Charley Maplewood is turning ten - today! - but his birthday party is a complete disaster. ("Why?") you might ask? Well, according to Charley, it all started about a month ago, when his nosy neighbor Mrs. Cleveland told him there was a letter in his mailbox. It was a birthday card from Charley's Dad ("Happy Birthday From Scotland") but Charley's birthday was still a month away. Even though his dad is absent-minded, Charley is excited to receive two issues of his favorite comic book ("Monsters & Maniacs - Part 1"). Charley is particularly fond of three monsters who have become his imaginary friends: Swampy, Zombie King, and Alien ("Monsters & Maniacs - Part 2").

When Charley reads the birthday card, Dad turns his whole world is turned upside down when he asks if Charley has plans for his big day. Charley thinks through what he would like to do for his birthday ("Some Sort Of Celebration"). When Charley's Mom gets home, she reminds him she must work on his birthday. Charley's teenaged sister Lorena points out that Dad is terrible with dates ("So Lame - Part 1"). Lorena also points out that, though Charley wants to throw a birthday party, he has no friends to invite ("So Lame - Part 2").

Charley thinks through the three things he needs for a party, which he learned from his mom's ex-boyfriend, obnoxious party planner extraordinaire Vince Champagne: a theme, cake, and friends. Charley's imaginary friends help him think through the party. His theme will be "Charley Maplewood's House of Horrors Birthday Celebration." Mom will take care of the cake. And as for the friends, Charley has enough construction paper to make six invitations to pass out at school.

At school, Charley's Monsters try to help him decide who he can count as a friend. He speaks to popular girls Donna, Dina, and Dana ("Can You Call This Person A Friend? - Part 1"), Darryl, the smartest kid in fourth grade ("Can You Call This Person A Friend? - Part 2"), and Cougar and Scottie, the elementary school's resident tough guys ("Can You Call This Person A Friend? - Part 3"). Then there's Jennifer, another new kid at school who Charley finds annoying ("Can You Call This Person A Friend? - Part 4"). Charley realizes that he can't call any of them friends and calls off the party, but the Kids talk him back into it ("This Is Big"). By the end of the song, Charley has given out his six invitations, accidentally leaving nothing for Jennifer. Mrs. Cleveland watches the scene unfold.

Feeling guilty, Charley finds Jennifer and apologizes, again thinking about cancelling his party. Jennifer talks him out of it ("That Could Be Me"). Charlie invites her to his birthday party as Mrs. Cleveland watches.

The next morning, Mom and Lorena wish Charley a happy birthday. Mom promises him a big cake when she gets home from work, and leaves. Charley confesses to Lorena that he's invited seven kids from school to his party without Mom's permission. To his surprise, Lorena is thrilled and offers to make his birthday cake. Charley decorates the house and begins to feel optimistic, but Lorena's cake is a disaster.

The guests arrive and all seems lost until the kids notice… there is a Ghost in the room! The lights go out and the kids get more and more scared, until Mom reveals herself! Mrs. Cleveland told her everything and has brought over a cake for Charley's birthday!

As he blows out his candles, Charley realizes he doesn't need his imaginary friends anymore - he's made real friends. Though everything went wrong for his big one-oh, somehow everything also went right ("Finale").