As the curtain opens, we meet Toby, the new kid in town, as he lets us in on his little secret: He desperately wants to be a part of the Main Street Kids' Club, or the M.S.K.C., as it's known to its members. With a motto of "math skills equal life skills," the M.S.K.C. are the coolest kids in town... so cool, in fact, that you have to be invited to join them. Toby isn't especially confident in his math skills but he thinks that he has found just the ticket to prove his worth... a treasure map! ("It's a Map / The M.S.K.C.").

As the official members of the M.S.K.C. enter their clubhouse, introduce themselves and share what their club is all about, Toby makes a mad dash for the exit, accidentally dropping the treasure map behind him. Matthew calls the day's meeting to order, and the members get down to business. Before they can get much done, however, Toby knocks at the door. Reluctant to let any non-official members in, Danny asks Toby if he knows the secret password. Not only does Toby not know it, neither do any of the other members, who look around at each other, surprised to have discovered that they have a secret password. Maggie suggests that Toby should have to answer three questions, instead, and although he is nervous, he passes the test with flying colors and is admitted into the M.S.K.C. clubhouse.

The members ask Toby about where he is from and tell him that... maybe someday... he can join them as an official member. When he asks them about the crazy t-shirts that they're all wearing, the group launches into a song about how they've made school cool for 100 days ("100 Days of Cool") by coming up with a theme or creative thing to do each day. Danny laments that, although they successfully made it through the 100 days of keeping school cool, this also means that he has been saving up for Heely skate shoes for more than that long and isn't even close to having enough money. Sheri teases him and tells him that he's like the character in a book that they've all read, Perry the Penguin, who spends more money than he saves. Danny pretends not to know what on earth she's talking about, and the gang launches into a reenactment of the book's plot ("Perry the Penguin").

When Danny goes to sit down, the chair collapses. The members begin to notice that their clubhouse could use a serious makeover, but a makeover requires money. The members set out to brainstorm things that they can do to raise funds. Toby thinks that this is his chance! He tries to join the conversation in order to tell everyone about his treasure map, but no one will listen to him. Suddenly, Danny sees the treasure map on the floor where Toby dropped it earlier. Toby tries to tell them that he found it, but Danny insists that finders are keepers. The gang sets out to find the hidden treasure, leaving a dejected Toby to stand guard at the clubhouse ("If I Could Just Be Me").

Meanwhile, the M.S.K.C. decodes the map, and Maggie leads the way to find the hidden treasure ("It's a Map – Reprise 1"). Toby continues to guard the clubhouse and fend off boredom by reading a book on how to guess someone's age while blindfolded and trying to use this new knowledge on various members of the audience. We bounce back and forth between Toby guessing ages and the official members hunting for treasure ("It's a Map – Reprise 2a, 2b, 3a").

After an exhaustive search, the gang finally finds the "X" that marks the spot ("It's a Map – Reprise 3b")! They uncover the hidden treasure excitedly – a small metal box – and decide to go back to the clubhouse so that Toby can watch them open it since he has been standing guard all this time.

Back with Toby at the club, they open the box. It reveals the treasures of a club from 60 years ago that was very similar to their own. Although they are a bit disappointed that it's not the gold for which they had hoped, they agree that the stuff is really cool, especially a copy of the comic book, Captain Invincible. Maggie strikes a pose, claiming to be Captain Invincible, and without missing a beat, the rest of the group joins in the energetic, imaginative reenactment of the comic book's story ("Captain Invincible 1, 2, 3, 4" and "Comet the Amazing Space Dog").

With the game complete, the members commiserate that they still don't have the money they need to fix up their clubhouse. When they discover a picture in the treasure chest of the old club and a lemonade stand, they decide to emulate. Duties and supplies are divided among the group, but when Toby tries to offer his help, he is again rejected for not being an official member ("If I Could Just Be Me – Reprise").

Over the next several days, the M.S.K.C.'s lemonade stand is a big hit. Each day, they excitedly chart the cups of lemonade that they've sold. All of a sudden, their success declines one day, and no one knows why until they see Toby on the opposing street corner, juggling and surrounded by a crowd of people.

Determined to make them both a success, the members propose that Toby join forces with them. Sure enough, the next day is the biggest success of all! While the official members are all dancing around in excitement, Toby totals the cups sold and resulting profit. Toby is not as bad at math as he thought. The club members are so impressed with Toby's help and math skills that they invite him to become an official member of the M.S.K.C., and the excitement abounds ("M.S.K.C. – Reprise").