The game is a beat-'em-up style Fighting Game where Grim, Billy, and Mandy find that everyone is starting to fight each other after being corrupted by Mojo Balls that were supposed to be in Grim's trunk. It's up to the trio to beat up every opponent they face and to find out who released the Mojo Balls from Grim's trunk.
The video game contains examples of the following tropes:
- And Your Reward Is Clothes: Each playable character has two alternate outfits. One is unlocked by beating story mode as the character and the other is unlocked by completing one of the missions in mission mode.
- Bad People Abuse Animals: Skarr says he kicks puppies.
- Big Ball of Violence: In the story mode's cutscenes, people being forced to fight each other by the Mojo Balls is represented by them becoming balls of smoke with fists and feet repeatedly poking out.
- Big "YES!": Fred Fredburger says this when he defeats pretty much anyone.
- Continuity Cavalcade: The game features many references and callbacks to characters and events from the show, such as Billy's dad Harold being playable as his Mogar persona from "Battle of the Bands" and the talking tree from "The Taking Tree" appearing as a background hazard in the stage taking place at Billy's house.
- Developer's Foresight:
- In the final boss battle against the Brain-Eating Meteor in story mode, every playable character has a different quote for when the battle starts.
- If you play through story mode while wearing an alternate costume (either the one obtained by beating story mode as the character or by completing one of the missions in mission mode), Mandy will appear in her own alternate costume during the game's credits where the player has her beat up Billy.
- Early-Bird Cameo:
- The game features references to the show's movie Billy & Mandy's Big Boogey Adventure, which wasn't released until less than a year after this game. Billy and Mandy's robot duplicates Billybot and Mandroid appear as alternate costumes for them, Boogey's alternate costumes have him wearing the pirate getup and his lawyer outfit from the movie, one of the locations the player can fight at is Boogey's pirate ship, and Horror's Hand is mentioned and shown during Boogey's Mojo Meltdown.
- Among the unlockable concept art is an image of the Underfist team, two years before the special would debut.
- Exact Words: After The Reveal in Story Mode, Mandy suggests that the game includes a mode where they beat up Billy. Cue the Creative Closing Credits, where you CAN beat up Billy.
- Fartillery: Billy's Mojo Meltdown has him fart at his opponent(s) and set them on fire.
- Fighting Game: The game is of the sort where you beat up other characters and nearly every character you fight is playable.
- Final Boss: The last boss faced in the game's story mode is the Brain-Eating Meteor from the show's episode "Little Rock of Horror".
- Gameplay and Story Segregation: The cutscene that plays after completing the "Clucking Doom" stage in story mode shows the main characters on the bus fleeing from the giant chicken, regardless of whether or not the player has made it to that part of the level by the time they've beaten all the opponents.
- Joke Character: The unlockable Mook characters only have a couple (laughably weak) basic attacks and can't pick up items or other characters.
- Large-Ham Announcer: When "Weird Al" Yankovic is the announcer, this is a given.
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Licensed Game: This is a video game based on a cartoon show. - Limit Break: Collecting Mojo Balls will fill up the player's Mojo Meter twice. When it's filled up once, the player can unleash a series of beatings toward their opponent if timed right or counter another opponent's attempt at doing the same. When filled up twice, the player can activate a Mojo Meltdown, which instantly takes away a life from every opponent currently faced.
- Mythology Gag: During the Asgard stage, a bearskin rug that appears to be Boskov is visible.
- One to an earlier Midway game — whenever a character is running out of health, Weird Al will announce "Player (x) needs ham badly!" akin to Gauntlet and the trope naming Wizard Needs Food Badly.
- Pass the Popcorn: When Grim agrees with Mandy that the fight between Billy and Irwin is getting good, he summons a box of nachos.
- The Reveal: Finishing story mode reveals that Billy was the one who released the Mojo Balls from Grim's trunk and that he did it because he wanted to free what he believed to be "Moe and Joe". In addition, it is explained that he left the trail of chicken feathers the other characters followed throughout the game and that the reason he had the chicken feathers was because the curse Grim placed on his trunk to keep people away from it made it so that whoever opened the trunk would sprout chicken feathers from their rear end.
- Secret Character:
- Hoss Delgado, Dracula, Jack O'Lantern, Nergal Jr., Boogey, Skarr, and Lord Pain are unlocked by completing certain missions.
- Completing story mode as Billy, Mandy, Grim, or Irwin unlocks Mogar, Eris, Nergal, and Fred Fredburger as playable characters for the story mode, though they're available from the start in versus mode.
- Completing story mode as all fifteen characters and completing additional requirements unlocks various mooks from the arenas as fully playable characters: Clown, Nergling, Chicken, Viking and Pumpkin.
- Spared by the Adaptation: Well, as spared as an immortal trickster can be. During the campaign, Mandy wonders if Jack is involved in the plot and suggests going to the Pumpkin Patch to investigate, suggesting that in the game’s continuity, Jack wasn’t banished to the Underworld.
- Super Move Portrait Attack: Whenever a character uses their Mojo Meltdown, a picture of them in a fighting stance briefly appears over the screen.
- Sure, Let's Go with That: In the cutscene taking place after the Clucking Doom stage in story mode, Mandy reacts to Irwin flirting with her by punching him in the face with enough force to send him flying into the distance. Grim asks if Mandy is infected by the Mojo Balls, too, and she hesitates before answering "Sure", implying that she isn't and merely used the Mojo Ball crisis as an excuse for her usual abuse toward Irwin.
- Unlockable Content: In addition to the alternate costumes unlocked by playing through story mode as a character or completing some of the missions, the other missions unlock different arenas for Vs. mode and concept art as well as the ability to play as Dracula, Skarr, Hoss Delgado, Jack O'Lantern, Lord Pain, and Nergal, Jr., while completing story mode as one of the four default playable characters Billy, Mandy, Grim, and Irwin unlocks Mogar, Eris, Nergal, and Fred Fredburger as playable characters.
- Version-Exclusive Content:
- The PS2 port came with "Little Rock of Horrors".
- The Wii port had additional weapons.
- Wham Shot: Billy produced chicken feathers from his rear.
- Weapon-Based Characterization: Every character comes with a weapon that suits their characters.
- The dim-but-tenacious Billy gets a giant wooden hammer.
- The ruthless and pragmatic Mandy uses her bare fists.
- The Grim Reaper uses his trademark scythe.
- Awkward nerd Irwin gets a giant spellbook he uses as a blunt instrument.
- Over-the-top rock star Mogar hits people with his guitar (or, in his fisherman's outfit, a swordfish.)
- Eris, the Goddess of Chaos, uses her Apple of Discord which can change form into whatever weapon she chooses.
- Nergal and Nergal Jr., two humanoid abominations, use the tentacles that grow out of their bodies.
- Incorrigible oddball Fred Fredburger uses a skillet.
- Macho tough-guy Hoss Delgado uses his cyborg hand that turns into a chainsaw.
- The foppish and old-fashioned General Skarr gets a Royal Rapier.
- Dracula fights with his cane.
- The incurable prankster Jack O'Lantern hits his enemies with his sack of tricks.
- The vaguely animalistic Boogey Man uses his massive talons.
- Demonic Knight Lord Pain dual wields a mace and shield.
- You Bastard!: In the opening cutscene, Grim asks what kind of sicko would make them fight for their amusement. He, Billy, and Mandy then give disapproving looks toward the player.
