa, but she worries about his drinking. When a plane crashes nearby, Tammy accompanies her Grandpa, who hopes to find something of value to salvage. What they end up finding is the small plane's pilot, PETER BRENT, unconscious but still alive, and they take him home to recuperate. Over the next few days, Peter finds Tammy's simple ways charming, and she develops a secret yearning for him, crushed when he's finally well enough to leave.
After Grandpa is arrested for making moonshine whiskey, he tells Tammy to look up Peter, who promised Grandpa his help should he ever need it. Tammy heads on foot to Brent's family mansion, taking along her pet goat. She meets Pete''s stuffy family, PROFESSOR and MRS. BRENT, the latter an unhappy, fearful and somewhat judgmental woman.
She also meets Peter's AUNT RENIE, a likable eccentric who takes to Tammy. Tammy impresses the household with her good cooking and charming homespun manner, even though her tendency to be outspoken often annoys Mrs. Brent. Meanwhile, Peter is trying to restore the mansion's lost splendor, hoping to make it self-sustaining again by developing a hardy new crop of tomato plants. Peter is engaged to socialite BARBARA, who'd prefer that he move to the city. Barbara's father, ALFRED BISSLE, a houseguest, wants Peter to work for him in advertising, but Peter turns him down, convinced he can make it farming.
Tammy grows closer to the family, trying to keep her jealousy of Barbara in check. Meanwhile, ERNIE, Peter's playboy friend, tries to move in on Tammy, but Peter puts a stop to it. Gradually, Tammy affects the whole household, figuring out what's bothering each of them. During an annual festival during which the history of the household is relived, complete with costumes of the era, Tammy enchants VISITORS with her charming performance as Peter's great-grandmother. Meanwhile, Barbara becomes aware that Tammy has enchanted her fiance. When she finds out that Tammy's grandfather is incarcerated, she hopes to use it against her.
When a huge storm hits, destroying Peter's crops, Tammy tries to keep him from giving up, but a weary Peter decides to take up Mr. Bissle's offer to work in advertising. That morning, Tammy takes off. In the meantime, her effect on the family is complete: Professor Brent comes out of his books to tend to the emotional needs of Mrs. Brent, and Aunt Renie has decided to move to New Orleans, where she'll pursue her life-long dream of being an artist. When Barbara guesses that Peter's in love with Tammy, not her, Peter confesses that she's right.
He goes off in search of Tammy, finding her at home on the river, where Grandpa, now out of jail, rejoices when he sees the two of them in each other's arms..