Billy accidentally sets off a powerful stink bomb in Mandy's house and, while the house is being disinfected and fumigated, Mandy's family goes to live with Billy's. Billy and Mandy have a dispute over how to split Billy's room, prompting Grim to intervene. Mandy takes over the entire room, and imprisons Billy inside a brick wall with his mouth gagged by a dark red handkerchief.

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. It appears that Mrs. Doolin is in fact a ghost, and when she meant "beating Grim", it meant that she "beaten death".

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. Billy-Irwin, and Mandy-Grim compete against Mindy, Sperg, and a mystery racer (Pud'n). 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 little red wagon.

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. Grim allows the kids to disappear by going before birth, but he cannot restore his own hourglass to its correct position and he disappears too.

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. She entrusts Grim the Apple of Discord to watch over for her, which he seals in his old trunk. The apple frees itself from the chest and goes to Billy. 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. 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.

Thanks to Mandy, Billy decides to get rid of his "Dinobonoid" action figures thinking he is too old for them. Grim makes the toys come to life, and shrinks Billy to their size so he can have a last adventure with them. 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.

When Billy finally gets a "secret decoder ring" in his Blasto Bits cereal, he receives a holographic message from a strange creature who speaks about finding the "Secret of the Universe" in an abandoned mine shaft. All the other children of Endsville, also with decoder rings, race to the mine to be the first one to get the Secret. The entire effort is revealed to be a trap laid by Lubbermouth, a worm-like creature from Grim's past, who wishes to free herself from the curse of guarding the Secret. When Mindy looks down at the pit, she becomes the new guardian of the Secret and has to stay there forever.

After Grim has a nice dream, a nightmare escapes the back of his head in the shape of a supernatural, invisible duck that makes farting noises. The duck follows Grim everywhere, causing him embarrassment with the non-existent farts, and to end up in jail. The duck repeats the strategy, following other citizens, including Irwin and Billy's dad, resulting in all of them going to jail too. The duck tries to annoy Mandy, but she remains imperturbable by its presence. The duck disappears and Principal Goodvibes is really angry, and he gives Mandy 75 days of detention.