This season has six regular episodes, and the 1-hour Christmas special, in which Billy and Mandy try to save Santa Claus, who has been turned into a vampire.
Characters that feature include Nigel Planter, trying to win the affections of a young witch; Principal Goodvibes, having a disastrous dinner with Billy, Hoss Delgado and Eris; Billy's parents, Harold and Gladys, becoming children again; Nergal, being a sore loser; the ghost of British poet Lord Byron, teaching Billy to be cool; the blue Schlubs (a parody of The Smurfs); and the ancient tentacled demon Cthulu.
This season includes My Fair Mandy, the only episode in which Mandy willingly smiles, which causes unforeseeable consequences.
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 can't believe Mandy when she tells them about the idea, but finally they decide to help her. Then, 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 strong and powerful that it alters the natural order of things, and The Grim Adventures universe transforms into the The Powerpuff Girls universe, with Billy, Mandy and Grim turning into the three Powerpuff girls, Irwin turning into Mojo Jojo, and Professor Utonium guest starring as himself in a cameo.
When Billy's parents are attacked by an anaconda, Billy suggest visiting boy wizard Nigel Planter (a parody of Harry Potter), because he can talk to snakes. However, Planter in fact doesn't 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). Then, Planter tries using a love potion and enchanted chocolates, but they don't work. In the end, the guys have forgotten completely about Billy's parents.
When Billy's parents leave for their second honey moon, 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. Then, Billy's dad suggest Billy to use Eris, the goddess of chaos, and Hoss Delgado, spectral exterminator, as his substitute parents during the dinner. Naturally, the dinner is a disaster with Delgado terrorizing Goodvibes, and Eris unleashing giant alien zombie lobsters. In the end, Goodvibes forcefully recommends Billy into Rollington Academy which, as it turns out, is actually a military school.
When Billy's mom, Gladys, discovers crow's feet on her face, she panics and doesn't want to leave the bathroom. As Grim needs to use the bathroom, he uses his scythe to make Gladys a young girl again, thus without wrinkles. Then, Gladys goes to see Billy, but neither he nor Mandy nor Irwin recognize her, and instead, Billy calls her "Corn", because of the white bread she gave him. Then, when Corn tells Billy to do his homework, Mandy tells him to go to the movie theater instead. In consequence, Corn and Mandy struggle over who gets to control Billy. In the end, Billy's father, Harold, plays with Grim's scythe and also becomes a child; he then goes to the movies and gets together with Gladys (Corn) again.
When Billy, Mandy and Grim notice things have been disappearing in the house, they set out to investigate using a supernatural detector. They find the culprit is a talking "taking tree" in Billy's yard, who has been stealing objects since Billy's dad was a child. In order to make the tree happy, Mandy and Grim set out to trade the stolen belongings for a highly-valued Mickey Munchle baseball card, which is worth eleventy million dollars. In the end, Mandy and Grim return from the baseball shop with the real Mickey Munchle. Then, the tree returns all of the stolen things, but keeps the player.
When the mall's surveillance cameras capture Grim's silhouette attacking endangered owls, a naturalist named Corey Nebraska blames Grim for the attacks, while Mandy suspects Grim is sleepwalking again. Then Billy, Mandy and Nebraska follow Grim while he sleepwalks, but they realize it is actually Gladys, Billy's mom, the one attacking the owls. When discovered, Gladys reveals her hatred toward the owls as she recounts the story on how an owl ate her pet rat, Sniffles, years ago. In the end, Billy's dad reveals that he was in fact the one who ate Sniffles merely to survive until lunch time. Note: this is the only episode without a title card
When Nergal, the fiend from the center of the Earth, 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. Then, Billy and Grim tell him to not worry about being a failure, but to embrace it. In the end, 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.
As Billy desires to be "cool" like Sperg, he has an ectoplasmic manifestation coming out of his mouth. Later, the ectoplasm grows full size and turns out to be the ghost of nineteenth-century British poet Lord Byron, who tries to teach Billy to be cool, using elaborate poetry instead of bare muscles. Subsequently, Billy uses a poem to insult Sperg's mother, and Sperg challenges Billy to a duel. Then, Billy and Lord Byron fight Sperg and his gang, but are badly beaten. In the end, Byron tells Billy he has been trained in the arts of coolness, and disappears.
The Schlubs: As Grim watches the entire season of My Troubled Pony TV series (parodying My Little Pony), he learns from Mandy that the show was cancelled and doesn't have an ending. Then, 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. However, Grim must get past Billy and the aggressive Schlubs themselves, including the single, large female (a parody of Smurfette). In the end, after Grim thinks he got the creatures with his scythe, Mandy appears to have used all the gold herself to become rich.
As Grim watches the entire season of My Troubled Pony TV series (parodying My Little Pony), he learns from Mandy that the show was cancelled and doesn't have an ending. Then, 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. However, Grim must get past Billy and the aggressive Schlubs themselves, including the single, large female (a parody of Smurfette). In the end, after Grim thinks he got the creatures with his scythe, Mandy appears to have used all the gold herself to become rich.
When Billy and Irwin start making prank calls, everybody knows that Billy is behind the calls because every telephone has a caller ID. Then, while Billy searches in Grim's trunk in the basement for something to defeat the caller ID, he finds a skull-shaped phone with no caller ID. Then, Grim warns him to not use the phone, because it is used to call the ancient demon Cthulu, whose sole appearance makes people go mad, and if awakened will cause the destruction of the world. Despite Grim's warning, Billy and Irwin use the phone anyway, and they are sucked into it by tentacles. After the commotion, Mandy and Grim go search for Billy, and they find themselves in a strange place with several other tentacled creatures. In this place, Billy and Irwin have been hired as official prank callers and steadily turn into tentacled monsters. However, their pranks actually make the people answering the phone to become monsters, so Mandy decides to confront Cthulu himself to put an end to this. In the end, Mandy tricks Cthulu into answering her cellphone, and when he is sucked into it, Mandy disconnects the call, trapping Cthulu in the telephone lines forever. However, just as Billy, Mandy, and Irwin are about to leave, Grim has destroyed the phone at the basement, and they are unable to return.
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. However, Grim assures Mandy that Santa is real, and they even went to college together. Then, they decide to travel to the North Pole and meet Santa, but when they arrive Mrs. Claus (Carol Kane) tells them that Santa Claus (Gilbert Gottfried) has been transformed into a vicious vampire. Then, to save Christmas, Mandy and Grim go to stake the Head Vampire, Baron Von Ghoulish (Malcolm McDowell), while Billy remains with Mrs. Claus to look after her. However, it turns out that the cleaning-obsessed Von Ghoulish is not the vampire they are looking for, so they set out to find the Head Head Vampire. Then, Mandy, Grim and Von Ghoulish go back to Santa's workshop where it is revealed that Mrs. Claus is in fact the Head Head Vampire, and it was she who transformed Santa, which she did so she could take a break from all her work. Then, Billy finds holographic instructions to bake cookies, which he gives to the vampire Santa Claus, restoring him to normal. Finally, Mrs. Claus and Mandy prepare for a final confrontation, but Santa arrives on time to reconcile with his wife. In the end, Santa acknowledges he married his wife even though she was a vampire long before they met, and that she has turned him into a vampire many times before, (six or seven times in fact), but he always manages to recover.