As Grim watches the entire season of "My Troubled Pony" (parodying My Little Pony), he learns from Mandy that the show was cancelled and does not have an ending. Grim decides to reap the "Schlubs" (a parody of The Smurfs), 100 creatures that live in Billy's yard, to melt them into gold so he can finance the creation of the series finale. Grim must get past Billy and the aggressive Schlubs themselves, including the single, large female (a parody of Smurfette). After Grim thinks he got the creatures with his scythe, Mandy appears to have used all the gold herself to become rich.

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.

General Skarr buys a home in Endsville and becomes Billy's neighbor. After Skarr invites Billy into his new house, Billy mentions Grim's scythe as ,he strongest and most powerful weapon in the entire universe, and Skarr tries to battle his desire to be evil again. Eventually Skarr steals the scythe but Mandy proves to be even scarier than he is, and takes it back.

When Billy decides he wants to grow up to be like Grim, he says nothing will stand in the way of their friendship. They meet a girl named Blandy, who looks very much like Billy, and Billy falls completely head-over-heels in love with her. When Blandy shows romantic interest in Grim, Billy is struck with mad jealousy and wants Grim out of the picture. Mandy consults Grim's encyclopaedia of monsters and realizes Blandy is actually a creature from another dimension. Despite Blandy showing her true form and leaving, Billy remains madly romantically infatuated with her; but only up until the following episode.

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

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.

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 and Mandy are utterly bored, Grim takes them to Underworld's "Circus of Fear". The ringleader, Dr. Fear, notices Billy and Mandy, and tries to recruit them into the circus to attract customers, who would pay to see real, live humans. After they go back home, Dr. Fear captures the two kids, and Grim has to get them back. During the show, Mandy breaks free and in a strange twist of the story, she reveals that the beastmaster, Clortho, is actually Dr. Fear, who in turn is just an earthworm, leaving Billy confused.

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.

Billy, Mandy and Grim visit Mighty Moe, an old man who has discovered the secret to add extra years to his lifespan: a good diet and exercise. Mighty Moe decides to publish his secret in a book, and Grim needs to stop him by offering junk food.

When Billy and Irwin want to watch classic giant monsters movies, Grim decides to take them to Japan to see real monsters. They meet the turtle-like Cragorah (a parody of Gamera) who engages in a fierce battle with the three-headed Kittirah (a cat version of Ghidorah). To help Cragorah, Grim transforms Billy into a giant monster too (a parody of Ultraman). Mandy soon arrives with the intent of punishing them all for organizing this expedition without her permission. Controlling Mecha-Gorillasaur (a giant gorilla robot) she defeats Kittirah and beats Billy.

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.

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.

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.

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 Mandy and Grim go to the warehouse store, Mandy notices that everything seems to be in order, lacking any presence of chaos. Hoss Delgado, spectral exterminator, arrives saying that everything has been that way since he started dating Eris, the goddess of chaos. Mandy assures Delgado he must break up with Eris to restore chaos, but fearing her temper, he is reluctant to do so. After failed attempts by Grim, Mandy, Billy, and Irwin to break their relationship, Delgado finally tells Eris the truth about he wanting to break up, and she complies by turning into a giant insect and devouring him.

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.

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.

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.

When Mindy mocks Mandy's dumb and coward dog, Saliva, Mandy decides to train her dog to be meaner. After Grim keeps mentioning his own dog, Cerberus, Mandy decides to use Grim's special whistle to call the gigantic, three-headed, fire-breathing hound. Cerberus marries Mindy's chihuahua, Little Delicious, and Saliva snaps against Mindy for a brief moment.

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.

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 ruins his eyes by playing too many video games and looking directly into the Sun, Mandy and Grim suggest he see an ophthalmologist. When Billy refuses wearing glasses or having laser surgery, Grim uses his own scythe to zap his eyes. Although Billy has sharp and clean eyesight, he can also see glimpses of the future, specially horrific accidents that occur to his best friends. Grim zaps Billy's eyes multiple times to restore them to normal. Billy ends up with weird-looking eyes.

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.

After watching a movie, Grim teaches Billy to possess people by jumping into their heads. Billy possesses Mandy and makes her do assorted things like flirting with Irwin, kissing Mandy's mother, going into both men's and women's restrooms and eating earthworms. In the end, Billy leaves Mandy's head, but later that night, Mandy possesses Billy's cat, Milkshakes, to maul him as a form of revenge.