Josie Geller is an insecure 25-year-old copy editor for the Chicago Sun-Times who has never had a real relationship. One day, her editor-in-chief, Rigfort, assigns her to report undercover at a high school to help parents become more aware of their children's lives.

Her first day at South Glen South High School is miserable. Josie reverts to the old geek persona that ruined her first high school experience. She also has an unfortunate run-in with three obnoxious popular girls, Kirsten, Gibby and Kristin, and the school's most attractive and popular student, Guy Perkins. Josie loses hope but is reassured when a kind-hearted nerd named Aldys befriends her. Aldys, who loathes Guy and his gang, invites Josie to join The Denominators, a group of intelligent students.

Josie falls in love with her English teacher, Sam Coulson, and becomes the top student in his class. After reciting a romantic excerpt from Shakespeare to Sam, Josie has horrible flashbacks to when she read a romantic poem aloud in class to her high school crush, a popular boy named Billy Prince, who later asked her to their senior prom, making her dream come true. However, on the night of the prom, Billy arrives with another girl and both of them hurl eggs and insults at Josie, humiliating her and breaking her heart.

One night while out driving with Aldys, Josie encounters Guy and his gang at a local hangout called "The Court" where promiscuity and underage drinking take place. Her managing editor Augustus "Gus" Strauss loses patience with Josie after a rival paper scoops The Court story, and orders Josie to become friends with the popular kids. He arranges for her to wear a hidden camera, and soon the whole office becomes obsessed with her story.

Josie confides in her brother Rob about her fears. Rob, who was their high school's most popular boy in his teens, urges her to let go of her old self and start anew. To help her, Rob enrolls as a student and becomes an instant hit. He then uses his influence to draw Josie into the cool crowd, much to the dismay of Aldys.

Sam and Josie grow closer, but Sam struggles with his feelings as he thinks she's a student. Guy and Josie attend the prom as Rosalind and Orlando from Shakespeare's As You Like It. Anita, Gus and Josie's other co-workers watch through the camera and are overjoyed as she is voted prom queen. As Guy dances with Aldys as an alleged act of friendship, the mean girls attempt to dump dog food over Aldys. Outraged, Josie prevents the incident, throws her crown away and reveals her true identity. She praises Aldys for her kindness and warns the students that one's persona in high school means nothing in the real world. Sam is hurt by her lies and states he wants nothing to do with her. Also angered is Rob, who as a phony student received a second chance at baseball. Josie, ultimately making amends, secures him a coaching job.

Josie vows to give Gus a story and writes an account of her experience. In it, she admits she's never been kissed, describes the students of South Glen South, and avows her love for Sam; the entire city is moved by it. She writes she will stand in the middle of the baseball field with a countdown and wait for Sam to come and kiss her. Josie waits, but the clock runs out with no sign of Sam. On the verge of giving up, soft cheers from the crowd give way to a booming roar, as Sam emerges to give her a romantic kiss. The movie ends with Sam telling her it took him forever to get here, a sentiment with whi