The Grim Adventures of Billy and Mandy

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.

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.

Billy, Irwin and Mandy go trick-or-treating on Halloween, and an evil marshmallow bunny attacks children with chocolate beasts.

Billy's pet hamster, Mr. Snuggles, celebrates his 10th birthday, so the Grim Reaper comes for him. However, Mandy, Billy's friend, is reluctant in allowing it, so she proposes a deal to the Grim Reaper: if he beats the kids in a game, he can take the hamster, but if he loses, he will spare the hamster's life and become their "best friend forever". The Grim Reaper transports the kids to limbo, where he challenges them to a game of limbo. Despite his best effort, Grim ends up losing because he is attacked by Mr. Snuggles at Mandy’s command.

Billy's mother, Gladys, first notices Grim in Billy's bed, then in the shower and subsequently starts to get panic surges thinking that Billy is in danger. Meanwhile, Harold, Billy's dad, just thinks that Gladys is imagining things. In the end, Gladys ends up going living with Billy's aunt, Sis to try and calm herself down.

Billy and Mandy tell Grim it is "Opposite Day" and that, when they say something, they really mean the opposite. Due to his difficulty understanding the concept of Opposite Day, Grim has to do Mandy's chores. Finally, Grim thinks he understands when Billy and Mandy say they "want" pie, meaning they "do not want" pie. However, Mandy then says that when she said it was Opposite Day, it really was not.

Grim gets Billy and Mandy into sorcery school.

Mindy befriends Grim.

Grim brings monster cards to life.

At a father-and-son picnic, Billy re-encounters with his spider "son", Jeff, in a dirty bathroom. Once again, Jeff tries to make Billy love him as a true father does, but Billy remains terrified of all bugs. Mandy tells Jeff he is being too nice, and to get Billy's attention he needs to be meaner. After being coached by Mandy, Jeff finally snaps and ensnares Billy in his spider web, forcing Billy to say he loves him.

When Billy crashes his bicycle, he obtains a free tricycle from a mysterious boy in a supernatural porta-potty. While Billy loves his tricycle, which he calls Trykie, he is also mocked because of it by Irwin, Pud'n, and Sperg. Trykie moves on its own and attacks the three of them. Mandy realizes the problem with Trykie, and Grim confirms that he is cursed. Billy realizes that Trykie is indeed evil as well, and he lets Mandy destroy it once and for all.

After spilling salt in a restaurant, and not throwing salt over his left shoulder, Billy starts having an unusual amount of extremely bad luck. The next day, Grim analyzes Billy and finds out he has a "Catastrophe Snail" attached to his brain, which is causing him the bad luck.

At school, Billy, Mandy, and Irwin look for a club to join. Irwin plans to join the Macrame club to meet girls; Billy inquires on the Junior CIA club, which forcefully recruits him; Mandy joins the "Secret Snake Club", a snake-themed club consisting of three geeks. The nerds reveal that their "secret agenda" is to resurrect Shnissugah, a giant snake which lived hundreds of years ago in Canada, to take revenge against the popular kids, but they lack the power of the Underworld to do so. Mandy calls for Grim, who proceeds to revive Shnissugah through the internet. Billy, as a junior agent of the CIA, stops a criminal plan by Irwin, who was using the Macrame club for personal gain; and Shnissugah turns out to be a tiny, 8-cm (3.14-in) long, harmless snake.

When Billy and Mandy's school is preparing for a big game, Mindy entrusts the care of the school's mascot, Fluffy Cat, to Billy, to protect him from being kidnapped by the rival team, the Booty Heads. At Billy's house, Fluffy Cat is a disaster, so Billy's mom throws him into the basement, where the cat gets inside Grim's trunk, absorbing its powers. A giant Fluffy Cat appears just before the game wreaking havoc.

As Billy is tired of all the rules in his house, he cannot resist the urge to try his father's new motorcycle. The motorcycle breaks down, and Billy asks Grim to repair it. Grim's scythe turns the motorcycle into a wild, supernatural vehicle, which transforms Billy's parents and everyone around it into undead creatures and motorized monsters. Mandy makes Billy tell the truth about breaking the motorcycle in the first place, at which point his parents and everyone else return to normal.

When it's scurvy-prevention day at school, Mindy parades in her float in anticipation for the Little Miss Scurvy beauty pageant, and she insults Mandy, telling her she is ugly. To prove her otherwise, and with Principal Goodvibes' encouragement, Mandy considers entering the pageant. Naturally, Billy and Grim cannot believe Mandy when she tells them about the idea, but finally they decide to help her. Grim calls for Crabina, the Underworld's greatest makeup artist, to give Mandy a complete makeover. Crabina is shocked by Mandy's sour attitude, so she sets up principally to make her smile. At the pageant, Mandy has a real chance at winning the contest, but she still refuses to smile during her heart-warming musical number. Ultimately she smiles, but this event is so powerful that it alters the laws of the universe, and The Grim Adventures universe transforms into the The Powerpuff Girls universe, with Billy, Mandy and Grim turning into the Powerpuff Girls, Irwin turning into Mojo Jojo, and Professor Utonium guest starring as himself in a brief speaking cameo.

When Billy's parents are attacked by an anaconda, Billy suggest visiting boy wizard Nigel Planter, because he can talk to snakes. Planter in fact does not speak to snakes, but to snacks, and instead he asks for Billy, Mandy and Grim's help to win the affections of Huffefnie Pfefferpfeffer, a beautiful witch in the School of Sorcery (and having a striking resemblance to Gladys). Planter tries using a romantic love potion and enchanted chocolates, but they do not work. The guys end up completely forgetting about Billy's parents.

When Billy's parents leave for their second honeymoon, Billy is left alone at the worst possible moment, since he has invited Principal Goodvibes over to dinner so he can recommend him into Rollington Academy for the Gifted. Billy's dad suggest Billy to use Eris, the goddess of chaos, and Hoss Delgado, spectral exterminator, as his substitute parents during the dinner. The dinner is a disaster with Delgado terrorizing Goodvibes, and Eris unleashing giant alien zombie lobsters. Goodvibes forcefully recommends Billy into Rollington Academy, which, as it turns out, is actually a military school.

When Billy, Mandy and Grim go to the beach, Billy is swallowed by a whale known as Blubbery Joe, which also houses in her stomach Mister Geppetto and Pinocchio. Tired of not having a real son, Geppetto convinces Billy to be his "real son", while Pinocchio becomes increasingly jealous and is set on eating Billy's flesh to become a real boy and gain Geppetto's fatherly love and affection. Mandy and Grim search for Billy on board Captain Deadwood's ship, who is made of wood and who has been hunting Blubbery Joe for years. Billy, Geppetto and Pinocchio are expelled from Blubbery Joe's stomach, and as Billy needs to go home, Blubbery Joe accepts to be Geppetto's son.

When Mandy and Grim tell Billy to quit watching TV and follow his life dreams, Billy gets into a chicken costume and decides to become a world famous "chicken juggler". When Billy's mom tries to convince him to be a chiropractor instead, Billy runs away to Uncle Chokey's farm to pursue his dream. Billy and Uncle Chokey juggle chickens at the rhythm of an old phonograph, whose music turns the farm into a 1920s black and white rubber hose cartoon. Due to a shortage of eggs in the city, Mandy and Grim go to the farm to investigate. They destroy the phonograph and things return to normal.

When Billy's dad gets a promotion at his job, he tells his family they are going to move to another town, starting a dispute between Billy and Mandy over who gets custody of Grim. Grim suggests they settle this in court, and hours later Billy, Mandy and Grim find themselves in the Underworld court, presided by Judge Roy Spleen, and which has Fred Fredburger, an incorrigible dumb character, as a juror.

As Billy behaves more reckless than ever and cannot stop breaking everything in sight, Mandy thinks he should go some place where he can release all his urge to destroy until he gets tired of it. Mandy, Grim, and Irwin go to Level 9, home of the fearsome warrior Lord Pain, to convince him to take Billy. Billy receives psychological treatment and it is revealed that his desire to destroy stems from lack of attention when he was younger. Lord Pain abducts him, and together they start destroying everything in Level 9, which seems to be is uninhabited. Billy is still not satisfied, so he and Lord Pain return to Endsville and carry on breaking everything.

When Grim leaves the house to meet someone he met on the internet, Billy looks inside Grim's trunk for something to entertain himself. Billy finds a magical talking Rubik's Cube, which releases the demon Pinface, who has bowling pins in his head. Pinface proceeds to call his minions to conquer Earth, but the minions are more interested in throwing a house party. It is revealed that Grim and Pinface already know each other, and Grim even broke up with Pinface's sister. Billy's father ends up cheating and solves the magical cube by rearranging the stickers, trapping Pinface and his minions inside it again.

As Billy realizes the three geeks of the Secret Snake Club get doctor's notes to avoid Physical Education (P.E.) class, he wants to know their secret to avoid the class too. All the boys in the class join the club and all, except Billy, get doctor's notes from Billy's dad, who poses as a doctor. While Billy is the only one exercising in the class, the Snake Club summons a snake to destroy the gym. Since the invoked snake is tiny, Sperg uses Grim's scythe to make it a giant. Mandy and the other girls beat the boys and their snake because they did appreciate the P.E. class as an outlet for their aggression.