Episode Title:Substitute Teacher / Which Came First.Episode Description:Substitute Teacher: Billy's first cousin from the center of the Earth, Nergal Jr. goes to school for the first time, but he is worried that people may not like him. Soon enough, Junior becomes bullied by Sperg, with Miss Butterbean's allowance. Billy suggests Junior to use his shapeshifting abilities to take on the form of Miss Butterbean, so Junior can help him pass a math test. Junior likes the idea, but he morphs into Miss Butterbean to get revenge on Sperg instead. Sperg is seen living in the center of the Earth as Junior's pet.
Which Came First: While going to a science museum, Billy and Mandy's school bus breaks down in the middle of the desert. After digging for fossils, Billy, Mandy, and Grim wander around and find an abandoned mine, where Billy is captured by a giant, mutant chicken. When Mandy and Grim go to rescue him, they find an underground laboratory from the days of the Cold War where mutant chickens were created. The mountain makes eruption, the chickens are roasted by lava, and the whole class obtains food so they do not have to resort to cannibalism.