After Grim's robe comes out of the washing machine, Billy and Mandy find Thromnambular, a talking skull with ruby eyes and a number nine imprinted on his forehead. Grim tells them that the skull is a magical artifact imprisoned on Earth that must grant his nine wishes to be free. When Mandy asks Grim why he has not used any of the wishes, Grim tells her that the skull will corrupt the wishes and end up affecting the wisher.

After being utterly bored of watching the same movie 60 times in a row, Grim takes Billy and Mandy to "Underworld Video" where they rent "The Legend of Chupacabra". While Billy watches the tape, a small, pink chupacabra emerges from his television and Billy decides to keep it as a pet, naming it Daisy. Mandy and Grim convince Billy that he is inept in taking good care of pets, and they return the blood-sucking chupacabra to the videotape.

When Billy and Irwin are yet again bullied by Sperg, they decide to play for the school's football team so they are not bullied anymore. During practice, Irwin shows a powerful arm and he becomes the starting quarterback, while Billy is relegated to being a punching bag. During their first game, Mandy directs the team into scoring their first ever touchdown, so Coach Kilgore gives her the job of captain cheerleader, taking it away from Mindy. With Irwin's arm and Mandy's cheerleading the team reaches the finals. Mindy tries different ways to get back at Mandy and recover her job, but she ends up affecting her own team, which eventually loses the game.

There is a new student in the school, named Dora (parodying Dora the Explorer), who insists on getting people to open her lunch box. Mandy befriends Dora once she gets into a fight with the snobby Mindy. While noting something strange with Dora, Grim does not recognize her as Pandora until it's too late and she tricks Mandy into opening Pandora's box, releasing all kinds of plagues upon humanity. Mandy restores everything to normal, and captures Pandora inside the box, saying that nobody tricks her, and that when the time is right it will be Mandy herself who will unleash humanity's doom and destruction.

When Mandy says even a trained monkey is more competent to do cleaning chores than Grim, and would complain less, Grim makes Dickie Galoot, a monkey appearing in a Japanese-styled animation, come to life to prove her wrong. As Dickie turns out to be extremely good at cleaning Mandy's house, Grim decides to leave the kids for good. Due to his friendship contract, Grim is unable to break free, and he is forced to battle Dickie to recover his place as the "cleaning monkey" of the group. Grim successfully defeats Dickie at cleaning Billy's house, but this all turns out to be an ingenious plan devised by Mandy to win a bet against Billy.

When Billy, Mandy and Grim attend an Ancient Egypt exhibit at the museum, they meet with Irwin who explains that, as his mother is a mummy, they are visiting Irwin's grandfather, the great Pharaoh King Popen Locken. Later that night, Irwin's grandfather visits Irwin in his room and offers him the Pharaoh's crown and the possibility of building his own pyramid to attract women; Irwin accepts, calling himself King Tooten Pooten, and soon they turn the citizens into slaves to build the pyramid. Against his mother's wishes, Irwin decides to finish the pyramid; however, there is something he did not anticipate, as he, the Pharaoh, needs to remain entombed inside it.

After Billy, Mandy and Grim are disappointed with the Sassy Cat amusement park submarine attraction, Grim uses his powers to take them to the bottom of the ocean to have a real magical adventure. They are sentenced to death for trespassing into the Googen Kingdom by King Triceps, a wrestling merman. The king's daughter, Hariel, also a mermaid, stops the sacrifice and tries to marry Billy to inherit the kingdom. It is revealed that King Triceps is actually Hariel's sister, the sea witch Uvula, who also tries inherit the kingdom. A cat fight ensues between the two ladies, which ends when Billy agrees to marry both of them. The plot is revealed to be a story Billy is telling to Miss Butterbean, who initially asked him what happened to his report.

A scene of Endsville two weeks into the future shows a dystopian world ruled by the "Lord of Horror", who wields the all-powerful artifact known as "Horror's Hand". At the same time, two robotic replicas of Billy and Mandy are sent into the past, to stop the real Billy and Mandy from interfering with the current events.orror's Hand to rule the world.

When Billy accidentally food-poisons Miss Butterbean, Principal Goodvibes offers Grim the job of substitute teacher, which Grim accepts as now Billy and Mandy have to obey him. Grim sends misbehaving students to the "Detention Dimension", a place of eternal classroom punishment, until only Mandy remains in his class. Grim tricks Mandy into opening a portal and she is dragged into the dimension too. Mandy proves to be more terrifying than Miss Slither, the giant, rattlesnake-like creature in charge of the Detention Dimension, that Mandy escapes the place and takes revenge on Grim by sending him there too.