Grim is bored of being left alone while Billy and Mandy are not with him. Grim decides to fulfill his dream by working as a swimsuit model, but, as he is a corpse, he is not hired. Subsequently, Billy and Mandy give him a makeover to look more human. In the end, Grim gets the job in a TV ad, but he is only in the commercial for a few seconds and is portrayed as an ugly person.

Grim misses his life as the reaper of souls and, due to his bet, he cannot return to the Underworld, much to the hilarity of his Underworld co-workers. Having a sudden epiphany, Grim decides to be good and help people rather than try to take their souls. However, Grim's attempts at being good are faced with resistance from people, who are scared off by his appearance.

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.

While Spring cleaning in the middle of November, Billy takes an old, cursed lamp from Grim's trunk. When he gives it a rub, he releases the curse Grim had warned them about: his own mother. Having three wishes to make, Billy wastes his wish by wishing to know what to wish for. Mandy wishes Billy and Grim would "shut up", so Grim never gets to make his wish. Then, Mandy decides to make the last wish to whoever makes her the happiest. In the end, Mandy wishes everyone in the world would go away, which she is glad about.

After having a nervous breakdown and leaving the house in the first episode, Billy's mother, Gladys, returns. She meets Grim again and, this time, she plans to destroy him once and for all. When Grim goes to the kitchen for a glass of water, Gladys attacks him and shatters his skeleton. In the end, Grim assures the kids that Gladys has won the battle but not the war.

With Irwin not answering the phone, Billy's parents missing and Mandy saying she wants "Billy for dinner", Billy and Grim assume Mandy is a cannibal who ate Billy's parents and friends. Looking for proof, Billy and Grim find tiki figures and arrangements for a celebration in Mandy's backyard. Ultimately, it is revealed that they are just celebrating Billy's birthday. Still, Mandy burps out Irwin's glasses.

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.

Billy, Mandy, and Grim go to the museum on a school trip. There, they meet Hoss Delgado, self-appointed hunter of the paranormal, who upon spotting Grim, tries to eliminate him. Subsequently, Grim and Delgado engage in a fight. Finally, Delgado quits hunting Grim when he realizes Billy and Mandy control him. In the end, Delgado continues his own adventures and is seen fighting a werewolf.

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.

Mandy and her parents leave the city for a day, which makes Billy feel lonely. Then, Grim uses his supernatural powers to create a fake Mandy with pieces of dolls and toys for Billy to play with. However, the "new" Mandy is too aggressive with Billy, and he starts fearing her. The next day, the real Mandy returns home, and promptly destroys the new Mandy while in the see-saw.