Screams! Sirens! Flashing lights! It's Charley Maplewood's tenth birthday, and he's made a terrible mess of everything.

Charley explains that it all began one month ago when he arrived home from school to be greeted by Boing Boing, his very vocal dog. Mrs. Cleveland, Charley's nosy neighbor, suggests that he check his mailbox, where a package is waiting from Charley's Dad. It's a birthday greeting, and, as usual, Dad has gotten Charley's birth date wrong ("Happy Birthday From Scotland"). But even though the gift is a month early, Charley is excited to receive two issues of his favorite comic book ("Monsters & Maniacs - Part 1"). Charley introduces his imaginary Monster friends - Zombie,King, Alien, and Swampy -- but is interrupted when his neighbor Garry Quarky rushes across the stage, screaming and juggling a smoking test tube. Charley explains that his Mom labeled Garry 'odd' ever since she baked him cookies when he first moved to the neighborhood and he never thanked her. Charley then reveals that, though he has his Monsters for company, he doesn't really have any friends at school ("Monsters & Maniacs - Part 2"). In Dad's package Charley also finds a card ("The Birthday Card") with a question that triggers Charley's subsequent adventures: "What are you going to do for your big day?" Charley contemplates what he might want to do for his birthday and the awesome significance of turning ten ("Some Sort of Celebration").

In the kitchen, Charley preps dinner. When Mom gets home, Zombie King noodges him to ask her what ideas she has for his 'big day.' Mom assures Charley that his birthday is a month away, and they still have plenty of time to plan. But his big sister Lorena, along with her friends Lilith, Lillian, and Leilani, mock Charley for his exuberance ("So Lame"). Seeing Charley cooking, Mom regretfully reminds him that she has dinner plans with Vince Champagne, her loud-mouthed party-planner boyfriend. On her way out to her job at the Chick-A-Dee, a fast food restaurant, Lorena wonders who Charley would invite to a birthday party because he has no friends. Concerned, Mom asks Charley if he has friends, and Charley replies, "Doesn't everybody?" After Mom leaves, Charley wistfully reminisces about spending time in the kitchen with the last person he truly considered a friend - his dad ("Delicious").

Swampy, Zombie King, and Alien help Charley as he attempts to compile a list of friends to invite to his party ("Can You Call This Person A Friend?"). But Donna Pointer, the most popular girl in school, can't even remember his name. Darryl Egbert, the smartest kid in fourth grade, is too busy to stop and talk. Cougar and Scottie, Fresno Elementary's resident "tough guys," regularly demand Charley's lunch money. And Jennifer Mobley, talkative and exhausting, only wants to blab about Monsters & Maniacs. Charley's list is blank, and he returns home disappointed and unsure whether he even wants a party.

Outside Charley's house, Boing Boing greets him with a human foot in his mouth! Running after Boing Boing is Garry, who explains that the foot isn't real but, rather, a "special effect," a prop he's made for a show at a local community theatre. When Charley goes inside, he finds Vince Champagne waiting to tell him what he has to do for his upcoming birthday ("The Perfect Party"). As Vince lists the three things he'll need -- friends, a party theme, and a cake-- Charley chops onions, which make him tear up. Thinking he's crying, Vince calls Charley a "freak!," and Mom asks Vince to leave - forever.

That night as Charley walks Boing Boing, he witnesses Stacy, Garry's girlfriend, break up with him after calling him a "freak!" Charley is disturbed by the symmetry between him and Garry and decides that now he's got to have a birthday party. So now he'll need a theme.

The next day at school, Jennifer relates the events of her own ill-fated, theatre-themed ninth birthday party ("Jennifer Is Gonna Have A Birthday").

On his way home, Charley is still worrying about his party when he runs into Garry, knocking grocery bags out of his arms. Seeing that Garry has only bought dozens of bags of potato chips, Charley offers to help him change his diet. Garry is so grateful that he shows Charley the workshop where he makes his "special effects."

They discover that they share a love of Monsters & Maniacs, and Charley is very impressed that Garry scares people for a living. Charley admits he can only scare Lorena by sneaking
up on her and yelling 'boo!' (So Lame - Reprise 1). But Garry explains to Charley how to make scary props out of latex, a flammable goo (Better Than A "Boo!"), which gives Charley - finally! - the inspiration for his own birthday theme: "Charley Maplewood's House of Horrors Birthday Celebration!"

Mom isn't happy when she finds Charley in Garry's spooky workshop and threatens to call off Charley's party unless he promises to stop speaking to Garry. Charley reluctantly agrees, although he'd been counting on Garry to help him make effects for his party. What's he supposed to do now?

Undeterred, Charley and his Monsters make invitations for the party ("Making Invitations"). Later, Mom comes to Charley's room after finding his Dad's mis-dated birthday card. Hoping to lift his spirits, she invites Charley to join her in singing his favorite lullaby, but he claims he is too old for that now ("Lullaby").

The next day at school, Charley hands out his invitations ("The Invitation"), but because Cougar and Scottie snatch the last two, Charley hasn't got one for Jennifer.

Back at home, Mrs. Cleveland asks Charley to hold onto a package marked FLAMMABLE that was left on Garry's porch until Garry can pick it up. Charley realizes the box is full of latex and decides that - if he can't count on Garry's help for his party-- he'll make his own special effects ("Making Special Effects"). Lorena, as usual, scoffs at his plans ("So Lame - Reprise 2"). Charley opts to dry the latex effects he creates by using the space heater in the garage. When Garry comes to the door asking for his package, Charley lies and says it was lost. But when smoke starts pouring from the garage, Garry springs into action, rescuing Boing Boing and putting out the fire Charley started with the latex and the space heater. Hurt that Charley lied to him, Garry says, "I thought we were friends," before turning and leaving. On top of that, Mom, upset by Charley's fire, announces that his party is cancelled.

The next day at school, Charley tells everyone that the party is off - but they won't accept his decision and try to impress upon Charley the importance of this specific birthday ("This Is Big - Encore"). They almost persuade him to defy his mom's order, but when Jennifer is crushed to learn that there is no invitation for her, Charley feels so awful he resolves to cancel his party once and for all. But then Jennifer convinces him not to ("That Could Be Me"), and Charley winds up inviting her to his party -- a party he'll now have to throw all by himself.

On the street in front of his house, Charley apologizes to Garry and invites him to his secret party ("Better Than A "Boo!" - Reprise"). But Garry declines, explaining that he has an appointment at the same time to audition his "effects" for the local community theatre.

The next morning, Mom and Lorena wake Charley with a birthday cupcake before Mom leaves for work. Lorena presents Charley with a bunch of balloons that Dad had ordered delivered, along with a note apologizing for initially getting the date wrong, But Charley is so upset about how badly everything's going that - in a burst of hurt and anger - he pops all the balloons. For the first time, Lorena behaves like a big sister, calming him down and reminding her brother of all that they share ("We've Got Each Other"). Charley confesses to Lorena that kids from school are coming to the house for a party that he's throwing without Mom's permission. Lorena is so impressed by Charley's moxie that she offers to help by making the cake. Which she promptly ruins.

The kids arrive and are disappointed to find that nothing about Charley's party is scary - until the lights go out and, in the dark, they realize the ghosts they're seeing all around them aren't Lorena! Chaos ensues until Mom and Garry reveal themselves! Garry explains he was so moved by Charley's apology that he brought the Community Theatre Folks and dressed them as ghosts to make Charley's party truly spooky. ("Better Than A "Boo!" - Reprise 2"). Garry presents Charley with an impressive horror-themed birthday cake he made himself, and Charley blows out all the candles before making his wish.

And the capper to this very awful, very eventful, very amazing day comes when Charley finally trades his imaginary friends... for real ones ("This Is Big - Finale")!