Set in the small town of Clover City, Texas, the story follows two protagonists: Millie Michalchuk, an optimistic, list-making fat girl who dreams of becoming a television news anchor, and Callie Reyes, a sharp-tongued half-Mexican, half-white co-assistant captain of the high school's competitive dance team, the Shamrocks.
Millie wakes before dawn each morning to open Down for the Count, a struggling boxing gym owned by her Uncle Vernon and Aunt Inga. The gym recently lost the backing of its parent franchise, leaving Vernon and Inga financially strained while caring for newborn twins. Millie is secretly working on a personal statement for a broadcast journalism summer camp at the University of Texas in Austin. Her mother, who bonds with Millie over dieting and sends her to Daisy Ranch Weight-Loss Camp every summer, expects this summer to be no different. After placing runner-up at the Miss Teen Blue Bonnet Pageant, Millie has decided she is done with weight-loss camp but cannot find the courage to tell her mother.
Meanwhile, Callie learns that the gym has dropped its sponsorship of the Shamrocks, leaving the dance team unable to afford travel to competitions. She competes with co-assistant captain Melissa Gutierrez for the captain position that will open when senior captain Sam Crawford graduates. Callie's mother, the school secretary, led the Shamrocks to a National championship in 1992, and Callie views carrying on that legacy as central to her identity. Her world revolves around two things: the team and her boyfriend, Bryce Dooley.
Millie proposes biweekly slumber parties with her best friend Amanda, along with Willowdean, Ellen, and Hannah, to maintain the friendships she formed during the pageant. At school, she sits next to Malik, a shy South Asian classmate, in AP Psychology. They share long conversations online every night, yet Malik barely speaks to her in person, leaving Millie frustrated.
Furious about losing their sponsor, several Shamrocks organize a midnight prank at the gym that escalates into full-scale vandalism when a teammate hurls a rock through a window. Callie participates in the destruction. The next morning, Millie arrives to find the gym wrecked. Reviewing security footage, she spots a distinctive gold C-pendant necklace on one of the masked vandals and identifies Callie to Sheriff Bell.
Callie refuses to name anyone else involved. Her mother grounds her completely. Vernon offers to let Callie work off the damages instead of pressing charges, but the school permanently removes her from the Shamrocks. Dance has defined Callie's identity since childhood, and losing it leaves her hollow. At the first slumber party, Amanda reveals she is biromantic asexual, meaning she does not experience sexual attraction but can feel other forms of attraction. Millie feels guilty for not knowing sooner but affirms her support.
Callie begins working alongside Millie at the gym. Their dynamic is prickly, but they develop an uneasy rapport. Callie gives Millie frank dating advice about Malik. She also makes an insensitive joke about Millie's ability to use the gym equipment in front of Mitch Lewis, a football player. While recovering from emergency wisdom-teeth surgery, Millie sends Malik an uninhibited text telling him he has a "kissable face." He responds warmly. Emboldened, Millie demands he talk to her in person the way he talks to her online, or stop entirely. Malik asks to see her. She sneaks out at midnight, and he drives her to a hidden diner, where he explains his shyness and the assumptions people project onto him as the only Indian kid at school. He tells her he likes her, and she initiates their first kiss.
Callie's life continues to unravel. Bryce breaks up with her publicly, saying she is no longer "one of us." Callie smashes his phone against a locker in front of a crowd. When she discovers that Sam and Melissa are spreading rumors she acted alone and was on drugs, she retaliates using the Shamrock Bible, a team scrapbook where every member records a personal secret upon joining. She photocopies the confessions onto green paper and plasters the school's hallway with them, using Millie as an unwitting driver. The school erupts in chaos, and Millie feels guilty for her role and for still not having told Callie that she identified her on the security footage.
Callie, further ostracized, begins sitting with Millie and Amanda at lunch. Millie shares her journalism camp dream, and Callie initially expresses doubt about the television industry's appearance standards. Millie is hurt. Callie corrects course, returning to tell Millie she is "really fucking cool" and can do anything she wants. Their friendship deepens through honest conversations about bodies, identity, and belonging. Millie teaches Callie to use the word "fat" without cruelty, and they discover they share the childhood nickname "Puddin'."
Malik helps Millie film a polished audition tape in the school's unused newsroom, and she mails her application. At Callie's seventeenth birthday weekend at her father and grandmother's house, Callie watches the tape and is moved to tears, telling Millie she was born for this work.
Then two blows land in quick succession. Millie's mother discovers she has been secretly dating Malik and forbids the relationship. The rejection letter from UT arrives. Her mother frames the rejection as proof that the world is too cruel for a girl her size and pushes her to return to Daisy Ranch. In a moment of defeat, Millie agrees. She withdraws from her friends and tries to break up with Malik via a note, but he tears it up and insists they talk face to face, urging her to fight for her dreams and their relationship.
At the gym, Sheriff Bell accidentally reveals that Millie identified Callie on the footage. Callie confronts Millie, furious that she withheld the truth, and storms out, blocking Millie's number.
Weeks later, Amanda, Willowdean, Ellen, and Hannah show up at Callie's door to tell her Millie did not get into camp and is heading back to Daisy Ranch. Callie decides she can help Millie even if she cannot fix the Shamrock situation. That night, she scratches on Millie's bedroom window and proposes they drive to Austin to contest the rejection in person. Millie's father catches them in the driveway and, overriding her mother's objections, gives his permission. On the road, the two reconcile, each apologizing for the hurt they caused.
At the UT School of Journalism, Millie meets Dr. Michelle Coffinder, the program head, who admits she cast the deciding vote against Millie. Dr. Coffinder compares Millie's situation to her own thwarted ballet career, arguing she is sparing Millie from an appearance-driven industry. Millie pushes back, telling her that one "yes" could change everything and asking her to open the door that was closed for her. Dr. Coffinder holds firm, but after Millie leaves, Iris, a teaching assistant who originally voted for Millie, advocates on her behalf. When an accepted student declines their spot, Dr. Coffinder relents. Iris runs after Millie with a welcome packet.
Back in Clover City, Millie presents the packet to her parents. Her father is immediately supportive, and her mother, after a tense pause, agrees, acknowledging that Millie is a different person than the girl she raised. Millie declares Malik is her boyfriend and nonnegotiable. Meanwhile, Callie addresses the school board about the funding disparity between the Shamrocks and the football team, presenting statistics she compiled with Melissa's help. The district agrees to increase the dance team's budget. Callie takes over Millie's shifts at the gym and begins dating Mitch, who initially declined her advances because of her past behavior but later recognized her growth. At a going-away pool party, Callie gives Millie a handmade cross-stitch reading "AUSTIN OR BUST." The two embrace in matching yellow bikinis, and Millie drives off toward Austin with Malik. Callie reflects that she does not know if she will get a happily ever after, but she is happy right now, and that feels good enough.