Sam Montgomery lives in the San Fernando Valley area of Los Angeles, with her wealthy widowed father Hal, who runs a popular sports-themed diner. Feeling Sam needs a mother, Hal marries a vain gold digger named Fiona, who has socially-awkward fraternal twin daughters, Brianna and Gabriella. During the 1994 Northridge earthquake, Hal is killed when he runs to save Fiona. Due to him having supposedly left no will, Fiona receives all of his belongings, including the house, the diner, and to her dismay, Sam.

Eight years later, Sam is employed as a waitress at the diner to save money to attend Princeton University, but she is regularly tormented by her step-family, who frequently insult her and treat her as if they were popular, despite being regarded as obnoxious. Even worse, Fiona, in her vanity, uses the inheritance to live as if they were insanely rich, including spending on minor facial surgeries, and even refuses to save water during the ongoing drought. She has also transformed Hal's beloved diner into something befitting of her own image, demands that salmon be served and included in over half the dishes and takes Sam's earnings. Sam struggles to cope socially at North Valley High School, where queen bee cheerleader Shelby Cummings also torments her and calls her "Diner Girl", along with other members of the popular clique.

Sam confides in her online pen pal "Nomad" about her dream to attend Princeton, a dream which he also shares. However, Nomad's true identity is Austin Ames, the popular, yet disenchanted, quarterback of the school's football team and Shelby's ex-boyfriend, although she refuses to accept that he has broken up with her. He is disappointed because his father planned for him to go to the University of Southern California with a football scholarship rather than going to Princeton. Nomad proposes that they meet in person at the school's Halloween-themed homecoming dance. On the night of the dance, Fiona orders Sam to work the night shift at the diner, then leaves to drive Brianna and Gabriella to the dance. Initially reluctant, Sam is convinced by her best friend, Carter Farrell, to go to the dance and meet her mysterious online friend.

Rhonda, the diner's head waitress who has looked after Sam, and the rest of the diner staff also convince her to disobey Fiona and go to the dance anyway. Sam, wearing a mask and Rhonda's old wedding dress, meets Nomad at the dance, and is surprised to learn that he is Austin, who had become smitten with her upon her entrance. The two decide to leave the party to walk alone and get to know each other a little better. While sharing a romantic dance, Sam and Austin begin to fall in love. But just as Austin is about to unmask her, Sam's cell phone alarm goes off, warning her to return to the diner before Fiona at midnight. She leaves without revealing her identity to Austin, and drops her phone on the way out.

Austin picks up her phone and begins a desperate search to figure out who his "Cinderella" really is, but every girl at school claims to be the mysterious owner of the phone. Sam is reluctant to reveal her identity to Austin, feeling that he will not accept her due to their different social circles. When Austin comes into the diner one day, Sam is forced to help him and, after a talk, she attempts to reveal her identity to him, but is cut off by Fiona. Sam's stepsisters end up discovering Sam and Austin's email relationship, and, after failing to convince Austin that one of them is the owner of the phone, convince Shelby that Sam tried to steal Austin from her. During a pep rally, they and the other cheerleaders humiliate Sam in front of the entire school and expose her identity, as well as naming her an impostor, horrifying the school staff, Carter and Sam. Austin, hurt by Sam's secrecy, is unable to step up to defend her, and Sam leaves the pep rally in tears.

Like Austin, Sam is revealed to have been accepted to Princeton, only to be duped by Fiona into believing that she was rejected by having a fake rejection letter made in order to keep Sam working at the diner and as her slave. Sam then decides to give up on her dreams and resigns herself to a lifetime of working at the diner, but Rhonda gives Sam a pep talk on not losing hope. When her stepsisters come into the diner, they slam the door, causing a guitar clock to fall off the wall, tearing the wallpaper down with it, and quickly blame it on Sam. However, the torn section reveals an inspirational quote Hal often said, "Never let the fear of striking out keep you from playing the game" (in real life, this quote came from Babe Ruth) and fills Sam with hope. She stands up to Fiona and her stepsisters, stating that she will no longer put up with their emotional abuse, quits her job at the diner, and moves in with Rhonda, who also quits along with the entire diner staff, all of whom had put up with Fiona's abuse for so many years after Hal's death only for Sam's sake, with Rhonda even threatening to beat up Fiona now that she was no longer bound to her job before Sam talks her out of it. The customers, who witness the entire scene, promptly leave in disgust as well.

Before the school's homecoming football game, Sam confronts Austin about his cowardice and not defending her at the pep rally, as well as for his seeming reluctance to show his true self. Before the final play of the game, he sees Sam making her way out of the stands, and finally stands up to his father, saying he wants to attend Princeton rather than simply play football all his life. He chases after Sam and apologizes. She accepts his apology, and they share their first kiss as rain falls over the drought-plagued valley; at the same time, the North Valley High Fighting Frogs win the football game. Soon after while finishing packing after moving in with Rhonda, Sam finds Hal's will hidden in her childhood fairy-tale book, stating that all of his money and possessions actually belong to her. Since this leaves her as the rightful and legal owner, Sam sells her step-family's fancy cars so that she can pay for college, and Fiona, who claims to have never seen the will before despite the fact she clearly signed it as a witness, is arrested by the L.A. District Attorney and L.A.P.D. for financial fraud and violating California's child labor laws for all the times she made Sam work long hours at the diner in spite of her being a minor.

Sam finds that she was in fact accepted to Princeton; the acceptance letter is retrieved from the garbage by her stepsisters, who knew where Fiona had "filed" it. As an alternative to going to prison, Fiona, Brianna, and Gabriella are made by the District Attorney to work off the money they stole from Sam at the diner, which is restored to its former glory by its new owners, Sam and Rhonda, who is made the new general manager of the restaurant. Also, Austin's father comes to accept his son's decision to attend Princeton. Things even work out in the end for Carter as he makes a commercial for a new acne medication. Shelby, having previously cheated off of Carter's school work and rejecting him for being an outcast, even after he was a proper gentleman to her at the Halloween dance, aims to pursue him since he is now popular, but after finally seeing her true colors at the pep rally, he turns her down for Astrid, the high school's goth DJ and announcer. The film ends with Sam and Austin, now officially a couple, driving off to Princeton together after Sam gets her phone back from Austin.