Underfist: Halloween Bash, aka Underfist is a Cartoon Network spin-off movie of the show The Grim Adventures of Billy and Mandy. It was first a joke but Maxwell Atoms's crew and Cartoon Network didn't deny it. Maxwell Atoms did most of the work for the movie since the Cartoon Network crew to work on Underfist was unconfirmed. It is actually the first of the Underfist movie series.
A real live living marshmallow bunny plans to destroy Endsville with real chocolate bars. Mindy is enslaved by Bunbun, a marshmallow bunny to take over the world from the Underworld power. Only the team of Underfist can stop the Trick-Or-Treater Eaters: Irwin, Fred Fredburger, Jeff the Spider, Hoss Delgado, and General Skarr. They rescue the new President of the United States, which turns out to be Mandy. Hoss Delgado refuses to be part of Underfist, but is forced to team up with the vampire-mummy Irwin after he realizes that Underfist is the only team he can trust. Evil chocolate-bars, Halloween-pumpkin fire-solidiers, and armed-ice cream tanks rampage through Endsville as Underfist must decide whether to accept their leadership or give up their attempt.
It is Halloween, and now the Trick-or-Treating Service is going slow. Dracula only gives Irwin, Mandy, and Billy pennies. Then, Mindy calls Irwin a Loser, Ha-Ha! Then, Irwin decides to quit the holiday, and the movie seems like its going to end, with credits breaking the fourth wall, until Mindy calls out Wait a second you idiot! I need your help!. Then, the movie goes really fast backwards to Irwin, where Mindy finally decides that Irwin is a someone because she needs someone to press a green gem on a spooky house because they're selling life-size chocolate bars. Irwin slowly pushes it with interrupting animation with foreshadowing. Then, after he presses the green gem crystal, a Vortex to the Underworld appears, a marshmellow bunny named Bunbun comes out, and his chocolate-bar solidiers come out of the Vortex with soda-rifles and hunting down the Trick-or-Treaters. The chocolate bars are about to destroy the town until Hoss Delgado comes out, and confronts the army. Then, Delgado's mommy comes out and tells Hoss to(No Mommy, Yes Mommy) play nice, while BunBun and the chocolate bars laugh that he's talking to his mommy. Then, Hoss uses his hand-gun to blow out the chocolate army, proceeding in destroying hundreds of them. BunBun later kidnaps Mindy while Irwin begs for help to save her. Several police cars rush until the chocolate bars aim ice cream at the police cars onto the roofs of houses. Later, chocolate army soldiers spray soda at Hoss Delgado, but soon, when they fight on one's lawn, it turns out that it is General Skarr who is the one to help Hoss destroy the chocolate bars because they ruined his #1 Trophy-Winning Lawn, using a robot-machine develop to fight the chocolate bars. Hoss later exclaims that the soda only affects him because they make his clothes all sticky & gross.
Then, Fred Fredburger and Jeff the Spider consider of helping Hoss and Mr. Skarr. But first, Fred suggests that he spell his name: F-R-E-D-F-R-E-D-B-U-R-G-E-(aaahhhh)..Jeff: Is it R? Fred: R! Yes! Fred Fredburger! Then, a chocolate bar with a Love and Hate lollipop who fights Hoss Delgado has his hands eaten by Fred Fredburger because he likes lollipops, and the skeleton candy pieces are taken down by Jeff the Spider's webbing. Then, Irwin joins the four and take control of Delgado's car: Stick. After the joining and combination of the Underifst members, Hoss Delgado reveals that the new President of the United States has ordered all candy shop services in Endsville demolished in case the candy in the shops are evil as well. Then, the Stick vehicle drives into a Vortex covered in dirt under the crust in the Earth to follow BunBun. Hoss Delgado and Irwin fall into a Vortex leading into the Underworld Lair for Bunbun and the Halloween candies.
Later, Mindy reveals herself that now she transformed into a witch because she believes Irwin gave her mummy cooties, turning her into a witch. Hoss Delgado frustrated at Irwin for what he is, yells out that the boy who is half monster and half human lied to him twice when he told him he's not a monster, a vampire-mummy half with a human half together. Mindy the Witch later reveals that the President of the United States is trapped in a tall Ginger-Bread Prison Tower. While chanting out the success and good starting of the plan to take over the Trick-or-Treating plan started by Bunbun, Jeff is so worried, General Skarr is bored and angry, and Fred Fredburger is calm because he thinks there's no danger in the situation. While singing over a pot of green slime and hedorah used as a chemical to take over the world in Halloween night, Mindy chants out a song: Trick-or-Treater Eaters, where the Halloween candy monsters will become 10 times more powerful than ever and fill the pot with all her dirty and unusable waste into the bowl.
Hoss Delgado almost falls into a giant cup of chocolate until Irwin, who morphs into a vampire bat, saves him. Nergal's sandwich picnic is interrupted by Stick, where that time Irwin is able to climb the Gingerbread Tower and rescue the President of the United States, and it turns out to be Mandy, with no surprise. Mindy tries to destroy both Irwin and Mandy, until Jeff the Spider rescues them by webbing the two from the tower. General Skarr launches a giant missile fist into the chocolate cup, destroying the entire hot chocolate. Mindy and BunBun tries to destroy Stick by aiming jawbreakers at the body of the vehicle. After getting into the real world again, Mandy supports evidence that Trick-or-Treating is important to the world since kids eat Halloween-candy from taking over the world, found in several magazines about Halloween topic.
Each of the Underfist team members get their costumes from Grim, and now a green diamond destroys the metal hand from the Understick vehicle. Jeff is now seen wearing a diaper while BunBun returns with his mutated chocolate bar army. Hoss Delgado quits Underfist and later tries to go off on his own to save the world. Irwin battles Hoss Delgado from fighting him, while Jeff and Fred entertain the chocolate bars with the song I Don't Wanna Grow Up, showing how much they don't wanna grow up. Mindy shows up again, and transforms the entire chocolate army into a giant mutated monster with a pumpkin on its head.
Afer Hoss Delgado and Irwin rip off the hand of the monster, it steals the Stick hand, showing that he can plug a drill into the hand. Irwin, using his vampire/mummy powers rips off the Stick hand, and Hoss Delgado fires Billy at the monster to eat all the candy in it. Hoss later apologizes to Irwin for never trusting him just because he was a monster. He only didn't trust monsters because in his childhood, a boogeyman had haunted a miserable 30 years of his life, making him so scared every night for the next 30 years, and he only remembers after 16 years that he was wearing a hat.
Soon, BunBun returns with his entire chocolate pot with now General Skarr on his side. He later reveals all the naughty things he did to the team:
- He made Mindy a witch by putting a witch worm into her hair, so that she can take him to Irwin.
- He was the monster who haunted Hoss Delgado when he was a kid.
- He made Jeff the Spider's dad, Billy, afraid of spiders by waving a toy spider in front of him when he was a baby.
- He sawed off Fred Fredburger's tusks when he was sleeping.
Soon, General Skarr reveals that he tricked BunBun so that he can prevent him from pouring hot chocolate onto the Earth by kicking him into the pot, and everyone is saved. Mandy rewards each of the Underfist team members, and now Dracula complains he only got a penny for Halloween. Soon, a squid life form monster comes to destroy the Sun with his digusting magic with his nephew. Underfist wastes no time and start to battle him. Throughout the ending credits, they spread their new song, Underfist, and show the future films what they'll appear in.
It started when Fred Fredburger made an art project with paint. Fred Fredburger drew an elephant, Captain Mannly that looks like him and can spell his name real good.
- Irwin, voiced by Vanessa Marshall
- Fred Fredburger, voiced by C. H. Greenblatt
- Jeff The Spider, voiced by Maxwell Atoms
- Mindy, voiced by Rachel MacFarlane
- Hoss Delgado, voiced by Diedrich Bader
- General Skarr, voiced by Armin Shimerman
- Mandy, voiced by Grey DeLisle
- Billy, voiced by Richard Steven Horvitz
- Grim, voiced by Greg Eagles
- Trick or Treater Eaters by Mindy and the Chocolate Bar Soldiers
- I Don't Wanna Grow Up by Jeff the Spider and Fred Fredburger
- Underfist sang by Hoss Delgado and spoken by the 5 Members of Underfist
- Fred Fredburger was redesigned to a peanut size. He is more green, smaller, and has a higher-pitched voice.
- Chowder makes a brief appearance in the film in a pumpkin when Hoss Delgado is seen looking around Endsville.
- There were supposed to be more sequels shown at the ending credits of Underfist: Halloween Bash. The sequels in order are "Underfist Vs. the Dinosaurs", "Underfist Against the Astro-Vampires", "Underfist: Easter Beatdown", "Underfist: Return of the Spider Queen", "Underfist: C.S.I. Miami", "Underfist Teaches Traffic Safety", and "Underfist Babies". However, none have ever been produced.
- Maxwell Atoms has stated that some of the sequels were simply jokes, but that at least the first two were actually planned.
- General Skarr seems to show signs of aging, as he now has a beard, mustache, and his military clothes are absent in the film (he's either in his underwear or in his Underfist uniform).
- Additionally, Hoss has also aged: he has grey streaks in his hair, he's put on weight, and it's confirmed that he's 48 years old in the movie (as opposed to being in his thirties in the series).
- The character Baron Von Ghoulish, from the episode "Billy & Mandy Save Christmas", makes a brief cameo in the film. He is seen being scared by Trick-or-Treaters in costumes, and falls back into his coffin.
- We learn how Hoss lost his hand, leg, and eye; he explains the he was running with scissors, despite what his mother told him.
- The science clothes Billy is wearing look similar to Dexter's typical clothes from Dexter's Laboratory.
- Maxwell Atoms wanted to turn the movie into a new series, but his contract with Cartoon Network was expired and it was too late for him to renew it; considering the cost, his series was dropped and the movie became the seires finale of The Grim Adventures of Billy & Mandy. This caused several petitions made to turn the show into a series.
- The characters who make up Underfist-Irwin, Hoss Delgado, General Skarr, Fred Fredburger, and Jeff the Spider-are all secondary characters who started gaining popularity after appearing in the series (sometimes called "Ensemble Darkhorses"). This is most likely why they were chosen as the main characters and Grim, Billy, and Mandy, are secondary characters this time.
- Irwin was, arguably, the most popular of these characters, and played a more significant role in later episodes of the series. This is probably why he is the main protagonist.