In this season, The Grim Adventures of Billy & Mandy is a standalone show, separate from Evil Con Carne, although the ninth episode, a sketch of Malo Con Carne is tied with other two episodes of Billy and Mandy.
There were 9 regular episodes in this season, and the half-hour Halloween special, in which Billy, Mandy and Grim face against Jack O'Lantern.
This season introduces the boy-wizard Nigel Planter and supporting characters such as Dean Toadblatt, which constitutes a parody of the Harry Potter franchise. Other characters that make major appearances include Mandy's nemesis, the popular and snobby Mindy; Nergal Jr., the son of Nergal and Billy's aunt, Sis; and Jeff the Spider (voiced by show creator Maxwell Atoms), who is hatched from an egg by Billy.
Other characters that return are Hoss Delgado, who battles zombie brownies alongside Billy; and Eris, who keeps scheming to create chaos
In a parody of the Harry Potter series, Grim sends Billy and Mandy to a sorcery school so the latter duo could avoid boring summer camps they were supposed to go to. The sorcery school is owned by Dean Toadblatt, an anthropomorphic frog. Billy and Mandy are sent to the most hated house on the school, Weaselthorpe, while Grim becomes a prefect for a more loved house, Gunderstank. Billy and Mandy then team up with boy wizard Nigel Planter to sabotage Gunderstank. During the episode, Planter takes all the credit for the sabotage (which was Mandy's idea in the first place), but in the end, he is punished for it once he is "accidentally" found out by Toadblatt.
Billy and Mandy take a reluctant Grim to school with them. There, Mandy's nemesis, Mindy, befriends Grim to annoy Mandy. In consequence, Billy calls for their other friend, who is Abraham Lincoln. In the end, Mandy devises a plan in which Mindy ditches Grim, and keeps Abraham as her friend instead.
After watching Billy and Irwin play with their "Hokey Monsters" cards (parodying Pokemon), Grim makes the monsters come to life. Soon, all the other kids want their own cards to be real too. Subsequently, the city is under attack by a multitude of Hokey Monsters. To put an end to the mayhem, Mandy draws her own card, Mandy's Monster, which defeats all the other monsters by turning them into toast.
Grim is lying sick in bed with "Encroaching Doom Syndrome". The Underworld doctors are not able to help Grim, and he becomes a human being. Back in Endsville, Grim is apparently attractive to women, but he ends up being punched by jealous boyfriends, especially Billy's dad Harold. In the end, Grim covers himself in the mucus that he produced when he was sick, and becomes a skeleton again. Billy tries it too.
Taking inspiration from his Pat the Baker video game, Billy decides to bake some brownies. However, Billy uses "Evil Powder" from Grim's old trunk instead of salt, and the resultant brownies stink. Mandy tells Grim to get rid of the brownies, and he reluctantly does, but he also stores some of them in his skull. Soon, the stench of the brownies spreads through the world and starts resurrecting dead people, animals, and even food, as zombies.
Billy and Hoss Delgado, spectral exterminator, team up to destroy all the zombies that congregate outside the house, but the zombies reassemble into a bigger zombie. Delgado realizes that the zombie is after the brownies that Grim has in his skull, so Delgado and Grim battle furiously for them. In the end, the zombie eats the remaining brownies and then falls apart.
On a hot day when Billy, Mandy, and Irwin are bored, Grim uses a crystal ball to show them the future. In the distant future, Mandy has turned herself into a giant worm-like creature to be immortal, and now rules the whole world. Mandy gives false information to a clone of Billy, in order to capture the members of a resistance that try to defeat her. The episode parodies the science fiction novel God Emperor of Dune by Frank Herbert.
When Billy, Mandy, and Grim go fishing, Eris, the goddess of chaos, gets Billy to help her use the Apple of Discord to create chaos (after her father grounded her). Instead of using the apple, Billy becomes distracted with his own idiotic errands, leading Eris to go insane. In the end, Billy finally decides to start creating chaos, but Eris is already too altered that she runs away.
Billy accidentally sets off a powerful stink bomb in Mandy's house and, while the house is being disinfected, Mandy's family goes to live with Billy's. Then, Billy and Mandy have a dispute over how to split Billy's room, prompting Grim to intervene. In the end, Mandy takes over the entire room, and traps Billy behind a brick wall.
After excluding Mandy from their board game Who Killed Who? for being a girl, Billy and Irwin mistakenly throw their dice into the backyard of a haunted house. Grim warns them about the ghost of Mrs. Doolin, a lady that lived there. Mandy dares to go inside to retrieve the dice, and she befriends Mrs. Doolin, who is alive and well, and who says that all the stories told about her (that anything that went beyond her fence into her garden never came back) are inventions from Grim, whom she beat long time ago at a staring contest. Fearing for Mandy's life, Billy also ventures into the house, but Mrs. Doolin and Mandy scare him away by posing as ghosts. However, it appears that Mrs. Doolin is in fact a ghost, and when she meant "beating Grim", it meant that she "beaten death".
After Grim fails to pull a werewolf out of his hat, Irwin is bitten by said werewolf. Then, Billy takes Irwin as his new pet dog, naming him Sprinkles, and enters him in the Endsville Dog Show to compete against Mandy's dog, Saliva. At the contest, Sprinkles is about to win, so Grim releases a "were-flea" whose bite causes the werewolf to revert to a naked Irwin. In the end, it appears that Irwin won the contest anyway.
When Billy, Mandy and Grim go to the beach, Grim falls in love with a goth woman named Malaria. Grim is having a good time with his girlfriend, but Mandy assures him that will change when he tells her that he is not human. In the following date, Grim demonstrates his supernatural powers while dancing and, as predicted, Miranda leaves in panic.
At school, Mandy rejects Irwin's invitation to go to the school dance with him. Then, Billy introduces his foreign and mysterious, new friend, "Piff", and Mandy becomes desperate when she realizes she has fallen in love with him. Subsequently, Mandy asks Grim to cut her heart out to stop being in love; however, Grim has a better idea and gives Mandy a makeover. In the end, Piff falls in love with Mandy, and Mandy takes the chance to brutally turn him down.
It's Valentine's Day and Nergal, the fiend living in Earth's core, appears in Billy's house to court Billy's spinster aunt, Sis. Then, Billy tries ruining Nergal's date, to avoid becoming his nephew. Although initially Sis doesn't show interest in Nergal, she changes her mind when Nergal is stung by bees and she sees his "dance moves". Then, aunt Sis accepts Nergal's marriage proposal, and accompanies him back to the center of the Earth.
Billy takes a big egg found in Grim's trunk and hatches it expecting a duck. Instead, Jeff the Spider is born, who considers Billy his father. Despite Jeff trying to please Billy in any possible way, Billy is terrified of Jeff and wants him squished. In the end, we see that the exterminator didn't squish Jeff because he is a nice bug.
Billy is worried about failing an upcoming Math test, so Grim gives him a "bookworm", which will make him much smarter. However, Milkshakes, Billy's cat, eats the worm instead, and is forced to tutor Billy. In the end, Billy is still an idiot.
Grim is upset after his favorite show, Atrocia's Late Night Atrocities, is cancelled. Then, he goes to the TV station and using his powers, turns the studio into a creepier place and becomes the host of the Grim Show. Grim's success on TV is interrupted when Mandy tells him he has neglected to do her chores. Then, Billy and Mandy go to the TV station and show embarrassing footage of Grim at home.
Billy and Mandy are sent to "Winter Camp", where they meet Nergal's son (and Billy's cousin), Nergal Junior. Just like his father, Junior tries making friends, although not in the best way possible. Junior attacks the campers, freezes them and, using his shape-shifting abilities, takes over their appearances. In the end, Billy decides to be Junior's friend, but he is scared away at the moment Junior shows him his true form.
After being accidentally hit by Billy, Grim lands in a can of white paint in a remote convent. Due to his appearance, still with the can in his head, Grim is mistaken for "Sister Big Mamma" by little nuns. Then, Grim has a great time with the friendly nuns, until Billy and Mandy come looking for him. When the nuns realize the truth about Grim they get angry at him, but also help him go back home by sending him flying away with another punch.
When Billy and Irwin exclude Mandy and Grim from helping them build a go-kart to enter the Annual Endsville Go-Kart Competition, the latter two decide to build their own. At the race, Billy-Irwin, and Mandy-Grim compete against Mindy, Sperg, and a mystery racer (Pud'n). In the end, Mindy gets stuck in highway traffic, Billy-Irwin's car is destroyed, Sperg and Grim mutually prevent each other from winning, and Pud'n wins in his litte red wagon.
After being impressed by Irwin's realistic armor at the medieval fair, Billy wishes for a fancy armor too. Then, Grim gives him the "Cursed Suit of the Black Knight". However, when Billy needs to go to the bathroom, the curse is revealed to be that the armor can't be taken off until the person wearing it beats someone in a competition. Therefore, Billy desperately tries to defeat other people at various contests but fails to come out victorious. In the end, Billy defeats Mandy at a breakdance contest, in which Mandy decided not to participate, but he doesn't need to go to the bathroom anymore.
Billy wants to join Sperg's garage band, "Purple Filth", and take part in the upcoming battle of the bands. However, after witnessing Grim's prowess with the electric guitar, Sperg invites Grim instead. Then, Billy's dad, adopting the rock and roll persona of "Mogar", decides he and Billy should go to the contest to make a surprise appearance. In the end, Billy impresses the audience with his armpit sounds.
After missing on a school trip, Grim takes Billy, Mandy and Irwin to the "Halls of Time" for a tour. There, Billy and Irwin mess the hourglasses that symbolize each person's lifespan. When they leave, they accidentally leave their own hourglasses, including Grim's, upside down, resulting in the group getting younger as time passes. In the end, Grim allows the kids to disappear by going before birth, but he can't restore his own hourglass to its correct position and he disappears too.
After a lousy night for Grim, Billy and Grim argue about who has the hardest time, the kid or the Reaper of Souls. Mandy proposes they switch places for the day to find out. Then, Grim seems to be having a good time, acting like a silly kid, while Billy is bored waiting for his old victim, Mr. Voorhees, to open the door. In the end, Billy transforms into a short skeleton, while Grim transforms into a taller version of Billy.
Grim's refusal to help Mandy win a karate tournament, and the $50 000 dollar prize it promises to the winner, leads her to a curio shop, where the shopkeeper, Kuan Ti, gives her an ancient "Chicken Ball" which, when eaten, allows her to become a great fighter. Subsequently, Mandy easily wins the tournament and doesn't care about the money anymore, only about fighting. It turns out Kuan Ti is actually Eris in disguise, and this was just another of her schemes to create chaos. To stop Mandy, Grim makes Billy a superb fighter too, and he and Mandy fight ferociously until the entire arena is destroyed, (note: this episode is an obvious parody of Dragon Ball Z). In the end, Mandy still demands the prize money, which she uses to buy Hector Con Carne's (from Evil Con Carne) Bunny Island.
It's Halloween, and Billy, Mandy and Grim go trick-or-treating, while Irwin is left behind because he needs a scarier costume. After pulling a trick on an old lady, and getting some candies, Grim tells the story of why people pull pranks, which he was partially responsible for. He tells about Jack, a prankster of the old Endsville who just didn't know when to stop. One day, the citizens decided to punish him for his constant pranks, and they pulled a prank on the queen, but signed it "From: Jack"; in consequence, the angry queen sent a knight to kill Jack. However, when Grim went for Jack's soul, Jack stole his scythe. In return for his scythe, Grim promised Jack eternal life, but after closing the deal, Grim cuts Jack's head off. Thus, Jack decided to use a pumpkin for head, becoming Jack O'Lantern, and every Halloween he goes out to pull pranks on the citizens of Endsville.
After hearing the story, Billy splits from the group and finds Jack's house downtown. Then, Jack realizes Billy is carrying the actual Reaper's scythe, so he takes it from him and uses it to open the gates of the Underworld and bring forth ghosts, which cause havoc in the city. Jack then plans to get revenge on Grim by cutting his head off which, being cut with the Reaper's scythe, will be cut permanently. To stop Jack from carrying on with his vengeance, Mandy sets up a series of pranks for him. However, the pranks affect Irwin instead, causing major laughter from Jack and his ghouls, which explode and return to the Underworld. In the end, Grim recovers his scythe and sends Jack to the Underworld too.