The Grim Adventures of Billy & Mandy — Season 3

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 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.

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.

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, which he names Little Porkchop. To make Billy happier, Grim makes the fish grow overnight to become a giant. Billy takes Porkchop to play under the Sun, and never gives him water again. He burns to ashes and is flushed down the toilet. Billy's dad returns form his fishing trip having caught Porkchop, now back to his normal size, but plans to cook him.

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.

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 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.

After Billy eats 19 creme-filled Atomic Frenzy Cakes he cannot sleep and wants Mandy to read him a bedtime story. 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. Billy departs from the story and follows Pinocchio, who wants to devour Billy to become a real boy. 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 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.

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 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.

After failing to scare a fifteen-year-old, the Boogeyman decides to visit his old middle-school victim, Grim. Grim remembers how Boogie used to be a bully, and demands an apology from him. Boogie disappears to the Underworld, taking Billy with him. Mandy and Grim follow Boogie, and to settle their differences Mandy suggests having a scaring contest, in which whoever can scare a kid the most wins. 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. Billy is taken to the mushroom lair of the dwarfs where King Beardbottom tells him the story on how the different races split the food-services industry, after the conference of Rivendell. The king sends Billy to infiltrate the elves' cookie factory to deactivate their force field, so the dwarves can assault the factory. 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.

While going to a science museum, Billy and Mandy's school bus breaks down in the middle of the desert. 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 mutant chickens were created. The mountain makes eruption, the chickens are roasted by lava, and the whole class obtains food so they do not have to resort to cannibalism.