Then Billy's mom, discovers crow's feet on her face, she panics and does not want to leave the bathroom. As Grim needs to use the bathroom, he uses his scythe to make Gladys a young girl again. Gladys goes to see Billy, but neither he nor Mandy nor Irwin recognize her, and instead, Billy calls her "Corn", because of the white bread she gave him. When Corn tells Billy to do his homework, Mandy tells him to go to the movie theater instead. Corn and Mandy struggle over who gets to control Billy. Billy's father plays with Grim's scythe and also becomes a child; he then goes to the movies and gets together with Gladys (Corn) again.

As Billy desires to be "cool" like Sperg, he has an ectoplasmic manifestation coming out of his mouth. The ectoplasm grows full size and turns out to be the ghost of nineteenth-century British poet Lord Byron, who tries to teach Billy to be cool, using elaborate poetry instead of bare muscles. Billy uses a poem to insult Sperg's mother, and Sperg challenges Billy to a duel. Billy and Lord Byron fight Sperg and his gang, but are badly beaten. Byron tells Billy he has been trained in the arts of coolness, and disappears.

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.

The day before Christmas, Billy, Mandy and Grim go to the shopping mall, where Mandy complains that Santa Claus is just an invention from corporations. Grim assures Mandy that Santa is real, and they even went to college together. They decide to travel to the North Pole and meet Santa, but when they arrive Mrs. Claus tells them that Santa Claus has been transformed into a vicious vampire.

As Billy behaves more reckless than ever and cannot stop breaking everything in sight, Mandy thinks he should go some place where he can release all his urge to destroy until he gets tired of it. Mandy, Grim, and Irwin go to Level 9, home of the fearsome warrior Lord Pain, to convince him to take Billy. Billy receives psychological treatment and it is revealed that his desire to destroy stems from lack of attention when he was younger. Lord Pain abducts him, and together they start destroying everything in Level 9, which seems to be is uninhabited. Billy is still not satisfied, so he and Lord Pain return to Endsville and carry on breaking everything.

When Billy desperately wants chewing gum at school, Grim gives him a bottomless gumball machine, but warns him not to swallow the gum. Billy ignores Grim's warning and starts swallowing the gum, causing him to constantly burp bubbles that fill the entire city. Mandy and Grim try to get Billy to cough the remaining gum that he has in his stomach, but instead Billy swallows the entire gumball machine. Billy coughs huge amounts of gum all over the city and ends up with a supernatural rash from another interdimensional world.

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 Billy realizes the three geeks of the Secret Snake Club get doctor's notes to avoid Physical Education (P.E.) class, he wants to know their secret to avoid the class too. All the boys in the class join the club and all, except Billy, get doctor's notes from Billy's dad, who poses as a doctor. While Billy is the only one exercising in the class, the Snake Club summons a snake to destroy the gym. Since the invoked snake is tiny, Sperg uses Grim's scythe to make it a giant. Mandy and the other girls beat the boys and their snake because they did appreciate the P.E. class as an outlet for their aggression.

As General Skarr has trouble finding a job, the recruiter tells him to attend anger management classes if he wants to work again. Skarr finds out Mandy and Grim are attending too, Mandy for staring at the school monitor, and Grim for enjoying his job too much. The instructor of the class, Larry, turns out to be a peace-loving, short man riding a tricycle, who teaches the trio ways to calm down when angered, including counting to ten, and using harmless weapons to let go of their anger. Larry invites Billy to the class, and Mandy, Grim, and Skarr all manage to contain themselves; however, Larry himself suffers an episode of rage over a parking space and he is arrested.

A scene of Endsville two weeks into the future shows a dystopian world ruled by the "Lord of Horror", who wields the all-powerful artifact known as "Horror's Hand". At the same time, two robotic replicas of Billy and Mandy are sent into the past, to stop the real Billy and Mandy from interfering with the current events.orror's Hand to rule the world.

When Billy is sent to bed after playing with his Dr. Brainiac action figure, he wishes he had an adventure. He notices a strange light outside and wanders into a corn field to investigate; he encounters Sperg, and both of them got caught by a UFO's tractor beam. Inside the extraterrestrials decide to experiment on them.

As Billy, Mandy, and Irwin need to write a school report on tall tales, Grim tells outrageous tall tales about Pecos Billy (based on Pecos Bill), Paula Bunyan (based on Paul Bunyan), and John Henry. In Grim's stories, Pecos Billy (Billy) tames a twister using his personal laser-shooting cow-like spacecraft, saving an Arizona trailer park; Paula Bunyan (Mandy) uses her blue ox companion, Babe, to stomp over a villain, creating the Grand Canyon in the process; John Henry (Irwin) digs through a tunnel so fast that he breaks the laws of physics and bursts into the "eighth dimension". After Grim tells them the stories, Mandy informs Billy and Irwin that he is trying to get them F's.

When Grim cannot take any more abuse from Mandy, Billy tries using Grim's scythe to shrink her to mouse size. Instead of shrinking, Mandy becomes a giant and destroys Billy's house. Although Mandy enjoys being a giant for a while, there are inconveniences, as she keeps growing as time passes, and she wants to return to her normal size. Billy is the only one who can cancel the curse he put on Mandy, and he refuses to do so. Mandy grows bigger than the planet, and she finds herself in the "Realm of the Infinite" with other creatures that grow at infinite pace. Mandy starts bossing them around to please her.