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, 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.

Grim tries to help Billy pass a test.

Grim tries to stop a show from being canceled.

Grim thinks he is a nun.

Grim and the children enter a kart competition.

Billy tries to show off at the Renaissance festival.

Grim sings in a band.

Billy and Mandy visit the Halls of Time.

Mandy eats a magical ball of chicken.