Billy, Mandy and Grim go to the beach where Grim falls asleep and Billy buries him in sand. When they leave, Billy only takes Grim's skull, leaving Grim's body to be found by a rich family that take it with them. With his adoptive family, Grim's body becomes a celebrity and refuses to go back with Billy and Mandy. Eventually, Grim's skeleton is reunited with his skull, but instants later it is shattered by dogs.

Lord Pain, a fearsome warrior from the Plain of Eternal Suffering, appears in Billy's house looking to serve his master, Grim. Lord Pain is put to clean the bathroom, and is confused as to why Billy and Mandy treat Grim the way they do. Eventually, Lord Pain realizes that Mandy is to be his real master, and prepares mashed potatoes for her.

In this short, set in the Stone Age, a child version of Grim undertakes his first assignments as the Grim Reaper. When he goes to visit a cave witch, she shows him his terrible future being abused by Billy and Mandy

When Billy, Mandy and Grim go playing golf, they encounter boy wizard Nigel Planter again, who is there hiding from the dark wizard Lord Moldybutt. They head back to the School of Sorcery, where they keep on running into Moldybutt, who warns them to stay away from the "Chamber Pot of Secrets" hidden in Dean Toadblatt's office. It is revealed that Lord Moldybutt is actually Toadblatt, who was disguising himself to scare off Planter. Note: The title is a spoof of Harry Potter and the Chamber of Secrets.

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.

After being bullied again by Sperg, Billy asks Grim to shrink him to enter Mandy's head to steal her "nerve". After Billy swallows the raw nerve, a polyp-like organism, Mandy becomes timid and scared of everything while Billy becomes tough and mean. While Billy goes to get revenge at Sperg, Mandy enters Billy's head to recover her nerve. Mandy recovers her tough and cynical attitude without her nerve, which is successfully retrieved from Billy and banished from Endsville.

When Billy forgets to do his history homework, Grim gives him a universal remote control to alter time and space so he can go back in time and finish his report. Billy and Mandy travel to the American Revolution, to prehistoric times, and even to the future, where they meet an aged Billy who is just finishing the report. After a struggle for the remote, Billy ends up again in the Prehistory and the remote breaks. Fast forward into the present, every person is dumb like he is and shares his characteristic big nose, including Mandy and Irwin.

After Billy wakes up from another nightmare involving clowns, he is more terrified than ever towards them, so Mandy and Grim try to cure him of this clown-a-phobia. Mandy asks Grim to bring to life a clown's head made on paper mache to entertain Billy, but the spell does not seem to work. They dress up as clowns in an attempt to look harmless. After consulting with his imaginary "Inner Frat Boy", Billy changes his feeling from being afraid to being angry at clowns, and attacks them with Grim's scythe. Billy is cured of his fear, but the paper mache clown attacks and eats them.

While reading an Underworld comic book, Billy finds an advertisement for "Chocolate Sailors", a delicious chocolate candy-which being Underworld candies are secretly hexed into enslaving the eater. Billy becomes a seller of Chocolate Sailors, but instead of selling them ends up eating them all by himself. The enchanted candies turn him into solid chocolate, and now Billy cannot stop eating himself. Billy, Mandy, and Grim go searching for the Chocolate Sailor to retrieve the antidote before Billy eats himself completely. Instead of eating the antidote, Billy eats the antidote along with other enchanted chocolates, and his head (which was all that was left of him) explodes as a result and covers Chocolate Sailor, Grim and Mandy in chocolate. Later Grim and Mandy are shown watching TV while drinking "Billy" Chocolate shakes.

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.

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 Billy finally gets a "secret decoder ring" in his Blasto Bits cereal, he receives a holographic message from a strange creature who speaks about finding the "Secret of the Universe" in an abandoned mine shaft. All the other children of Endsville, also with decoder rings, race to the mine to be the first one to get the Secret. The entire effort is revealed to be a trap laid by Lubbermouth, a worm-like creature from Grim's past, who wishes to free herself from the curse of guarding the Secret. When Mindy looks down at the pit, she becomes the new guardian of the Secret and has to stay there forever.

One night a strange odor travels through the city, prompting the noses of all the people, including Billy's, to follow it. The next night, Mandy, who is noseless, and Grim decide to wait outside Billy's house for the event to repeat. When the odor reappears, the nose parade leads them to the shopping mall, where they meet a "Nasalmancer", a nose wizard who is trying to take over other people's noses since he lost his own. Billy recovers his nose, but slices it in two, and gives one half to the Nasalmancer.

When Billy gets the opportunity to adopt one of four dogs, he says he wants them all. Grim uses his scythe to magically combine the dogs and create Billy's dream dog, who calls himself Wiggy Jiggy Jed. Although initially Billy and Jed are best friends, their relationship falls apart when Billy tells Jed to sleep outside. Jed uses the powers of Grim's trunk to build a giant machine to capture the beds of all people. Billy throws Jed a dish of primordial ooze and he dissolves.

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 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 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 Nergal goes to visit Billy's family (his in-laws) for the weekend, he ends up staying with them over a year. When Billy finally asks his uncle to leave, Nergal says he cannot return home because he is a complete failure, having lost at a board game 599 times in a row against his own family, and experiencing constant disappointment elsewhere. Billy and Grim tell him to not worry about being a failure, but to embrace it. After training with Billy and Grim, Nergal returns home and he finally wins at the board game, but he also makes a crack in the ceiling and his home starts filling with lava.

The day before Christmas, Billy, Mandy and Grim go to the shopping mall, where Mandy complains that Santa Claus is just an invention from corporations. Grim assures Mandy that Santa is real, and they even went to college together. They decide to travel to the North Pole and meet Santa, but when they arrive Mrs. Claus tells them that Santa Claus has been transformed into a vicious vampire.

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 digs his front yard he finds a frozen caveman, who is actually Fred Flintstone (from The Flintstones), but whom Billy names Jake Steele. Billy tries to acclimate Fred to the modern life but the job is not easy. When Fred is taken to school, he predictably goes out of control, kidnapping Miss Butterbean and running away in Principal Goodvibes' car. The ensuing chase results in Fred crashing into an ice cream truck, becoming frozen again. Instead of reaping him, Billy decides to bury Fred in the yard again. A future is shown where both Billy and Fred are defrosted by brain-eating creatures.

When General Skarr tries to get rid of a weed in his otherwise perfect garden, he asks Grim to use his scythe to do the job. Billy drives a lawnmower and ruins the garden; also, the scythe's magic accidentally shoots Skarr, transforming him into a plant-like creature. Using his new plant powers, Skarr rebuilds his garden, and tries to exact revenge on Billy by throwing a party, and attacking him. Skarr gets mulched by a helicopter rotor, but he is reborn as a small weed.

When Billy goes to visit his maternal grandmother and Mandy decides to have the day for herself, Grim finds himself free of the kids to do whatever he wants to do. Since Grim quickly gets bored he goes to visit General Skarr, and lets him take his magical scythe, which Skarr uses to build an army of giant lawn gnomes. Since Grim is so used to having two kids with him when they go on an adventure, he enlists Mindy and Pud'n to help him combat Skarr and retrieve his scythe. Mindy and Pud'n prove to be worthy substitutes but they are scared away by Mandy when she notices the unusual trio

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 Grim decides to visit again his childhood idol, the aged king of the vampires, Dracula, the latter suggest they drive to an early bird buffet, only to get lost in the scorching desert. Grim and Dracula need to find their way back home while dealing with situations like meeting a creepy truck driver and being attacked by a giant scorpion. After what they have gone through, and thinking that Dracula is just a selfish, old man, Grim decides to go a separate way. After finding that Dracula in fact helped him by sucking out the scorpion's venom, Grim rushes to help Dracula, and they finally make it to the diner. However, it appears that Dracula left without paying the check.

When Billy buys a new cereal, a phoenix emerges from the box and attaches to his head, annoying everybody. Since the phoenix cannot be removed because it revives every time it is destroyed, Billy, Mandy and Grim seek help by going to the factory where the cereal is produced, which is run by Eris, the goddess of chaos. Mandy tricks the phoenix into leaving Billy and attacking Eris instead. It is revealed that all the events were really an episode-within-an-episode.

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.

As Billy notices Mandy and Grim's usual negative attitude, he wishes they were in a magical land of happiness, so Grim transports them to an enchanted forest. Billy enjoys the relaxed surroundings until he meets a cantankerous and sour unicorn named Mary-Frances. Billy tries to change Mary-Frances attitude, but she is reluctant due to an earlier trauma of not being able to swim and follow the other unicorns into the sea. After Billy teaches Mary-Frances to swim, she feels happy and finally can join the other unicorns, but they turn out to be bullies, the real reason she never joined them in the first place. Billy stays with the unicorns and Mary-Frances goes back with Mandy and Grim to watch TV.

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.

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.

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