Because of a silly misunderstanding, Stella is booted off the annual carnival committee for Harper Valley High, but cooks up a plot with best friend Cassie to get even with the snobbish PTA members.
A misunderstanding leads the PTA members to believe that unmarried Stella is having a baby, and they set out to find her a husband to protect the reputation of their upstanding town.
Flora and the PTA members keep tabs on Stella by monitoring her mail, but Stella gets even by planting a phony letter from an imaginary middle eastern princess who just happens to be visiting Harper Valley.
After Wanda accuses Stella of trying to destroy her marriage, mother hen Flora hires an actor to romance Stella and break her heart.
Stella fakes amnesia to get the goods on Flora Reilly, who is involved in a corporate cover up that is illegally diverting water from Harper Creek.
Stella appears in a TV commercial for Kitchen Queen Paper Towels, unaware that she is working for her enemy, Flora Simpson Reilly who is the CEO of The Reilly Paper Company.
The Harper Valley PTA members accuse Stella of being an alcoholic when she is seen in town gathering beer cans. When Cassie (Fannie Flagg) refuses to throw Stella out of her hair salon, Flora (the town villain) tries to shut her shop down by setting up her snooty daughter Wanda in the beauty salon business.
Sentimental Stella chains herself to the oldest living tree in Harper Valley to save it from being bulldozed by Flora's workmen.
Stella picks up some extra shifts in the evening doing all the book-keeping for the local business people in town. But, the PTA members jump to the wrong conclusion and think that Stella is practicing the world's oldest profession when she is seen leaving the Harper Valley Motel late at night !
Flora Reilly thinks that Stella has inherited millions of dollars and promptly installs her as an honorary member of the Harper Valley PTA Board.
Mayor Harper approves an entertainment license for Kelley's Bar against town matriarch, Flora Reilly's advice. Angered by the mayor's actions, Flora tries to unseat the kindly Mayor Harper by backing her sleazy son-in-law (Bobby Taylor) in the upcoming mayoral election.
A bungling con man holds Stella, Cassie and all of the PTA board members hostage after a failed bank robbery attempt. When he realizes that he has wealthy Flora Simpson Reilly's daughter Wanda among the hostages, he demands 100,000 dollars and a 747 airliner to guarantee safe passage out of Harper Valley.
While taking diction lessons from the married English teacher at Harper Valley High, Stella becomes furious when she receives a sweater with a scarlet A on it from an anonymous member of the PTA board who assumes that she is having an affair