List of ''The Grim Adventures of Billy & Mandy'' episodes

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.

In the present, Grim goes to reap General Skarr, who accidentally got a big wound in his torso. Due to Billy and Mandy's meddling, Skarr escapes, and they make a big mess in the city. Following his failure to be an effective Reaper, Grim is taken to the Underworld Court, presided by Judge Roy Spleen, where he is sued for misuse of his powers by his nemesis, the Boogeyman. Grim is found guilty, is stripped of his rank and magical scythe, and he, Billy, Mandy, and Irwin are sentenced to be exiled. Boogey volunteers to carry on the sentence by taking the prisoners aboard his ship, and dropping them off in a river of fire; he plans to do this while he sails to obtain Horror's Hand, an object capable of transforming him into the scariest, strongest and most powerful being in existence.

The group of Billy, Mandy, Irwin, and the powerless Grim, escapes from Boogey's ship when Billy distracts Boogey and his crew with the song "Scary-o", and set themselves to reach Horror's Hand before Boogey does. They eventually meet Horror, the guardian of the Hand, at the same time Boogey and his crew arrive. To decide who will have the honor to battle Horror for his Hand, the heroes and Boogey participate in a challenging obstacle course, which the heroes win against all odds. After dispatching Horror with ease, Billy, Mandy and Irwin reach for the Hand, but it defends itself by making their worst nightmares come to life. Boogey gets hold of Horror's Hand, but he turns out to be the least scariest creature ever, seriously injuring himself, and being abandoned by his own crew.

On a rainy day, giant spiders appear in Billy and Mandy's school and start kidnapping the children one by one. Grim presumes this is somehow related to his past, but does not figure it out until he receives a message saying that Velma, the Spider Queen, an old good friend from high school, is responsible for it. Meanwhile, Billy meets with Jeff the Spider, who informs him that he is precisely marrying Velma. After trying to recover a talking carton of chocolate milk from Jeff, Billy and Jeff get into a fight, and Jeff looks angrier than ever before.

Upon meeting in the school's football field, Velma tells Grim that she has finally come to devour his skull and absorb his magical and supernatural powers, since one hundred thousand years ago, Grim had cheated and robbed her from the possibility of becoming the Reaper in a scare election. Converting the school into her own palace, and with help from the Dark Elves, Velma captures Mandy, Mindy, and Grim, and proceeds to devour the latter's skull. When Velma cracks open Grim's skull, his thoughts start coming out in the form of light blue light, and she is able to see his past memories.

Grim's skull is connected to a projection screen, and the story on how he became the Reaper is shown. It is revealed that an election was held in the Underworld's high school to choose the Reaper, in which students had to vote on the scariest participant. Velma was the scariest contestant and she was getting most of the votes over the pathetic Grim and the Boogeyman. Boogey decided to tamper with the ballots, and Grim in turn decided to help Velma by filling the ballot with votes for her, but she got the impression that Grim was the cheater and ran away. In a fit of anger for what Boogey had caused, Grim used the prototype scythe in a spectacular fashion to scare Boogey off. The people that had not yet cast their vote voted for Grim, who became the new Reaper. While the story is being told, Mandy and Mindy form a temporary alliance to fight the spiders, and also Billy conquers his arachnophobia, and fights the spider-cow centaur-like extraterrestrial from the milk carton.