Samantha 'Sam' Montgomery (Hilary Duff) lived a semi-fairytale life with her widowed father when she was 8 years old in a suburb in California. Later, her father opened up a Diner named after him and fell in love with a woman named Fiona (Jennifer Coolidge) who he later married. One day, a falling support beam kills her father in the 1994 Northridge earthquake and leaves her with her plastic-surgery-salmon-eating-obsessed stepmother and her two cruel stepsisters Brianna (Madeline Zima) and Gabriella (Andrea Avery). 10 years later, Sam is a straight-A student enduring her uncaring stepmother's wrath, who turned her father's cafe into a girly hot-pink hot spot and forces her to work as a "diner girl". In addition to her family, Sam must deal with the "popular" girls at school, led by the lead cheerleader, Shelby (Julie Gonzalo) who, according to Carter (Dan Byrd) "wants him so bad". Sam confides in her best friend Carter and cyber-space, pen-pal, her "secret admirer" who wants to attend Princeton with her. What she doesn't know is that her secret admirer turns out to be Austin Ames (Chad Michael Murray), quarterback of the football team and the most popular boy at school who is currently dating Shelby.
As they text each other back and forth, Austin (or, as he refers to himself in cyberspace, "Nomad") invites Sam to the Halloween Dance so they can meet face-to-face. Austin dumps Shelby a few hours before the dance. At the same time, Fiona forces Sam to work late at the diner that night until midnight. Carter, Rhonda (Regina King) - Sam's mentor and friend at the diner who also works there, and the other cafe employees help Sam get to the dance, masquerading as Cinderella in a mask and Rhonda's wedding dress. She meets Austin in the middle of the dance floor and is at first put off by the fact that her "Nomad" is Austin. He soon wins her over, and they move outside together. They play "10 Questions", Sam never revealing her identity, as they dance outside in a gazebo. However, Sam's cell phone alarm goes off 15 minutes before midnight, warning her to run back to the diner before she's caught, leaving Austin without Sam's real name. Austin and Sam, or Prince Charming and Cinderella are chosen as the Homecoming Prince and Princess but "Cinderella is playing hard to get". As Sam and Carter flee the dance, once Sam has managed to pull Carter (or Zorro who he went as) away from Shelby who he is kissing after he prevented David, one of Austin's friends, from making a move on her, Sam drops her cell phone and Austin picks it up, determined to find out who this "Cinderella" is.
Sam goes to school the next day to find Austin performing a school-wide search for his Cinderella. Sam is still feeling uncomfortable about revealing herself to Austin, who in turn is still searching obsessively. One afternoon, Sam's stepsisters find her emails while snooping around on her computer. After unsuccessfully trying to pass themselves off as Cinderella to Austin, they end up showing the emails to Shelby and her friends. The stepsisters make her believe Sam stole Austin on purpose, so she puts together a cruel skit to perform at a football pep rally in order to humiliate Sam. The skit is performed in front of the whole school, as well as Austin's father who is present at the pep rally and has been pressuring Austin to attend college to play football at USC, his alma mater. Austin has never told his father he really wants to attend Princeton University.
Hurt and humiliated, Sam goes back home and Fiona brings her a letter from Princeton saying that she wasn't accepted. This is, in actuality, a fake letter as Fiona and the stepsisters threw out the real letter from Princeton which said she was accepted (most likely so she wouldn't leave the diner). The next day, after Brianna and Gabriella slam the door so the guitar with Elvis on falls down, ripping off the wallpaper and exposing a quote that says "Never let the fear of striking out, keep you from playing the game" and blaming this on Sam, she reads this aloud and decides she has had enough of her step-family and their abusive treatment. She quits working at the diner, tells Fiona that "she can mess with her hair, her nose, her face and even her dad's diner, but she's through messing with her", and moves in with Rhonda. The night of the big football game, Sam storms into the boys locker room and confronts Austin, saying that she's fed up with his two different identities, one to her and one to his friends "Waiting for you is like waiting for rain in this drought, useless and disappointing." In the last few moments of the game, as the game is about to start again and Austin is going to score the touchdown the North Valley Fighting Frogs need to go to the semi-finals of the State Championships, Austin sees Sam leaving and abandons the game, but as he is running off the pitch, his father grabs him and asks him what he is doing, throwing away his dream like that, to which Austin replies "no dad, I'm throwing away yours", and makes up with Sam and kisses her, at the very same time it starts raining.
In the end, Sam finds her father's hidden will which leaves everything to her. She has Fiona taken for questioning by the district attorney for keeping it a secret when her signature was plainly on the document. The stepsisters manage to find Sam's real Princeton-acceptance letter in the garbage, where they'd "filed" it. Hal's Diner is finally restored to normal, and, after making a deal with the district attorney, Fiona, Brianna, and Gabriella are forced work at the diner to pay off their community service debts, under supervision from Rhonda. Carter gets a role in a commercial, which becomes a success, thus landing Carter, Shelby's interest, but he sees her for what she really is and rejects her for the school's radio presenter Astrid. The last scene is Sam and Austin sitting where at the start Sam is shown with her dad, and Austin is returning her phone. As a testament to the dance he starts attaching it to her ankle, which is how it was attached at the party, but she takes it before he can finish and they kiss then ride off to Princeton and their new future together.