Imagine waking up to find that your life is not what you last remember, in fact, you are seventeen years older. In 13 Going on 30, Jenna Rink (Jennifer Garner) wakes up one morning only to find that she is now a thirty year old magazine editor of a popular magazine - Poise, living in a luxurious New York City apartment, dating a NHL star athlete.
Jenna's last memory prior to this strange morning was her 13th birthday party, where she is humiliated by the "six chicks" - the popular girls - following this humiliation, Jenna wishes away her awkward teenage years, wishing to be "thirty, flirty, and thriving".
To most, it would seem that all of Jenna's childhood wishes had come true, however, the viewer soon learns that fame and fortune are not all Jenna truly needs. Very confused by the revelation of her life, Jenna asks her assistant to seek out her childhood best friend Matt (Mark Ruffalo). After tracking down Matt, Jenna soon learns her life was not at all as she remembers it to be. She learns that she was not the awkward teenage girl, dying to be part of the "in crown" in high school, she was in fact the leader of the in-crowd. She also learns that her current co-worker, Lucy (Judy Greer), is actually her best friend. Jenna does not recall that Lucy was the ring leader of the pranks played against her at her thirteenth birthday party. Jenna also learns that Matt and her were not best friends and had not spoken since her thirteenth birthday party. Jenna, shocked by the news of her and Matt's non-existent friendship makes numerous attempts to reconnect with Matt throughout the movie. Jenna also learns that Matt is engaged to be married to Wendy (Lynn Collins).
The discovery of Matt and her friendship is not the only disappointing discovery that Jenna is faced with, as she quickly finds herself conflicted with the person she has become. A young-spirited Jenna is shocked to learn that she rarely speaks to her parents, that she has been rude to her teenage neighbors, and that she has been involved with her co-worker's husband.
The viewer is taken through the adventures of a young-hearted Jenna's discovery of herself as an adult and the hardships faced as she discovers she is not fond of her apparent past behavior. Enjoy Jenna's innocent nature as she makes her world right with a youthful flare, preaching the words of Pat Benetar's Love is a Battlefield.