Billy and Mandy are sent to "Winter Camp", where they meet Nergal's son, Nergal Jr. Just like his father, Junior tries making friends, although not in the best way possible. Junior attacks the campers, freezes them and, using his shape-shifting abilities, takes over their appearances. Billy decides to be Junior's friend, but he is scared away at the moment Junior shows him his true form.

It's Valentine's Day and Nergal, the fiend living in Earth's core, appears in Billy's house to court Billy's spinster aunt Sis. Billy tries ruining Nergal's date, to avoid becoming his nephew. Although initially Sis does not show interest in Nergal, she changes her mind when Nergal is stung by bees and she sees his "dance moves". Aunt Sis accepts Nergal's marriage proposal and accompanies him back to the center of the Earth.

When Nergal comes to tell the guys about his new pizza joint, Grim thinks he can do better by using his Granny's recipe. A pizza war ensues between Nergal's Pizza and Grim's Pizza, with each pizzeria launching TV ads and discrediting each other. Nergal Jr. uses his shape-shifting abilities to infiltrate Grim's pizza and replace Granny Grim's hot sauce with a "ghastly elixir". The result is pizzas becoming monsters and attacking the city, and the citizens developing parasites similar to Nergal. Nergal and his family leave the city just as Grim's sauce explodes.