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

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.

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.

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.

After being accidentally hit by Billy, Grim lands in a can of white paint in a remote convent. Due to his appearance, still with the can in his head, Grim is mistaken for "Sister Big Mamma" by little nuns. Grim has a great time with the friendly nuns, until Billy and Mandy come looking for him. When the nuns realize the truth about Grim they get angry at him, but also help him go back home by sending him flying away with another punch.

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.

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.

After he turns down his dad's invitation to go fishing, Billy gets a tiny, weakling, pet fish, which he names Little Porkchop. To make Billy happier, Grim makes the fish grow overnight to become a giant. Billy takes Porkchop to play under the Sun, and never gives him water again. He burns to ashes and is flushed down the toilet. Billy's dad returns form his fishing trip having caught Porkchop, now back to his normal size, but plans to cook him.

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.

After watching a TV show, starring Captain Heifer and his Heifer Farms Brand Yogurt, Billy asks Grim to turn him into a superhero with the power of squeaking and throwing yogurt from his armpits. With his wish granted, Billy goes to outer space to fight the Evil Empire, but the planet he visits is mostly inhabited. Despite being free of Billy, Grim still has to deal with Billy's equally dumb father, Harold. After Billy fills the alien planet with yogurt, he returns home to be "normal".

Grim is annoyed by Billy who thinks that a shadow is following him. Grim takes Billy and Mandy to the Shadows' World, where their opposites live. Grim tells them to do whatever they want, but to be back at 5 o'clock, when the portal to the normal world opens. Grim also warns Billy about the Door to Other Dimensions, but Billy opens it and a multitude of Billys come out an wreak havoc.

When Billy has a loose tooth he cannot wait for the Tooth Fairy to appear to give him money. Mandy, who does not believe in the Tooth Fairy, asks Sperg to end Billy's waiting by knocking his tooth off. Sperg's punch leaves Billy unconscious, who has a dream about being in the wild west, where the Tooth Fairy has taken his tooth. Mandy challenges the Tooth Fairy to a showdown to recover the tooth, and she comes out victorious just as Billy awakens. Billy does find money under his pillow, while the aged Tooth Fairy watches through the window.

As Billy is tired of all the rules in his house, he cannot resist the urge to try his father's new motorcycle. The motorcycle breaks down, and Billy asks Grim to repair it. Grim's scythe turns the motorcycle into a wild, supernatural vehicle, which transforms Billy's parents and everyone around it into undead creatures and motorized monsters. Mandy makes Billy tell the truth about breaking the motorcycle in the first place, at which point his parents and everyone else return to normal.

When Billy goes into the mud for the fifth time in the day while wearing his white tuxedo before a wedding, Grim tries to figure out why Billy is so stupid. Billy's dad recounts Billy's life from his birth until he met Mandy and explains that Billy is not dumb, but actually a genius with a certified "Honorary Genius Degree". It is quite evident to Grim that Billy's problem is his equally dumb father.

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.

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.

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.

When Mandy and Grim tell Billy to quit watching TV and follow his life dreams, Billy gets into a chicken costume and decides to become a world famous "chicken juggler". When Billy's mom tries to convince him to be a chiropractor instead, Billy runs away to Uncle Chokey's farm to pursue his dream. Billy and Uncle Chokey juggle chickens at the rhythm of an old phonograph, whose music turns the farm into a 1920s black and white rubber hose cartoon. Due to a shortage of eggs in the city, Mandy and Grim go to the farm to investigate. They destroy the phonograph and things return to normal.

Billy, Mandy and Grim visit Mighty Moe, an old man who has discovered the secret to add extra years to his lifespan: a good diet and exercise. Mighty Moe decides to publish his secret in a book, and Grim needs to stop him by offering junk food.

As Billy and Mandy celebrate Father's Day, Grim becomes very depressed that he sits inside Billy's closet only listening to country rock music. Grim explains that, since his own father cared more about country rock than his own son, he has been living a lie; telling his father he is a famous country rock singer, and not the actual Grim Reaper. Mandy arranges for Grim's father to come visit and she tells him the truth, but still the old man does not believe her. Grim decides that the only way for his dad to listen to him is while performing country music. At an outdoor concert, Billy, Mandy, and Grim start performing a country rock song, in which eventually Grim reveals his supernatural abilities as the mighty Grim Reaper to his father. Grim's dad accepts his son is the Reaper, but he still does not forgive him for lying to him.

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.