Stacy Friedman is a girl who dreams about her upcoming bat mitzvah, including sharing the moment with her BFF Lydia Rodriguez Katz and sharing a dance with her crush Andy Goldfarb. She clashes with her parents Bree and Danny about having a "perfect" party, which she feels will set her up for her future, and gets support from her big sister Ronnie.
When Lydia is invited to popular girl Kym Chang Cohen's house, Stacy tags along. The girls all go out to a high ledge, where Andy and his friends meet up with them. On a dare, and to impress Andy, Stacy agrees to jump off the ledge into the water below. Her happiness at getting Andy and everyone else's approval is short-lived when her bloody maxi-pad floats up next to her. Angry at Andy laughing at her and thinking Lydia was laughing with the group, Stacy decides to "call it", having Lydia announce the time when her crush on Andy "died".
Once school starts again, Stacy soon finds that Lydia and Andy are going out with each other. Feeling betrayed by Lydia, Stacy stops talking to her. Stacy sends a text message to an anonymous group that Lydia has hair on her nipples. Stacy even goes so far as to sabotage an "entrance video" she was making for Lydia's upcoming bat mitzvah with humiliating personal video clips. However, Stacy does not send the video to Lydia.
Stacy continues to try getting Andy's attention, also visiting the retirement home where Andy's grandmother is in. Stacy also continues to try making herself look older and more popular than she is, arguing with her parents over attractions to have at her bat mitzvah and what clothes she will be allowed to wear. Andy eventually offers to give Stacy her first kiss. Hiding behind the curtains of a Torah ark, the two are interrupted by their teacher Rabbi Rebecca. This leads to more fighting between Stacy and Danny.
Lydia's mother Gabi, who does not know that Stacy and Lydia are fighting, comes over to the Friedman home. She asks Bree for the entrance video for Lydia's bat mitzvah. Not realizing what is in the video, Bree sends the video to Gabi. On the day of Lydia's bat mitzvah, the entrance video is played by DJ Schmuley. Although Stacy tries to get DJ Schmuley to stop the video, the damage is done - when Lydia is about to walk into her party, she runs off and angrily tells Stacy off.
When Stacy's bat mitzvah day comes, she goes up to read from the Torah from her parsha of Ki Tissa. Seeing Lydia and her parents not there, she stops and confesses everything to the gathered audience, and tells off Andy for being horrible and driving her and Lydia apart. After her confession, she finishes her Torah reading at the urging of Danny.
Afterwards, Stacy runs off to apologize to Lydia and ask if Lydia will come to her party later that night. When Lydia and her parents come, they are surprised to see that Bree and Danny have changed the party to be a new party for Lydia mixed with Stacy's party. Stacy and Lydia make up with each other, while Stacy starts dancing with Mateo, a different boy at her school who had often sought Stacy's attention.
Sometime later, Stacy, Lydia, and their friends have a bake sale to help the retirement home.
