Willowdean Dickson grows up in a small Texas town, raised primarily by her Aunt Lucy. Her mother Rosie – who calls her “Dumplin’”, much to Will’s chagrin – is a former beauty queen, too committed to various pageants to spend time with her daughter, and remains the planner of the town’s annual Miss Teen Bluebonnet Pageant.

Lucy introduces Will to Ellen Dryver, who becomes her best friend. Six months before they start their final year of high school, Lucy dies. Rosie finds difficulty connecting with Will, who often feels judged about her weight and is embarrassed by her mother’s pageant world.

On the first day of school, as Will is being dropped off by Rosie, everyone hears Rosie call her Dumplin, which results in being made fun of by students. Will is suspended for defending another higher-weight girl, Millie Mitchellchuck, from a bully. Will accuses Rosie of resenting her for her looks, and is upset when Rosie states Lucy should have taken better care of her health. After finding Lucy’s application for the Miss Teen Bluebonnet Pageant when she was 16, Will enters the pageant as an act of “protest in heels”. Ellen signs up as well, along with the enthusiastic Millie and their classmate Hannah Perez, an edgy feminist.

Rosie interprets Will’s application as a mockery of the pageant, and warns her that pageants are harder than she thinks. Will becomes jealous when Ellen warms to the pageant and the other contestants, and they quarrel after she tells her to quit.

Bo, who works with Will at the local diner, asks her out to watch a meteor shower. They kiss, but she panics when he innocently touches her back, and leaves abruptly. She feels further alienated when Rosie mentors Bekah Colter, the pageant frontrunner, who plans to ask Bo to the Sadie Hawkins dance.

After finding a flyer for a bar’s Dolly Parton-themed drag show Lucy frequented, Will takes Millie and Hannah to it. There she meets drag queen Lee Wayne, who was a close friend of Lucy’s.

During the pageant talent tryouts, Will’s choice to perform a magic trick goes poorly. Bo confesses his feelings for Will, having declined to go with Bekah to the dance, but she is quick to question his attraction; hurt, he accuses her of caring too much about what others think.

Grappling with her insecurities, Will is finally able to revisit the childhood playroom she shared with Ellen and Lucy. Rediscovering a special brooch among Lucy’s belongings, Will is inspired to take the pageant seriously, and she, Millie, and Hannah are coached by Lee and friends.

At the preliminary pageant event, Rosie is impressed by Will’s speech about loyalty, which leads Will and Ellen to repair their friendship. At home, Rosie and Will reconcile over memories of Lucy, and Rosie regrets asking Will to donate Lucy’s things, having given “too much of her away”.

At the pageant, Millie is confronted by her mother for lying about entering the competition, but Millie stands firm. Will incorporates a Dolly Parton tribute into her magic act, to the joy of the audience and her mother; Millie’s performance is also a success. While Rosie is overwhelmed with pride, Will admits she is disqualified for unapproved changes to her costume and song and knows her mom can’t make an exception for her daughter.

When Ellen’s boyfriend is unable to escort her for the formalwear presentation, Rosie suggests Will do so, allowing them to complete the pageant together. Bekah wins the pageant as expected, but Millie is awarded first runner-up, while Will leaves to reconcile with Bo, and they share another kiss. The movie ends as Will and her friends take Rosie and Millie’s mother to the bar where Aunt Lucy brought so much joy.