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.

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 seeing Billy, Mandy and Grim battling a strange creature, Irwin again feels rejected by Mandy. Hoss Delgado, spectral exterminator, appears to help Mandy destroy the monster. Delgado decides to have Irwin as his sidekick and train him to be "cool" to win Mandy's love. Irwin gets an attire like Delgado's, and attaches an egg beater to his right hand. Delgado releases a monster so Irwin can fight it and impress Mandy, but he is humiliated anyway. Mandy tells Irwin he is just a nerd, and Billy tries to assure him that being a nerd is not such a bad thing after all.

When Mandy says even a trained monkey is more competent to do cleaning chores than Grim, and would complain less, Grim makes Dickie Galoot, a monkey appearing in a Japanese-styled animation, come to life to prove her wrong. As Dickie turns out to be extremely good at cleaning Mandy's house, Grim decides to leave the kids for good. Due to his friendship contract, Grim is unable to break free, and he is forced to battle Dickie to recover his place as the "cleaning monkey" of the group. Grim successfully defeats Dickie at cleaning Billy's house, but this all turns out to be an ingenious plan devised by Mandy to win a bet against Billy.