This season includes 16 regular half-hour episodes. For the most part, from this season on each episode includes only two segments, each of 11 minutes (without commercials), instead of three of 7 minutes each.
In this season, new characters that are introduced are Sir Raven (Julian Stone), a humanoid raven that serves as the storyteller for some segments; Lord Pain, the fearsome warrior looking to serve his master, the Grim Reaper; Cerberus, the gigantic, three-headed, fire-breathing dog of Grim; Pinocchio, the puppet made of wood who wants to eat Billy's flesh to become a real boy; Lord Moldy Butt, a parody of the Harry Potter villain, Lord Voldemort; and Boogey Man, the Boogeyman himself and a long time adversary of Grim.Irwin's parents are also intreoduced, his father Dick, and his mother Judy, who is revealed to be a mummy.General Skarr, a character from Evil Con Carne, moves to Endsville after the Hector Con Carne's company was sold to oher company who doesn't want to have competition to plan to rule the world.
Other characters that feature prominently include Billy's "son", Jeff the Spider; Billy's neglected pet fish, Little Porkchop; Billy and Mandy's school teacher, Eleanor Butterbean; Billy's cousin, Nergal Junior, and his shape-shifting powers;and his acne problems; the Chocolate Sailor, and his The stories presented are varied, including Billy helping out dwarves in their war against the elves; Billy becoming the superhero the Green Squeaker, having the power of being green, squeaking when he moves, and shooting yoghurt from his armpits; and Hoss Delgado, spectral exterminator; Odin, Thor, and other Norse gods, who battle against Billy in Valhalla; Eris and the Apple of Discord; Billy facing his phobia of clowns; Sperg,Billy stealing Mandy's "nerve", to become tough and mean;enchanted candies; the Tooth Fairy, who is an Old West cowboy; Mandy getting a new friend to replace Billy; and Billy getting rid of his "Dinobonoid" action figures.
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, but Billy remains terrified of bugs. Then, Mandy tells Jeff he is being too nice, and in order 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. Following this, Trykie moves on his own, and attacks the three of them. Later, Mandy realizes the problem with Trykie, and Grim confirms that he is cursed. Finally, Billy realizes that Trykie is evil, and he lets Mandy destroy him 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 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.
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.
After he turns down his dad's invitation to go fishing, Billy gets a tiny, weakling, pet fish(Arnold Schwarzenegger), which he names Little Porkchop. Then, to make Billy happier, Grim makes the fish grow overnight to become a giant. However, Billy takes Porkchop to play under the Sun, and never gives him water again, so he burns to ashes and is flushed down the toilet. In the end, Billy's dad returns form his fishing trip having caught Porkchop, which returned to his normal size, but plans to cook him.
General Skarr (originally appearing in Evil Con Carne) buys a home in Endsville and becomes Billy's neighbour. Then, after Skarr invites Billy to his new house, Billy mentions Grim's scythe as the most powerful weapon in the universe, and Skarr tries to battle his desire to be evil again. Eventually, Skarr steals the scythe, but Mandy proves to be scarier than he is and takes it back.
Lord Pain, a fearsome warrior from the Plain of Eternal Suffering, appears in Billy's house looking to serve his master, Grim. However, 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 Mindy mocks Mandy's dumb and coward dog, Saliva, Mandy decides to train her dog to be meaner. However, 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. In the end, Cerberus marries Mindy's chihuahua, Little Delicious, and Saliva snaps against Mindy for a brief moment.
After Billy eats 19 creme-filled Atomic Frenzy Cakes he can't sleep and wants Mandy to read him a bedtime story. Then, Mandy, Billy and Grim each tell a story. For his story, Grim uses a supernatural storybook from the Underworld, which makes Billy and Mandy enter the book and reprise the tale of Hansel and Gretel. However, Billy departs from the story and follows Pinocchio who, as is later revealed, wants to devour Billy to become a real boy. Eventually, Mandy saves Billy from Pinocchio, but she and Billy remain trapped in the book as Grim, the storyteller, falls asleep.
When Billy ruins his eyes by playing too much video games and looking directly into the Sun, Mandy and Grim suggest he see an ophthalmologist. However, when Billy refuses wearing glasses or having laser surgery, Grim uses his own scythe to zap his eyes. Now, although Billy has a clean eyesight, he can also see glimpses of the future, specially horrific accidents that occur to his friends. In consequence, Grim zaps Billy's eyes multiple times to restore them to normal. In the end, Billy ends up with weird-looking eyes.
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 (a parody of Lord Voldemort from the Harry Potter books). Then, 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. In the end, it is revealed that Lord Moldybutt is actually Toadblatt, who was disguising himself to scare off Planter. Note: This is a parody of Harry Potter and the Chamber of Secrets.
When Billy and Mandy are utterly bored, Grim takes them to Underworld's "Circus of Fear". There, 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.
After failing to scare a 15-year-old, the Boogieman decides to visit his old middle-school victim, Grim. Then, Grim remembers how Boogie used to be a bully, and demands an apology from him. Instead, Boogie disappears to the Underworld, taking Billy with him. Mandy and Grim follow Boogie, and to settle their differences Mandy suggest having a scaring contest, in which whoever can scare a kid the most wins. In the end, Boogie's transformation as a werewolf is not enough to scare the kid, but Grim's transformation into a giant, horrific rubber duck is.
As Billy walks into the woods to have a picnic, Yogi and a giant, feral version of Boo-Boo (from The Yogi Bear Show) come to steal his basket, and he escapes by hiding in a cave. Then, Billy is taken to the mushroom lair of the dwarves where King Beardbottom tells him the story on how the different races split the food-services industry,after the conference of Rivendell after the The Cold War,in which Ronald Reagan allows each race to chose which food to make (The meeting is a parody from the Lord of the Rings.)Then, the king sends Billy to infiltrate the elves' cookie factory to deactivate their force field, so the dwarves can assault the factory. Finally, after a fierce battle (censored out "for younger viewers", by a single clip of a pink koala continuously licking a eucalyptus leaf) and Billy's words, the elves and dwarves decide to work together and split the profits.
Billy's 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. Then, Billy suggests Junior to use his shape-shifting powers to take 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. In the end, Sperg is seen living in the center of the Earth as Junior's pet.
While going to a science museum, Billy and Mandy's schoolbus breaks down in the middle of the desert. Then, after digging for fossils, Billy, Mandy and Grim wander around and find an abandoned mine, where Billy is captured by a giant, mutant chicken. When Mandy and Grim go to rescue him, they find an underground laboratory from the days of the Cold War, where many mutant chickens were created. In the end, the mountain makes eruption, the chickens are roasted by lava, and the whole class obtains food so they don't have to resort to cannibalism. This episode might be a parody of Godzilla (1998) and Raptor Island.
After watching a TV show, starring Captain Heifer and his Heifer Farms Brand Yoghurt, Billy asks Grim to turn him into a superhero with the power of squeaking and throwing yoghurt from his armpits. Then, with his wish granted, Billy goes to outer space to fight the Evil Empire, but the planet he visits is mostly inhabited. At home, despite being free of Billy, Grim still has to deal with Billy's equally dumb father, Harold. In the end, after Billy fills the alien planet with yoghurt, he returns home to be "normal".
Trying to get revenge at Mandy for bossing them all the time, Billy and Grim put her in the "Mask of the Beast". Now, Mandy must be nice and friendly while wearing the mask, or else she'll become a hideous monster. Then, failing to remove the mask herself, and struggling to be kind to Billy and Grim, Mandy gradually turns into the beast. In the end, Billy and Grim ask Mandy to smile while she gives them a sponge bath, but this turns out to be too hard for Mandy, then she fully transforms into the beast and punishes them instead.
When Billy sees his father shaving, he decides having a beard is cool and buys "Hair Raiser Tonique", a hair-growing formula. Then, Grim casts a spell on the tonic to make its effects appear faster, but Billy exaggerates in the quantity and grows hair in all his body. Consequently, Billy is mistaken for the Sasquatch, and a reward is offered for his capture. In the end, Hoss Delgado also receives the tonic and is captured and exhibited as the Sasquatch, while Mandy collects the reward.
After being bullied again by Sperg, Billy asks Grim to shrink him to enter Mandy's head to steal her "nerve". Then, after Billy swallows the raw nerve, a polyp-like organism, Mandy becomes a coward while Billy becomes tough and mean. Later, while Billy goes to get revenge at Sperg, Mandy enters Billy's head to recover her nerve. In the end, 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. Then, 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.
During a thunderstorm, a lightning hits Grim's scythe, transporting Billy to Asgard, land of the Norse gods. Then, Mandy and Grim go searching for him, while a viking wearing Billy's shirt remains at his house as a substitute. In Asgard, Billy learns about the endless battle in Valhalla, in which if a warrior dies, he is resurrected to continue fighting. In the end, Billy is surprisingly good at killing other Vikings, so Mandy and Grim are unable to convince him to return home.
Grim is annoyed by Billy who thinks that a shadow is following him. Subsequently, Grim takes Billy and Mandy to the Shadows' World, where their opposites live. Grim tells them to do whatever they want, but to be back at 5 o'clock, when the portal to the normal world opens. Grim also warns Billy about the door to other dimensions, but Billy opens it and a multitude of Billys come out an wreak havoc.
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. First, Mandy asks Grim to bring to life a clown's head made on paper mache to entertain Billy, but the spell apparently doesn't work. Then, they dress up as clowns in an attempt to look harmless. However, 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. In the end, Billy is apparently cured of his fear, but then the paper mache clown attacks and eats them.
Thinking that the world is chaotic enough as it is, Eris, the goddess of chaos, decides to quit chaos to work at a hamburger restaurant. Then, she entrusts Grim the Apple of Discord to watch over for her, which he seals in his old trunk. However, the apple frees itself from the chest and goes to Billy. Later, Mandy is attracted by the apple's powers, and after being persuaded by Billy's talking burrito, Billy, Mandy and Grim battle for control of the apple, which divides in three pieces. In the end, the battle results in them being fused together into an amorphous mass, and, with an evil grin on her face, Eris decides to offer the apple to Irwin too.
This episode is unusual in some aspects: it starts being called Billy gets Dumber, but then the Apple of Discord swallows the screen and the real title Complete and Utter Chaos is shown instead; the episode starts at the end, and the story is told as a flashback; the talking burrito is featured prominently, but other than a brief mention, its origin is not explained.
When Billy and Mandy have a brief fight, Billy says he never wants to speak to Mandy again. The next day, Billy is over his grudge, but Mandy has already decided to find a replacement for him. Later, Mandy introduces Bobby, the new "Billy", who turns out to be a better friend than Billy, even gaining the affection of his parents, but who also wants to take over Billy's life. In the end, a jealous Billy uses Grim's scythe to open a portal and send Bobby to another dimension.
When a doctor is unable to cure Sperg of his severe case of acne, Sperg goes to see a fortune teller that deals with skin problems. Then, following a Burmese recipe in her book, the psychic tells Sperg to get the Grim Reaper's bones and rub them upon his face. In consequence, Sperg has to tolerate being friends with Billy and Grim to get access to the latter's bones. In the end, one of Grim's arms gets into a stew and when Sperg is immersed in it, the mix cures completely his acne, but it also erases his nose and mouth.
While reading an Underworld comic book, Billy finds an advertisement for "Chocolate Sailors", a delicious chocolate candy. Billy then becomes a seller of Chocolate Sailors, but ends up eating them all by himself. The enchanted candies turn him into solid chocolate, and now Billy can't stop eating himself. Then, Billy, Mandy and Grim go searching for the Chocolate Sailor to retrieve the antidote before Billy eats himself completely. In the end, Billy eats the antidote along with other enchanted chocolates and, his head (it was all that was left of him) explodes as a result.
When Billy has a loose tooth he can't wait for the Tooth Fairy to appear to give him money. Then, Mandy, who doesn't believe in the Tooth Fairy, asks Sperg to end Billy's waiting by knocking his tooth off. However, Sperg's punch leaves Billy unconscious, who then has a dream about being in the wild west, where the Tooth Fairy has taken his tooth. Then, Mandy challenges the Tooth Fairy to a showdown to recover the tooth, and she comes out victorious just as Billy awakens. In the end, Billy does find money under his pillow, while the aged Tooth Fairy watches through the window.
When Billy hangs out at Irwin's house, he finds a living mummy in the bathroom. Seeing as Irwin gets nervous, and as Billy has never met Irwin's mother, he suspects she is the mummy. Then, to scare him off, Irwin's dad tells Billy he has the "curse of the mummy". Subsequently, Billy is attacked by insects and snakes while he tries to find out the truth about the mummy. In the end, it is confirmed that Irwin's mother is in fact the mummy, but no further explanation is given as to how Irwin's family works, or how he was conceived.
Thanks to Mandy, Billy decides to get rid of his "Dinobonoid" action figures thinking he is too old for them. Subsequently, Grim makes the toys come to life, and shrinks Billy to their size so he can have a last adventure with them. Later, the "Jurassic Creeps" become aware of Grim's scythe and steal it from him to use its powers to rule the world; however, the magic that made them come alive runs out just in time, and the toys become inanimate once again. Billy reverts to his normal size too, but he stays trapped inside the box in which the action figures came.