Billy's mother, Gladys, first notices Grim in Billy's bed, then in the shower and subsequently starts to get panic surges thinking that Billy is in danger. Meanwhile, Harold, Billy's dad, just thinks that Gladys is imagining things. In the end, Gladys ends up going living with Billy's aunt, Sis to try and calm herself down.

Billy and Mandy tell Grim it is "Opposite Day" and that, when they say something, they really mean the opposite. Due to his difficulty understanding the concept of Opposite Day, Grim has to do Mandy's chores. Finally, Grim thinks he understands when Billy and Mandy say they "want" pie, meaning they "do not want" pie. However, Mandy then says that when she said it was Opposite Day, it really was not.

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, Mandy, Grim and Pud'n are part of the "Person Scouts" (a Boy Scouts-esque organization) and need to sell cookies in order to win a badge. However, as Billy eats all of the cookies, Grim proposes to bake their own cookies using his aunt Kali's secret recipe. The cookies are a success and the troupe wins the contest. In the end, at the Person Scouts awards ceremony, aunt Kali reveals that the secret recipe contains insects, but everyone continues eating the cookies anyway.

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.

Billy tells Grim to help him throw a surprise birthday party for Mandy. Then, a reluctant Grim helps Billy prepare a cake while keeping it a secret from Mandy. In the end, a somewhat surprised Mandy reveals that her birthday isn't for the next five months, something that Billy already knew about.

Billy and Irwin have been playing their favorite video game, Beasts and Barbarians, for three days non-stop. In an attempt to stop their video game addiction, Grim transports them all to a real-life version of the game. Seeing as Billy and Irwin are still doing good in their quest, Grim increases the difficulty by manipulating the traps himself. In the end, Billy, Mandy, and Grim return home escaping from a giant cyclops, while Irwin remains trapped.

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.

While going to the shopping mall with Billy and Mandy, Grim spots the beautiful Eris, the goddess of discord and chaos, and tries to ask her out. However, Eris is not interested in the skinless Grim, but in the muscular Adonis. Then, Eris invites Mandy to join her in creating chaos using the Apple of Discord. Although initially they get along, Eris pulls a prank on Mandy, who subsequently tries to get revenge. In the end, Mandy fools Eris into thinking that Adonis is in the movie theater, and Eris accidentally kisses Grim.

After taking the Underworld's "Bad Book" from Grim, Billy becomes possessed by an evil entity. Then, Billy steals Grim's scythe, and uses it to open a portal to summon Yog-Sothoth. In consequence, Mandy calls for Hoss Delgado to battle Billy. It is revealed that the portal can only be sealed by throwing the book into it. As Billy eats the book, Mandy throws Billy himself into the portal, restoring everything to normal. Instants later, a portal opens up and Billy comes out and says they didn't want him there.

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.

In this musical episode, which pays homage to Little Shop of Horrors, Billy befriends a singing brain-eating meteor (voiced by Voltaire) that falls from the sky. With Billy's help, the creature grows bigger and eventually eats the brains of all people in Endsville (turning them into zombies with glowing green eyes). The last brain he eats is Mandy's, which results in the creature transforming into a brain-eating Mandy. The song featured is BRAINS!, composed by goth artist Voltaire specifically for this episode.

Billy, Mandy, and Grim eat moldy pizza leftovers, and after falling asleep, they have nightmares. Billy dreams of cute bunnies which turn into monsters; Grim dreams of an endless number of bossy Mandys; Mandy dreams of being pulled into a volcano and then commanding demons at the rhythm of music, in a scene that reminisces the Night on Bald Mountain segment of Fantasia. In the end, Mandy reveals that her dream was actually very pleasant, and she eats the last slice of the rotten pizza.

Mindy befriends Grim.

Grim brings monster cards to life.

Zombies crave the brownies Billy makes.

Zombies crave the brownies Billy makes.

Billy helps Eris spread chaos.

Mandy and Billy are forced to share a room.

Irwin turns into a werewolf.

Mandy develops a crush.

Nergal attempts to woo Billy's aunt.