Nergal, the fiend from the center of the Earth, returns to the surface, emerging a carnival with him, trying once more to make friends. In the following, Billy tries to help Nergal make friends by being considerate, but Nergal fails to apply correctly his advice. When Nergal is told that Grim was forced to be Billy and Mandy's friend-slave, Nergal then decides to force the world to be his friends by turning them into dark beings similar to him. In the end, Nergal turns Billy, Mandy, Grim and even the viewer (thus breaking the fourth wall) into one before the episode ends.

Billy tells Grim to help him throw a surprise birthday party for Mandy. Then, a reluctant Grim helps Billy prepare a cake while keeping it a secret from Mandy. In the end, a somewhat surprised Mandy reveals that her birthday isn't for the next five months, something that Billy already knew about.

Billy is constantly provoked by a bully named Sperg and runs over to Grim for help.

After taking the Underworld's "Bad Book" from Grim, Billy becomes possessed by an evil entity. Then, Billy steals Grim's scythe, and uses it to open a portal to summon Yog-Sothoth. In consequence, Mandy calls for Hoss Delgado to battle Billy. It is revealed that the portal can only be sealed by throwing the book into it. As Billy eats the book, Mandy throws Billy himself into the portal, restoring everything to normal. Instants later, a portal opens up and Billy comes out and says they didn't want him there.

In this musical episode, which pays homage to Little Shop of Horrors, Billy befriends a singing brain-eating meteor (voiced by Voltaire) that falls from the sky. With Billy's help, the creature grows bigger and eventually eats the brains of all people in Endsville (turning them into zombies with glowing green eyes). The last brain he eats is Mandy's, which results in the creature transforming into a brain-eating Mandy. The song featured is BRAINS!, composed by goth artist Voltaire specifically for this episode.

In a parody of the Harry Potter series, Grim sends Billy and Mandy to a sorcery school so the latter duo could avoid boring summer camps they were supposed to go to. The sorcery school is owned by Dean Toadblatt, an anthropomorphic frog. Billy and Mandy are sent to the most hated house on the school, Weaselthorpe, while Grim becomes a prefect for a more loved house, Gunderstank. Billy and Mandy team up with boy wizard Nigel Planter to sabotage Gunderstank. Planter takes all the credit for the sabotage (which was Mandy's idea), but he is punished once he is found out by Toadblatt.

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.

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.

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.

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 and Mandy are sent to "Winter Camp", where they meet Nergal's son, Nergal Jr. Just like his father, Junior tries making friends, although not in the best way possible. Junior attacks the campers, freezes them and, using his shape-shifting abilities, takes over their appearances. Billy decides to be Junior's friend, but he is scared away at the moment Junior shows him his true form.

Billy wants to join Sperg's garage band, "Purple Filth", and take part in the upcoming battle of the bands. After witnessing Grim's prowess with the electric guitar, Sperg invites Grim instead. Billy's dad, adopting the rock and roll persona of "Mogar", decides he and Billy should go to the contest to make a surprise appearance. Billy impresses the audience with his armpit sounds.

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.

Billy, Mandy and Grim go trick-or-treating for Halloween, and Irwin is left behind because he needs a scarier costume. After pulling a trick on an old lady, and getting some candies, Grim tells the story of why people pull pranks, which he was partially responsible for.

At a father-and-son picnic, Billy re-encounters with his spider "son", Jeff, in a dirty bathroom. Once again, Jeff tries to make Billy love him as a true father does, but Billy remains terrified of all bugs. Mandy tells Jeff he is being too nice, and to get Billy's attention he needs to be meaner. After being coached by Mandy, Jeff finally snaps and ensnares Billy in his spider web, forcing Billy to say he loves him.

When Mindy mocks Mandy's dumb and coward dog, Saliva, Mandy decides to train her dog to be meaner. After Grim keeps mentioning his own dog, Cerberus, Mandy decides to use Grim's special whistle to call the gigantic, three-headed, fire-breathing hound. Cerberus marries Mindy's chihuahua, Little Delicious, and Saliva snaps against Mindy for a brief moment.

As Billy walks into the woods to have a picnic, Yogi and a giant, feral version of Boo-Boo (from The Yogi Bear Show) come to steal his basket, and he escapes by hiding in a cave. Billy is taken to the mushroom lair of the dwarfs where King Beardbottom tells him the story on how the different races split the food-services industry, after the conference of Rivendell. The king sends Billy to infiltrate the elves' cookie factory to deactivate their force field, so the dwarves can assault the factory. After a fierce battle (censored out "for younger viewers", by a single clip of a pink koala continuously licking a eucalyptus leaf) and Billy's words, the elves and dwarves decide to work together and split the profits.

Trying to get revenge at Mandy for bossing them all the time, Billy and Grim put her in the "Mask of the Beast". Mandy must be nice and friendly while wearing the mask, or else she will become a hideous monster. Failing to remove the mask herself, and struggling to be kind to Billy and Grim, Mandy gradually turns into the beast. Billy and Grim ask Mandy to smile while she gives them a sponge bath, but this turns out to be too hard for Mandy. She fully transforms into the beast and punishes them instead.

When Billy forgets to do his history homework, Grim gives him a universal remote control to alter time and space so he can go back in time and finish his report. Billy and Mandy travel to the American Revolution, to prehistoric times, and even to the future, where they meet an aged Billy who is just finishing the report. After a struggle for the remote, Billy ends up again in the Prehistory and the remote breaks. Fast forward into the present, every person is dumb like he is and shares his characteristic big nose, including Mandy and Irwin.

During a thunderstorm, a lightning bolt hits Grim's scythe, transporting Billy to Asgard, land of the Norse gods. Mandy and Grim go searching for him, while a Viking wearing Billy's shirt remains at his house as a substitute. In Asgard, Billy learns about the endless battle in Valhalla, in which if a warrior dies, he is resurrected to continue fighting. Billy is surprisingly good at killing other Vikings, so Mandy and Grim are unable to convince him to return home.

When a doctor is unable to cure Sperg of his severe case of acne, Sperg goes to see a fortune teller that deals with skin problems. Following a Burmese recipe in her book, the psychic tells Sperg to get the Grim Reaper's bones and rub them upon his face. Sperg has to tolerate being friends with Billy and Grim to get access to the latter's bones. One of Grim's arms gets into a stew and when Sperg is immersed in it, the mix cures completely his acne, but it also erases his nose and mouth.

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.

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.

When Billy finally gets a "secret decoder ring" in his Blasto Bits cereal, he receives a holographic message from a strange creature who speaks about finding the "Secret of the Universe" in an abandoned mine shaft. All the other children of Endsville, also with decoder rings, race to the mine to be the first one to get the Secret. The entire effort is revealed to be a trap laid by Lubbermouth, a worm-like creature from Grim's past, who wishes to free herself from the curse of guarding the Secret. When Mindy looks down at the pit, she becomes the new guardian of the Secret and has to stay there forever.

To raise money to buy Sea-Critters (similar to Sea-Monkeys), Billy has a garage sale where he sells Grim's scythe to Irwin for less than two cents. With the great mystical strength and power of the scythe, Irwin hypnotizes Mandy and takes her on a romantic date. Billy and Grim try to recover the scythe, and when they do Mandy gets out of the trance. Irwin is punished by being transformed into a tiny sea-critter and placed in Billy's fish bowl.

When Billy gets under his house's foundation, he encounters again his "son", Jeff the Spider, who has just laid eggs. Jeff tries to make things right with Billy so he can take care of the baby spiders, as he is going to die after the eggs hatch. Naturally terrified of seeing Jeff, Billy runs away and a wild car chase ensues. The eggs hatch, and Grim decides to refill Jeff's hourglass that represents his lifetime so he does not die and can continue to scare Billy.

When Grim misuses his powers to play bowling with asteroids, he is taken to the Underworld court and sentenced to 50 hours of community service. At a retirement home, Billy, Mandy, and Grim go to the supernatural wing, which is the Home of the Ancients, a place for retired, geriatric monsters. Grim meets his childhood idols, the Bride of Frankenstein, Dracula, and the Wolf Man. Despite Billy's comments that the old monsters are finished, Grim sets out to prove that they are still scary. Grim tries to scare a bunch of kids in the woods, but the old monsters ruin his attempt and he walks out on them.

When the mall's surveillance cameras capture Grim's silhouette attacking endangered owls, a naturalist named Corey Nebraska blames Grim for the attacks, while Mandy suspects Grim is sleepwalking again. Billy, Mandy and Nebraska follow Grim while he sleepwalks, but they realize it is actually Billy's mom, the one attacking the owls. When discovered, Gladys reveals her hatred toward the owls as she recounts the story on how an owl ate her pet rat, Sniffles, years ago. Billy's dad reveals that he was in fact the one who ate Sniffles merely to survive until lunch time.

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.