En una parodia de la serie de Harry Potter, Grim envía a Billy y Mandy a una escuela de brujería para que el último dúo evite los aburridos campamentos de verano a los que se supone que deben ir. La escuela de brujería es propiedad de Dean Toadblatt, una rana antropomórfica. Billy y Mandy son enviados a la casa más odiada de la escuela, Weaselthorpe, mientras que Grim se convierte en prefecto de una casa más querida, Gunderstank. Billy y Mandy se asocian con el niño mago Nigel Planter para sabotear las esperanzas de Gunderstank de ganar el campeonato de la casa. Plantador se lleva todo el mérito del sabotaje (que fue idea de Mandy), pero es castigado una vez que es descubierto por Toadblatt.

Billy and Mandy take a reluctant Grim to school with them. Mandy's nemesis, Mindy, befriends Grim to annoy Mandy, and Billy calls for their other friend, who is Abraham Lincoln. Mandy devises a plan in which Mindy ditches Grim, and keeps Abraham as her friend instead.

After watching Billy and Irwin play with their "Hokey Monsters" cards (parodying the Pokémon Trading Card Game), Grim makes the monsters come to life. Soon, all the other kids want their own cards to be real too, and the city is soon under attack by a multitude of Hokey Monsters. To put an end to the mayhem, Mandy draws her own card, Mandy's Monster, which defeats all the other monsters by turning them into toast.

Grim está acostado enfermo en la cama con el "Síndrome de Enroaching Doom". Los doctores del Inframundo no pueden ayudar a Grim y se convierte en un ser humano. De vuelta en Endsville, Grim parece atractivo para las mujeres, pero termina siendo golpeado por novios celosos, especialmente el padre de Billy, Harold. Grim se cubre con la mucosidad que produjo cuando estaba enfermo y se convierte en un esqueleto de nuevo, y Billy lo intenta también.

Taking inspiration from his Pat the Baker video game, Billy decides to bake chocolate brownies. Billy uses "Evil Powder" from Grim's old trunk instead of salt, and the resultant brownies stink. Mandy tells Grim to get rid of the brownies, and he reluctantly does, but he also stores some of them in his skull. Soon, the stench of the brownies spreads through the world and starts resurrecting dead people, animals, and even food, as zombies.

Billy and Hoss Delgado, spectral exterminator, team up to destroy all the zombies that congregate outside the house, but the zombies reassemble into a bigger zombie. Delgado realizes that the zombie is after the brownies that Grim has in his skull, so Delgado and Grim battle furiously for them. The zombie eats the remaining brownies and falls apart.

En un día caluroso cuando Billy, Mandy e Irwin están aburridos, Grim usa una bola de cristal para mostrarles el futuro. En un futuro lejano, Mandy se ha convertido en una criatura gigantesca parecida a un gusano para ser inmortal, y ahora gobierna todo el mundo. Mandy da información falsa a un clon de Billy para capturar a los miembros de una resistencia que intentan derrotarla. El episodio parodia la novela de ciencia ficción God Emperor of Dune, de Frank Herbert.

When Billy, Mandy, and Grim go fishing, Eris, the goddess of chaos, gets Billy to help her use the Apple of Discord to create chaos (after her father grounded her). Instead of using the apple, Billy becomes distracted with his own idiotic errands, leading Eris to go insane. Billy finally decides to start creating chaos, but Eris is already too altered that she runs away.

Billy accidentally sets off a powerful stink bomb in Mandy's house and, while the house is being disinfected and fumigated, Mandy's family goes to live with Billy's. Billy and Mandy have a dispute over how to split Billy's room, prompting Grim to intervene. Mandy takes over the entire room, and imprisons Billy inside a brick wall with his mouth gagged by a dark red handkerchief.

Después de excluir a Mandy de su juego de mesa ¿Quién mató a quién? por ser una niña, Billy e Irwin equivocadamente tiran sus dados en el patio trasero de una casa embrujada. Grim les advierte sobre el fantasma de la señora Doolin, una dama que vivía allí. Mandy se atreve a entrar para recuperar los dados, y se hace amiga de la Sra. Doolin, que está viva y bien, y que dice que todas las historias contadas sobre ella (que cualquier cosa que traspasara la valla de su jardín nunca volvió) son inventos de Grim, a quien venció hace mucho tiempo en un concurso de miradas. Temiendo por la vida de Mandy, Billy también se aventura en la casa, pero la Sra. Doolin y Mandy lo asustan haciéndose pasar por fantasmas. Parece que la Sra. Doolin es en realidad un fantasma, y cuando quiso decir "golpear a Grim", significaba que ella "venció a la muerte".

After Grim fails to pull a werewolf out of his hat, Irwin is bitten by said werewolf. Billy takes Irwin as his new pet dog, naming him Sprinkles, and enters him in the Endsville Dog Show to compete against Mandy's dog, Saliva. At the contest, Sprinkles is about to win, so Grim releases a "were-flea" whose bite causes the werewolf to revert to a naked Irwin. It appears that Irwin won the contest anyway.

When Billy, Mandy and Grim go to the beach, Grim falls in love with a goth woman named Malaria. Grim is having a good time with his girlfriend, but Mandy assures him that will change when he tells her that he is not human. In the following date, Grim demonstrates his supernatural powers while dancing and, as predicted, Malaria leaves in panic.

At school, Mandy rejects Irwin's invitation to go to the school dance with him. Billy introduces his foreign and mysterious, new friend, "Piff", and Mandy becomes desperate when she realizes that she has developed a crush on him. Mandy asks Grim to cut her heart out to stop being in love, but Grim has a better idea and gives Mandy a makeover. Piff gets a crush on Mandy, who takes the opportunity to brutally turn him down.

It's Valentine's Day and Nergal, the fiend living in Earth's core, appears in Billy's house to court Billy's spinster aunt Sis. Billy tries ruining Nergal's date, to avoid becoming his nephew. Although initially Sis does not show interest in Nergal, she changes her mind when Nergal is stung by bees and she sees his "dance moves". Aunt Sis accepts Nergal's marriage proposal and accompanies him back to the center of the Earth.

Billy takes a big egg found in Grim's trunk and hatches it expecting a duck. Instead, Jeff the Spider is born, who considers Billy his father. Despite Jeff trying to please Billy in any possible way, Billy is terrified of Jeff and wants him squished. The exterminator chose not to squish Jeff because he is a nice bug.

Billy is worried about failing an upcoming Math test, so Grim gives him a "bookworm", which will make him much smarter. Milkshakes, Billy's cat, eats the worm instead, and is forced to tutor Billy. Billy is still an idiot in the end.

Grim is upset after his favorite show, Atrocia's Late Night Atrocities, is cancelled. He goes to the TV station and using his powers, turns the studio into a creepier place and becomes the host of the Grim Show. Grim's success on TV is interrupted when Mandy tells him he has neglected to do her chores. Billy and Mandy go to the TV station and show embarrassing footage of Grim at home.

Billy y Mandy son enviados al "Campamento de Invierno", donde conocen al hijo de Nergal, Nergal Jr. Justo como su padre, Junior intenta hacer amigos, aunque no de la mejor manera posible. Junior ataca a los campistas, los congela y, usando sus habilidades de cambio de forma, se hace cargo de sus apariciones. Billy decide ser el amigo de Junior, pero está asustado en el momento en que Junior le muestra su verdadera forma.

When Billy and Irwin exclude Mandy and Grim from helping them build a go-kart to enter the Annual Endsville Go-Kart Competition, the latter two decide to build their own. Billy-Irwin, and Mandy-Grim compete against Mindy, Sperg, and a mystery racer (Pud'n). Mindy gets stuck in highway traffic, Billy-Irwin's car is destroyed, Sperg and Grim mutually prevent each other from winning, and Pud'n wins in his little red wagon.

After being impressed by Irwin's realistic armor at the medieval fair, Billy wishes for a fancy armor too. Grim gives him the "Cursed Suit of the Black Knight". When Billy needs to go to the bathroom, the curse is revealed to be that the armor cannot be taken off until the person wearing it beats someone in a competition. Billy desperately tries to defeat other people at various contests, he but fails to come out victorious. He defeats Mandy at a breakdance contest in which Mandy decided not to participate, but he does not need to go to the bathroom anymore.

Billy quiere unirse a la banda de garage de Sperg, "Purple Filth", y tomar parte en la próxima batalla de las bandas. Después de presenciar la destreza de Grim con la guitarra eléctrica, Sperg invita a Grim en su lugar. El padre de Billy, adoptando la personalidad del rock and roll de "Mogar", decide que él y Billy deberían ir al concurso para hacer una aparición sorpresa. Billy impresiona a la audiencia con sus sonidos de axila.

After missing on a school trip, Grim takes Billy, Mandy and Irwin to the "Halls of Time" for a tour. There, Billy and Irwin mess the hourglasses that symbolize each person's lifespan. When they leave, they accidentally leave their own hourglasses, including Grim's, upside down, resulting in the group getting younger as time passes. Grim allows the kids to disappear by going before birth, but he cannot restore his own hourglass to its correct position and he disappears too.

After a lousy night for Grim, Billy and Grim argue about who has the hardest time, the kid or the Reaper of Souls. Mandy proposes they switch places for the day to find out. Grim seems to be having a good time, acting like a silly kid, while Billy is bored waiting for his old victim, Mr. Voorhees, to open the door. Billy transforms into a short skeleton, while Grim transforms into a taller version of Billy.

Grim's refusal to help Mandy win a karate tournament, and the $50,000 prize it promises to the winner, leads her to a curio shop, where the shopkeeper, Kuan Ti, gives her an ancient "Chicken Ball" which, when eaten, allows her to become a great fighter. Mandy easily wins the tournament and does not care about the money anymore, only about fighting. It turns out Kuan Ti is actually Eris in disguise, and this was just another of her schemes to create chaos. Seeing how he has a chance to get rid of the kids and finally be free, he tries to take advantage over this by making Billy a superb fighter too, so he and Mandy can fight ferociously to the death until the entire arena is destroyed. With both still alive, Mandy still demands the prize money, which she uses to buy Hector Con Carne's Bunny Island.

Un hombre sin cabeza usa a Billy para robar la guadaña de Grim.