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.

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, 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.

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.

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.

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.