When Billy's parents are attacked by an anaconda, Billy suggest visiting boy wizard Nigel Planter, because he can talk to snakes. Planter in fact does not speak to snakes, but to snacks, and instead he asks for Billy, Mandy and Grim's help to win the affections of Huffefnie Pfefferpfeffer, a beautiful witch in the School of Sorcery (and having a striking resemblance to Gladys). Planter tries using a romantic love potion and enchanted chocolates, but they do not work. The guys end up completely forgetting about Billy's parents.

When Billy's dad gets a promotion at his job, he tells his family they are going to move to another town, starting a dispute between Billy and Mandy over who gets custody of Grim. Grim suggests they settle this in court, and hours later Billy, Mandy and Grim find themselves in the Underworld court, presided by Judge Roy Spleen, and which has Fred Fredburger, an incorrigible dumb character, as a juror.

General Skarr buys a home in Endsville and becomes Billy's neighbor. After Skarr invites Billy into his new house, Billy mentions Grim's scythe as ,he strongest and most powerful weapon in the entire universe, and Skarr tries to battle his desire to be evil again. Eventually Skarr steals the scythe but Mandy proves to be even scarier than he is, and takes it back.

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.

As Grim watches the entire season of "My Troubled Pony" (parodying My Little Pony), he learns from Mandy that the show was cancelled and does not have an ending. Grim decides to reap the "Schlubs" (a parody of The Smurfs), 100 creatures that live in Billy's yard, to melt them into gold so he can finance the creation of the series finale. Grim must get past Billy and the aggressive Schlubs themselves, including the single, large female (a parody of Smurfette). After Grim thinks he got the creatures with his scythe, Mandy appears to have used all the gold herself to become rich.

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 Nergal goes to visit Billy's family (his in-laws) for the weekend, he ends up staying with them over a year. When Billy finally asks his uncle to leave, Nergal says he cannot return home because he is a complete failure, having lost at a board game 599 times in a row against his own family, and experiencing constant disappointment elsewhere. Billy and Grim tell him to not worry about being a failure, but to embrace it. After training with Billy and Grim, Nergal returns home and he finally wins at the board game, but he also makes a crack in the ceiling and his home starts filling with lava.

When Billy and Mandy are utterly bored, Grim takes them to Underworld's "Circus of Fear". The ringleader, Dr. Fear, notices Billy and Mandy, and tries to recruit them into the circus to attract customers, who would pay to see real, live humans. After they go back home, Dr. Fear captures the two kids, and Grim has to get them back. During the show, Mandy breaks free and in a strange twist of the story, she reveals that the beastmaster, Clortho, is actually Dr. Fear, who in turn is just an earthworm, leaving Billy confused.

After spilling salt in a restaurant, and not throwing salt over his left shoulder, Billy starts having an unusual amount of extremely bad luck. The next day, Grim analyzes Billy and finds out he has a "Catastrophe Snail" attached to his brain, which is causing him the bad luck.

Billy, Mandy and Grim go to the beach where Grim falls asleep and Billy buries him in sand. When they leave, Billy only takes Grim's skull, leaving Grim's body to be found by a rich family that take it with them. With his adoptive family, Grim's body becomes a celebrity and refuses to go back with Billy and Mandy. Eventually, Grim's skeleton is reunited with his skull, but instants later it is shattered by dogs.

Lord Pain, a fearsome warrior from the Plain of Eternal Suffering, appears in Billy's house looking to serve his master, Grim. Lord Pain is put to clean the bathroom, and is confused as to why Billy and Mandy treat Grim the way they do. Eventually, Lord Pain realizes that Mandy is to be his real master, and prepares mashed potatoes for her.

When Billy, Mandy and Grim notice things have been disappearing in the house, they set out to investigate using a supernatural detector. They find the culprit is a talking "taking tree" in Billy's yard, who has been stealing objects since Billy's dad was a child. To make the tree happy, Mandy and Grim set out to trade the stolen belongings for a highly-valued Mickey Munchle baseball card, which is worth eleventy million dollars. Mandy and Grim return from the baseball shop with the real Mickey Munchle. The tree returns all of the stolen things, but keeps the player.

When Billy crashes his bicycle, he obtains a free tricycle from a mysterious boy in a supernatural porta-potty. While Billy loves his tricycle, which he calls Trykie, he is also mocked because of it by Irwin, Pud'n, and Sperg. Trykie moves on its own and attacks the three of them. Mandy realizes the problem with Trykie, and Grim confirms that he is cursed. Billy realizes that Trykie is indeed evil as well, and he lets Mandy destroy it once and for all.

When Billy, Mandy and Grim go playing golf, they encounter boy wizard Nigel Planter again, who is there hiding from the dark wizard Lord Moldybutt. They head back to the School of Sorcery, where they keep on running into Moldybutt, who warns them to stay away from the "Chamber Pot of Secrets" hidden in Dean Toadblatt's office. It is revealed that Lord Moldybutt is actually Toadblatt, who was disguising himself to scare off Planter. Note: The title is a spoof of Harry Potter and the Chamber of Secrets.

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.

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.

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 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.

When Billy ruins his eyes by playing too many video games and looking directly into the Sun, Mandy and Grim suggest he see an ophthalmologist. When Billy refuses wearing glasses or having laser surgery, Grim uses his own scythe to zap his eyes. Although Billy has sharp and clean eyesight, he can also see glimpses of the future, specially horrific accidents that occur to his best friends. Grim zaps Billy's eyes multiple times to restore them to normal. Billy ends up with weird-looking eyes.

When Billy's parents leave for their second honeymoon, Billy is left alone at the worst possible moment, since he has invited Principal Goodvibes over to dinner so he can recommend him into Rollington Academy for the Gifted. Billy's dad suggest Billy to use Eris, the goddess of chaos, and Hoss Delgado, spectral exterminator, as his substitute parents during the dinner. The dinner is a disaster with Delgado terrorizing Goodvibes, and Eris unleashing giant alien zombie lobsters. Goodvibes forcefully recommends Billy into Rollington Academy, which, as it turns out, is actually a military school.

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.

After Billy eats 19 creme-filled Atomic Frenzy Cakes he cannot sleep and wants Mandy to read him a bedtime story. Mandy, Billy and Grim each tell a story. For his story, Grim uses a supernatural storybook from the Underworld, which makes Billy and Mandy enter the book and reprise the tale of Hansel and Gretel. Billy departs from the story and follows Pinocchio, who wants to devour Billy to become a real boy. Mandy saves Billy from Pinocchio, but she and Billy remain trapped in the book as Grim, the storyteller, falls asleep.

Billy's pet hamster, Mr. Snuggles, celebrates his 10th birthday, so the Grim Reaper comes for him. However, Mandy, Billy's friend, is reluctant in allowing it, so she proposes a deal to the Grim Reaper: if he beats the kids in a game, he can take the hamster, but if he loses, he will spare the hamster's life and become their "best friend forever". The Grim Reaper transports the kids to limbo, where he challenges them to a game of limbo. Despite his best effort, Grim ends up losing because he is attacked by Mr. Snuggles at Mandy’s command.

After watching a movie, Grim teaches Billy to possess people by jumping into their heads. Billy possesses Mandy and makes her do assorted things like flirting with Irwin, kissing Mandy's mother, going into both men's and women's restrooms and eating earthworms. In the end, Billy leaves Mandy's head, but later that night, Mandy possesses Billy's cat, Milkshakes, to maul him as a form of revenge.

Billy wanders into a junkyard, where a massive hole in the earth opens up and pulls him down in. Later, Mandy and Grim get pulled in as well. They land at the center of the Earth, which is home of Nergal, a devilish creature. The lonely Nergal, who has been watching Billy and Mandy for some time, wants Grim to hand over the kids to him so they can be his friends forever. Grim happily leaves the kids behind as he returns to the surface; however, he later goes back for them because he can't handle the guilt of abandoning his friends. In the end, Grim is defeated by Nergal's electrified tentacles, but Mandy defeats Nergal merely by kicking him in the shin.