Twelve-year-old New Yorker, Evan Goldman, is surrounded by rabbis. After chanting, one of them explains that, when a boy has his Bar Mitzvah, he becomes an adult. Evan breaks from the scene and expresses his anguish to the audience. The mounting pressure of puberty, coupled with his parent's ugly divorce, has created unneeded stress in his life ("Thirteen / Becoming a Man"). Just as Evan begins to feel hopeful, his mother calls to tell him that they're moving to Indiana. Once there, however, Evan finds a friend in his neighbor, Patrice. Evan is disappointed that there aren't any cool places in Indiana to have his Bar Mitzvah; Patrice attempts to encourage him ("The Lamest Place in the World").
Next, we join Evan for the start of the school year. Brett, the most popular kid in school, has summoned up the courage to ask Kendra, the prettiest girl in school, to a scary movie on Friday night so that he can kiss her ("Hey, Kendra"). Despite her best friend, Lucy's, attempts to dissuade her, Kendra consents. Inviting Kendra to a scary movie was Evan's idea, so Brett decides that Evan is cool and refers to him as the "Brain." As the new kid, Evan is thrilled. Patrice, however, is not amused. As the popular kids begin considering Evan's invite to his Bar Mitzvah, however, he becomes pressured to renege his invitation to Patrice – no one will attend if she is there. Seeing the group is not bluffing, he hastily rips up her invitation. Patrice is devastated.
Later that day, we meet Archie, a high school student who suffers from a degenerative illness. He has run into Evan in the school and expresses anger at Evan for humiliating Patrice, his best and only friend. In an attempt to capitalize on the incident, Archie promises to help mend things between him and Patrice, but only if Evan can get him a date with Kendra ("Get Me What I Need"). At cheerleading practice, Kendra teaches a new cheer while Lucy resolves to make Brett her boyfriend ("Opportunity").
Elsewhere, Archie tries to talk Patrice into giving Evan a second chance, especially since she has a crush on him. Patrice feels betrayed by Evan and finds it difficult to forgive him ("What It Means to Be a Friend"). In class, Brett tells Evan that he should get his mom to buy them all tickets to the new thriller, "The Bloodmaster." Evan protests that his mother won't buy them tickets to an R-rated movie, but Brett reminds him that, if she doesn't, nobody will go to his Bar Mitzvah. Evan cycles through possible plans, eventually settling on having Archie use his illness to guilt his mother into relenting ("All Hail the Brain / Terminal Illness"). In an effort to make things better with Patrice, Evan eventually asks her to go to the movie with him as his date. He doesn't realize until later on that he has set up Archie's date with Kendra on the same day and movie as Brett's date! Realizing his mistake, Evan makes Archie promise not to do anything more than sit next to Kendra during the movie, or else he will screw up Brett's date. Archie agrees, and everyone prepares for Friday night ("Getting Ready").
At the movie, Patrice is upset that Evan isn't even sitting next to her. Throughout the bloody scenes, we see into Brett and Kendra's thoughts as they prepare for the impending kiss ("Any Minute"). Just as they're about to lock lips, Archie ruins it. Brett swears revenge on Evan and Archie. Lucy, seeing her chance, kisses Brett. Patrice tells off Evan when she sees that all he cares about is Brett being angry at him ("Good Enough"). With no one to whom he can turn, Evan realizes that he's in danger of forever being a lonely geek ("Being a Geek").
As Lucy and Brett begin dating, she forces him to spend more and more time with her. Brett's friends recognize that Lucy isn't good for Brett... or for them at all ("Bad Bad News"). Evan promises to try to bring Brett and Kendra back together so that he can get on everyone's good side. Archie, fearing that this is an impossible mission, begs Patrice to help Evan. She shows up just in time to tell Brett to talk to Kendra, and she and Evan suggest things to say – while resolving their own issues ("Tell Her").
Brett takes their advice and mends his relationship with Kendra. Lucy, however, won't stand for this. She spreads a rumor that Kendra is cheating on Brett with Evan, and then gets Evan and Kendra in the same place so Brett can catch them ("It Can't Be True"). As Kendra runs after Brett, Evan realizes that he didn't really want to be friends with Brett anyway and that Archie and Patrice are his true friends. He wants to call off the Bar Mitzvah because it would only be the three of them, but Archie and Patrice insist that it won't be that bad ("If That's What It Is"). Finally, Evan has his Bar Mitzvah, and it turns out to be a success. Reflecting on the lessons of becoming a man, he begins to accept what growing up is all about ("A Little More Homework"). The show ends with the cast in celebration ("Brand New You").