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.

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.

It's Halloween, Billy, Mandy and Grim go trick-or-treating. Grim mistakes a woman for Atrocia, the host of the all-night Halloween marathon on TV and goes to talk to her. Upon looking at her face, Grim realizes his mistake; however, the woman mistakes Grim for her son, Gregory, and takes him home Gregory is now Grim, and does not have a stomach. At the same time, Billy mistakes Gregory for Grim and he and Mandy take him with them. In the end, Billy and Mandy take Gregory back to his mother and Grim goes back with them.

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 eats too much junk food that makes him feel sick. Grim prepares him a remedy using his granny's recipe, but this medicine makes a parasite to start growing in Billy's back, which Billy names Yap-yap. In the following, Billy and Yap-yap continue eating junk food, so Mandy suggest to use the antidote to the original remedy. Eventually, Yap-yap, now looking like a smaller version of Billy, detaches himself from Billy (which would have technically cured Billy without the need of him drinking the anitdote), and tries to escape from the antidote. In the end, Yap-yap drinks the antidote and is destroyed.

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

Grim gets sick.

Grim falls for a Goth girl.

Billy hatches a giant spider.

Billy and Mandy contend with Nergal Jr.

Billy and Grim switch places for a day.

A headless man from Grim's past uses Billy to steal Grim's scythe.

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.

After Billy's baseball team loses badly against Mindy's, Grim decides to coach them. Mandy is not allowed on the team, as Billy is reluctant to let girls play, although he does not realize Mindy is a girl too. Mandy puts on a baseball cap and, assuming the role of "Manfred", pitches for Billy's team anyway. With additional help from ghoulish players which played in the Underworld Little Leagues alongside Grim, Billy's team wins when Pud'n hits a home run.

After a trip to the mall, Mandy becomes the spokesperson for Happy Huggy Stuffy Bears, a new stuffed toy in town. After the success of her commercial, Mandy gives away stuffed bears to other children, who start wreaking havoc in the city. Eris, the goddess of chaos, is behind the toys, which mind-control the children into creating chaos. Mandy decides not to share control of the world with Eris, and orders Grim to destroy the toys.

One night a strange odor travels through the city, prompting the noses of all the people, including Billy's, to follow it. The next night, Mandy, who is noseless, and Grim decide to wait outside Billy's house for the event to repeat. When the odor reappears, the nose parade leads them to the shopping mall, where they meet a "Nasalmancer", a nose wizard who is trying to take over other people's noses since he lost his own. Billy recovers his nose, but slices it in two, and gives one half to the Nasalmancer.

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.

When Billy gets the opportunity to adopt one of four dogs, he says he wants them all. Grim uses his scythe to magically combine the dogs and create Billy's dream dog, who calls himself Wiggy Jiggy Jed. Although initially Billy and Jed are best friends, their relationship falls apart when Billy tells Jed to sleep outside. Jed uses the powers of Grim's trunk to build a giant machine to capture the beds of all people. Billy throws Jed a dish of primordial ooze and he dissolves.

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.

After Grim has a nice dream, a nightmare escapes the back of his head in the shape of a supernatural, invisible duck that makes farting noises. The duck follows Grim everywhere, causing him embarrassment with the non-existent farts, and to end up in jail. The duck repeats the strategy, following other citizens, including Irwin and Billy's dad, resulting in all of them going to jail too. The duck tries to annoy Mandy, but she remains imperturbable by its presence. The duck disappears and Principal Goodvibes is really angry, and he gives Mandy 75 days of detention.

Billy and Mandy find a supernatural zipper from Grim's robe, and while Billy uses it to create mischief, Mandy gets herself in a sticky situation.

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 buys a new cereal, a phoenix emerges from the box and attaches to his head, annoying everybody. Since the phoenix cannot be removed because it revives every time it is destroyed, Billy, Mandy and Grim seek help by going to the factory where the cereal is produced, which is run by Eris, the goddess of chaos. Mandy tricks the phoenix into leaving Billy and attacking Eris instead. It is revealed that all the events were really an episode-within-an-episode.