Samantha "Sam" Montgomery (Hilary Duff) lives in the San Fernando Valley with her widowed father Hal (Whip Hubley), who runs a popular sports-themed diner. Hal meets and eventually marries a vain, self-absorbed woman named Fiona (Jennifer Coolidge). When the 1994 Northridge earthquake strikes, Hal is killed when the roof caves in on him. As he supposedly left no will, Fiona receives all of his belongings, including the diner. She reveals her true hatred for Sam, and banishes her to the attic room, spoils her own daughters, and forces Sam into becoming little more than a servant.
Eight years later, Sam is employed to work at the diner and tormented by Fiona and her two dim-witted daughters, Brianna and Gabriella who aren't really as popular, or as well liked as they think they are. Fiona is so self-absorbed that she uses more water than necessary despite the fact that the San Fernando Valley is in the middle of a drought. Brianna and Gabriella constantly try to play up to all of the popular students. A running gag that occurs throughout the film shows Sam's stepsisters taking lessons in Synchronized Swimming for which they are found to have absolutely no talent. Sam struggles to cope at North Valley High School (despite maintaining straight "A's"), where queen bee cheerleader Shelby Cummings (Julie Gonzalo) also torments her, like her stepmother and stepsisters, and calls her names such as "Diner Girl."
Sam in her online pen pal "Nomad," who shares her dream to attend Princeton University. Little does Sam know, "Nomad"'s true identity is Austin Ames (Chad Michael Murray), the popular - yet unhappy - quarterback of the school's football team and Shelby's boyfriend (although he breaks up with her, she chooses to ignore that). "Nomad" proposes that they meet in person at the school's Halloween dance. Initially reluctant, Samantha is convinced by her aspiring-actor friend Carter Ferrell (Dan Byrd) to go to the dance and meet her mysterious online friend.
On the night of the dance, Fiona forces Sam to work the night shift at the diner, then leaves to drive Brianna and Gabriella to the dance. Carter and Rhonda (Regina King), Sam's amiable co-worker at the diner, take Sam to find a costume for the dance. Sam, wearing a mask and a beautiful white dress, meets "Nomad" at the dance, and is surprised and shocked to learn that he is Austin. The two decide to leave the party to walk alone and get to know each other. After sharing a romantic dance together, Sam and Austin begin to fall in love. But just as Austin is about to kiss her, Sam's cell phone alarm goes off, warning her to return to the diner before Fiona comes back at midnight. She leaves without revealing her identity to Austin, and drops her phone on her way out.
Austin picks it up and begins a desperate search to figure out who his "Cinderella" really is, the result being that every girl in school claims to be the mysterious owner of the phone. Carter, dressed as Zorro, rescues Shelby from the unwanted amorous advances of Austin's friend, David (Brad Bufanda). Shelby falls for "Zorro" not knowing who he is and Carter resolves to tell her who he really is the next day.
Later, Brianna and Gabriella discover Sam's emails to Austin and realize that Sam is Cinderella. They later present them to Shelby and convince her that Sam persuaded him to leave Shelby to be with her. To retaliate, Shelby, Brianna and Gabriella perform a mean-spirited skit at a school pep rally where the emails are read aloud and Sam's identity is revealed to Austin. Carter even experiences some humiliation when he sees Shelby and her friends lounging by the pool. He reveals that it was he who was dressed as "Zorro" and Shelby, showing her true colors, denies that anything ever happened between them at the dance.
Sam, who (like Austin) had been accepted to Princeton University, was then duped by Fiona into believing she was rejected. Shortly after, fed up with Fiona's persistent emotional abuse and her schoolwide humiliation, Sam confronts Fiona about the mistreatment she's given her, quits her job at the diner and moves out to live with Rhonda. Simultaneously, Rhonda and the rest of the diner's staff quit along with Sam, having put up with Fiona's abuse for Sam's sake all those years. The customers, who watched the entire scene, leave the diner as well.
Before a school football game, Sam confronts Austin about his cowardice and lies. Before the final play of the game, he sees Sam making her way out of the stands, and finally faces up to his father that he wants to attend Princeton rather than simply play football all his life. He hands his helmet to Ryan, then chases after Sam and apologizes. She accepts his apology and they share their first kiss as rain falls over the drought-plagued valley.
Soon after, Sam finds her father's will hidden in her childhood fairytale book, which stated that all of his belongings would go to her. Sam sells her step-family's fancy cars so that she can pay for college, and Fiona, who signed the will as a witness, is arrested after being chased through the front yard of what is now Sam's house by the police.
Fiona and her daughters are made to work off the money they stole from Sam at the diner, which is restored to the way it was before Hal's death by its new owners, Sam and Rhonda. Sam then finds out that she was, in fact, accepted by Princeton after her acceptance letter was found in the garbage by her stepsisters.
Things also work out in the end for Carter as he makes a commercial for acne medication. After being dumped by Austin, Shelby then makes a play for Carter only to be spurned in favor of Astrid, the school's DJ. Sam and Austin begin a relationship and attend Princeton together (After Austin's Dad approves his son's choice). The film ends as they drive away together torwards New Jersey.