Hog Wild (S04E03)

Frustrated with the strict rules at home, Billy succumbs to temptation and takes his father's new motorcycle for a spin. When the bike malfunctions, he enlists Grim's help to fix it. However, Grim's scythe accidentally enchants the motorcycle, turning it into a chaotic, supernatural machine that curses Billy's parents and those nearby into zombie-like beings and monstrous vehicles. Mandy insists that Billy confess to his initial mishap with the bike, and once he does, everyone—including his parents—returns to their former selves.