Screams! Sirens! Flashing lights! It's Charley Maplewood's tenth birthday, and he's made a terrible mess of everything. ("House of Horrors" Prologue)

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, who now lives in Scotland. It's a birthday greeting, and, as usual, Dad has gotten Charley's birth date wrong. But even though the gift is a month early, Charley is happy to receive two issues of his favorite comic book series ("Monsters & Maniacs"), and he excitedly introduces his imaginary Monster friends -- Zombie King, Alien, and Swampy.

In Dad's package Charley also finds a birthday card with a question -"What are you going to do for your big day?" - that causes Charley to contemplate the awesome significance of turning ten ("Some Sort of Celebration").

In the kitchen, Charley - who's quite an impressive chef - preps dinner. When Mom gets home, Zombie King nudges 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-the L's-mock Charley for his exuberance and his lack of friends ("So Lame").

Before leaving, Mom asks Charley if he has friends, which Charley dodges by answering, "Doesn't everyone?" Alone now, Charley reminisces about spending time in the kitchen with the last person he truly considered a friend - his dad ("Delicious").

At school the next day, 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, doesn't 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 what appears to be a human foot in his mouth! Running after Boing Boing is Garry Quarky, Charley's scatterbrained next-door neighbor, 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 enters his house, he finds Mom and her loud-mouthed boyfriend, Vince Champagne, an event-planner, whom Mom asks to advise Charley on how to properly celebrate his upcoming birthday ("The Perfect Party"). As Vince expounds, Charley chops onions, which make him tear up. Thinking he's crying, Vince labels Charley a "freak!," and Mom asks Vince to leave - forever.

That night as Charley walks Boing Boing, he witnesses Stacey, Garry's girlfriend, break up with him and move out 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 bumps into Garry, knocking several grocery bags out of his arms. Seeing that Garry - without Stacy to cook for him -- has simply bought dozens of bags of potato chips to eat, Charley offers to help him change his diet. Garry is so grateful that he shows Charley the workshop where he makes his "special effects." Charley, impressed that Garry scares people for a living, admits he can only scare Lorena by sneaking up behind her and yelling 'boo!' So Garry - with the help of the creations and creatures in his workshop - explains to Charley how he makes scary props out of latex, a flammable goo ("Better Than A "Boo!""). And that's what finally gives Charley the inspiration for his own birthday theme: "Charley Maplewood's House of Horrors Birthday Celebration!"

But Mom is so upset when she finds Charley in Garry's spooky workshop that she threatens to call off Charley's party unless he promises to stops 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, which they hide when Mom comes to Charley's bedroom after finding Dad's premature birthday card. To lift his spirits, she invites Charley to join her in singing his favorite bedtime song ("Lullaby").

The next day at school, Charley hands out his invites ("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 that was left on Garry's porch until Garry can pick it up. Seeing that the box is marked FLAMMABLE, Charley realizes it's full of jars of latex and decides that - if he can't count on Garry's help for his party - he'll make his own special effects. Once he does, he opts to dry them 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, 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. And to make matters worse, Mom is so upset by Charley's fire that she announces his birthday party is cancelled.

At school the next day, 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"). They almost persuade him to defy his mom's order, but when he sees how crushed Jennifer is 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 points out that if he did, he'd miss the best part of a birthday - blowing out the candles and making a wish ("That Could Be Me"). So 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 their houses, Charley apologizes to Garry for lying about the latex and invites him to his 'secret' party. 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 gone 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 plotted with Charley's Mom to bring the community theatre folks to help make Charley's party truly spooky. 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! ("Finale")