When Billy and Mandy have a brief fight, Billy says he never wants to speak to Mandy again. The next day, Billy is over his grudge, but Mandy has already decided to find a replacement for him. Mandy introduces Bobby, the new "Billy", who turns out to be a better friend than Billy, even gaining the affection of his parents, but who also wants to take over Billy's life. A jealous Billy uses Grim's scythe to open a portal and send Bobby to another dimension.

When Billy hangs out at Irwin's house, he finds a living mummy in the bathroom. Seeing as Irwin gets nervous, and as Billy has never met Irwin's mother, he suspects she is the mummy. To scare him off, Irwin's dad tells Billy he has the "curse of the mummy". Billy is attacked by insects and snakes while he tries to find out the truth about the mummy. It is later confirmed that Irwin's mother is in fact the mummy, but no further explanation is given as to how Irwin's family works or how he was conceived.

When Billy and Mandy's school is preparing for a big game, Mindy entrusts the care of the school's mascot, Fluffy Cat, to Billy, to protect him from being kidnapped by the rival team, the Booty Heads. At Billy's house, Fluffy Cat is a disaster, so Billy's mom throws him into the basement, where the cat gets inside Grim's trunk, absorbing its powers. A giant Fluffy Cat appears just before the game wreaking havoc.

While at Sassy Cat Land, the theme park dedicated to Sassy Cat (and to ridiculously long lines), Billy, Mandy, and Grim stumble upon a closed attraction, a futuristic "house of tomorrow". Inside, they find Master-Control, a sentient and autonomous, thinking, feeling artificial intelligence computer program claiming to know everything (and wishing death upon the trio when Billy angers him), and Tom Smith, a tour guide that became trapped in the house when it closed forty years ago. Tom explains that the attraction closed when a kid, actually Billy's dad, annoyed Master-Control with his dumb questions, and the robots started attacking the humans. Billy, Mandy and Grim exit the house unharmed, but Tom misses his opportunity to escape and is left behind again.

After Grim's robe comes out of the washing machine, Billy and Mandy find Thromnambular, a talking skull with ruby eyes and a number nine imprinted on his forehead. Grim tells them that the skull is a magical artifact imprisoned on Earth that must grant his nine wishes to be free. When Mandy asks Grim why he has not used any of the wishes, Grim tells her that the skull will corrupt the wishes and end up affecting the wisher.

At school it is time for the "Presidents Fitness Test", but Billy is absolutely unfit for exercise. On the other hand, Nergal Junior uses his shape-shifting powers to become fit at will and excel in every test. Billy asks Junior to transform into a pair of pants that he can wear, so he can run fast and win the 50-meter race. Since Junior has never attempted to transform into inanimate objects he cannot stop running and cannot revert to his human form. Billy keeps running forever while he becomes a national sensation known as the "Fitness Lad". It is revealed that the story was just all a fabrication created by the narrator, who parodies The Twilight Zone.

When Billy is accepted to Mrs. Pollywinkle's dancing academy, his father tries to persuade him to forget about dancing because of his own bad experience as a child during a recital. At the academy it is revealed that Mrs. Pollywinkle is actually a powerful, dark witch set on stealing Billy's soul. Billy's dad unexpectedly comes to rescue, challenging Mrs. Pollywinkle to a dance competition, which he wins using a powerful dance move. Billy's dad tells Billy to follow his dream of becoming a dancer if he so wishes, but Billy changes his mind saying that dancing is for girls. Note: This episode was inspired by the movie Billy Elliot.

When Billy and Irwin start making prank calls, everybody knows that Billy is behind the calls because every telephone has a caller ID. While Billy searches in Grim's trunk in the basement for something to defeat the caller ID, he finds a skull-shaped phone with no caller ID. Grim warns him to not use the phone, because it is used to call the ancient demon Cthulhu, whose sole appearance makes people go mad, and if awakened will cause the destruction of the world. Despite Grim's warning, Billy and Irwin use the phone anyway, and they are sucked into it by tentacles.

When Billy, Mandy and Grim go to the beach, Billy is swallowed by a whale known as Blubbery Joe, which also houses in her stomach Mister Geppetto and Pinocchio. Tired of not having a real son, Geppetto convinces Billy to be his "real son", while Pinocchio becomes increasingly jealous and is set on eating Billy's flesh to become a real boy and gain Geppetto's fatherly love and affection. Mandy and Grim search for Billy on board Captain Deadwood's ship, who is made of wood and who has been hunting Blubbery Joe for years. Billy, Geppetto and Pinocchio are expelled from Blubbery Joe's stomach, and as Billy needs to go home, Blubbery Joe accepts to be Geppetto's son.

When Billy decides he wants to grow up to be like Grim, he says nothing will stand in the way of their friendship. They meet a girl named Blandy, who looks very much like Billy, and Billy falls completely head-over-heels in love with her. When Blandy shows romantic interest in Grim, Billy is struck with mad jealousy and wants Grim out of the picture. Mandy consults Grim's encyclopaedia of monsters and realizes Blandy is actually a creature from another dimension. Despite Blandy showing her true form and leaving, Billy remains madly romantically infatuated with her; but only up until the following episode.

When Billy digs his front yard he finds a frozen caveman, who is actually Fred Flintstone (from The Flintstones), but whom Billy names Jake Steele. Billy tries to acclimate Fred to the modern life but the job is not easy. When Fred is taken to school, he predictably goes out of control, kidnapping Miss Butterbean and running away in Principal Goodvibes' car. The ensuing chase results in Fred crashing into an ice cream truck, becoming frozen again. Instead of reaping him, Billy decides to bury Fred in the yard again. A future is shown where both Billy and Fred are defrosted by brain-eating creatures.

When Billy fails an important test, the result of which will go into his permanent record, he asks Grim to magically alter his grade from an "F" to an "A"; as Grim reluctantly changes the grade, Billy becomes a genius, but reality is thrown completely off balance. Mandy and Grim go back in time to stop the past Grim from helping Billy, but it does not work. They keep going back in time to different epochs, each time meeting another Grim, eventually trying to stop Billy's ancestors from conceiving him, but nothing works. After meeting Billy's distant dinosaur relative, the episode ends abruptly as Grim wonders what the end of this could be.

As Billy sees Hoss Delgado's manliness is action again, Billy wishes to grow up to be a hero like him. Grim tells Billy that in older times an evident way to become a hero was to slay a dragon. They travel to the Middle Ages so Billy can train under the supervision of the best dragon slayer, Sir Boss Del Guapo, who resembles the present time Delgado. Billy becomes friends with the dragon he was supposed to kill and spares his life, becoming the dragon's hero. Del Guapo finds himself transported to the present, where he has a brief encounter with Delgado.

When Grim leaves the house to meet someone he met on the internet, Billy looks inside Grim's trunk for something to entertain himself. Billy finds a magical talking Rubik's Cube, which releases the demon Pinface, who has bowling pins in his head. Pinface proceeds to call his minions to conquer Earth, but the minions are more interested in throwing a house party. It is revealed that Grim and Pinface already know each other, and Grim even broke up with Pinface's sister. Billy's father ends up cheating and solves the magical cube by rearranging the stickers, trapping Pinface and his minions inside it again.

At a school fair, Grim's scythe accidentally transforms the dull Principal Goodvibes and his hippopotamus into gangster rappers, Goodvibes now calling himself Principal Goodbling. In the following, Goodbling changes the entire school to be more hip-hop-oriented, to the annoyance of Mandy. Although initially Grim likes the new style of the school principal, he later thinks Goodbling is just a poseur and goes to talk to him, but Goodbling attacks with yo-mama jokes. Irwin's grandmother, Tanya, confronts Goodbling and proves to be better than he is at yo-mama jokes that Goodbling reverts to being the boring Goodvibes.

As General Skarr is once again annoyed by Billy, Hector Con Carne and Major Dr. Ghastly arrive in his house with the intention of luring him back to the recently relaunched Evil Con Carne world domination organization. Although initially refusing to rejoin, Skarr later accepts with the condition that Billy and the other neighbors be destroyed. After a few weeks of living together and preparations, Evil Con Carne's ultimate weapon is revealed to be an army of tanks that shoot giant rubber bands. Skarr's all-out attack over the neighborhood is foiled when Billy tricks Hector into shooting Skarr's garden and they get into a fight.