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.

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.

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.

While going to a science museum, Billy and Mandy's school bus breaks down in the middle of the desert. After digging for fossils, Billy, Mandy and Grim wander around and find an abandoned mine, where Billy is captured by a giant, mutant chicken. When Mandy and Grim go to rescue him, they find an underground laboratory from the days of the Cold War where mutant chickens were created. The mountain makes eruption, the chickens are roasted by lava, and the whole class obtains food so they do not have to resort to cannibalism.

When Billy sees his father shaving, he decides having a beard is cool and buys "Hair Raiser Tonique", a hair-growing formula. Grim casts a spell on the tonic to make its effects appear faster, but Billy exaggerates the quantity and grows hair in all his body. Billy is mistaken for the Sasquatch, and a reward is offered for his capture. Hoss Delgado also receives the tonic and is captured and exhibited as the Sasquatch, while Mandy collects the reward.

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 Billy and Mandy have a brief fight, Billy says he never wants to speak to Mandy again. The next day, Billy is over his grudge, but Mandy has already decided to find a replacement for him. Mandy introduces Bobby, the new "Billy", who turns out to be a better friend than Billy, even gaining the affection of his parents, but who also wants to take over Billy's life. A jealous Billy uses Grim's scythe to open a portal and send Bobby to another dimension.

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.

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.