He attempts to have Star heal him with magic, but unfortunately, Star does not know any bone healing magic. She consults her wand's magic manual—a giant, musty, disorganized collection of personal notes from her ancestors and former wielders of the wand—and manages to find a bone healing spell. (Or at least she thinks it's one. It's in an ancient language.)
Star casts the spell, and ends up turning Marco's broken arm into a giant, mutated, and monstrous tentacle. She attempts to cast a reverse spell, but at Marco's insistence, test it out first. Star proceeds to perform a long, long series of trial and error on numerous potted plants, while Marco tries to adapt.
The next day starts out rather horrible for Marco: he wakes up and accidentally smacks himself in the face with his slimy monster arm; the arm keeps knocking everything away as he's trying to take a shower, along with turning the water dial all the way to hot, scalding him; and at breakfast, it keeps smacking his cereal away and smashing his glass of orange juice on him.
Star arrives in no better shape; she'd stayed up the whole night practicing the counter-spell to limited success. She promises, however, that she'll figure it out eventually, and the two go to school.
At lunch, the monster arm situation hasn't improved, though Marco's become adept at keeping it away from his lunch tray. Just before they can find a seat, however, Echo Creek's resident bully Lars is seen terrorizing an unfortunate victim but knocking down his lunch, pushing his face into his pudding, and regaling him with his awful (lack of) humor.
Before Marco can react, the monster arm smacks Lars in the face. At first indignant and violent Lars is quickly beaten up and runs away crying. Marco and his monster arm are regaled as heroes, even earning the attention and praise of the school's most popular girl and Marco's crush, Jackie Lynn-Thomas. His opinion on the monster arm changes dramatically, as does the arm's uncontrollable nature.
Later, Star has finally figured out the counter-spell and excitedly goes to Marco to give him the good news. However, Marco's no longer thrilled to hear it. Having gained full control of the monster arm, he now plans to take it to the karate tournament and use it to kick Jeremy's butt. When Star mentions that it's cheating, Marco suddenly turns hostile and shoves her out of his room.
The door shuts, and Marco says about what does she know of the arm when the Monster Arm suddenly grows a mouth and gains a voice, and starts telling him he doesn't need Star; with the Monster Arm, he can do so much more, like beat Jeremy to a pulp, win the heart of Jackie Lynn-Thomas, and slaughter all of humanity and devour their bowels! Uh, it meant, win the karate tournament!
Thus convinced, Marco brings the monster arm to the tournament. His rival, Jeremy, comes as expected, and is revealed to be little more than a spoiled brat of nine years of age.
In the stands, Star arrives looking like a zombie. Marco's two friends voice their concern, and Star explains that she'd been up for two days straight figuring out the counter spell for Marco's monster arm—along with staying awake to attend Marco's tournament, because he's her earth bestie.
The tournament begins, and as expected, Marco effortlessly beats the ever loving daylights out of Jeremy. It's all going according to plan until the monster arm picks up the beaten Jeremy and prepares to devour his bowels. Horrified, Marco tells it it's not part of the plan he had in mind. Unfortunately, the monster arm decides to turn against him for stopping the arm from eating Jeremy's bowels!
Marco struggles to subdue the monster arm, all while calling for Star to cast the reverse spell. Unfortunately, Star's finally fallen asleep from her extended research bender; Marco's friends frantically try to wake her up. All the while, the monster arm keeps giving Marco pain!
Eventually, Star is woken up, and after a brutal struggle between Marco and the Monster Arm, and with the help of many misfires and objects turned to human arms, the counter spell is cast and the limb starts to go back to normal. It utters a disturbing warning, however:
- You can't get rid of me, I'm a part of you now! You can treat the symptoms but you'll never cure the virus...!
Cryptic parting words aside, Marco's arm is back to normal! And back to being totally broken.
With Jeremy knowing that Marco's arm is broken, Jeremy has won the tournament! ...Until a pair of gold, diamond-encrusted knuckles falls out of his glove, and their Dojo master noticed it. Having brought shame to strip mall dojos everywhere, the two are immediately disqualified.
Marco worries about the monster arm's parting words, but Star tells him not to worry. But for how long?
- All Anime Is Naughty Tentacles: Quite a few of the girls at school are very excited about Marco's new tentacle appendage. One of them, later revealed in "Mewberty" to be Janna, even says she wants the tentacle arm to be her boyfriend.
- Bread, Eggs, Milk, Squick: Monster Arm boasts that he can help Marco defeat Jeremy, win over Jackie, and feast on the bowels of their enemies.
- Combat Tentacles: The eponymous Monster Arm is a sentient one.
- Comically Missing the Point: When the master discovers that Jeremy has been cheating.Master: Brass knucles? Seriously?
Jeremy: Brass? They are gold.
- Corrupt the Cutie: The Monster Arm does this to Marco, though eventually Everyone Has Standards.
- Doorstopper: The Butterfly Magic Wand's Manual. Justified as it's a collection of personal notes from previous users, all compiled together, instead of any edited, supervised work.
- The End... Or Is It?: The Monster Arm warns that it will always be a part of Marco, with a zoom on the arm to punctuate it.
- Evil All Along: The Monster Arm is cool, gives you strength and confidence...and wants to eat the bowels of anyone not attached to it.
- Evil Hand: One that encompasses the entire arm.
- Foreshadowing: When Star is flipping through the book, Glossaryck can briefly be seen.
- Flexible Tourney Rules: Comically inverted. The Master is fine with Marco beating Jeremy with an powerful tentacle but later disqualifies Jeremy for hiding brass knuckles in his gloves.
- Heroic RRoD: Star stays up for two days perfecting the arm-fixing spell, so she falls asleep during Marco's dojo match and is very hard to awaken.
- Jerkass Has a Point: Marco during his "The Reason You Suck" Speech to Star points out that she always uses her magic to benefit herself, mainly for Mundane Utility, but she's judging him for wanting to do the same.
- Lovecraftian Superpower: Downplayed. The way the arm is introduced (Involving Star casting a spell from what appears to be a disorganized version of the Necronomicon) seem almost like a Lovecraft Homage. The arm itself fits 1 (being able to corrupt Marco and have him use it to harm other people), 2 (being a sentient tentacle with a mouth makes it rather unique) and 4 (because a sentient tentacle with a mouth and the ability to control himself is a little off). Unlike most examples, neither Marco or the tentacle have Reality Warper powers, and Star manages to return Marco's hand to normal.
- Mundane Utility: Subverted; the Monster Arm is useless with everyday normal activities.
- Naughty Tentacles: Quite a few of the girls at school are very excited about Marco's new tentacle appendage. One of them, later revealed in "Mewberty" to be Janna, even says she wants the tentacle arm to be her boyfriend.
- Nice Job Breaking It, Hero!: If Marco hadn't asked Star to heal his arm, he wouldn't have gotten into this mess.
- "The Reason You Suck" Speech: Marco after the Monster Arm corrupts him delivers such a speech to Star when she finally perfects the spell to restore his arm to normal. This puts her off, but she still agrees to attend his dojo tournament.
- Skewed Priorities: Marco would rather kick his rival's butt than take care of his arm and let it heal naturally, given Star's track record with spells going right.
- We Will Meet Again: As Marco's arm returns to normal, Monster Arm claims he will always be a part of Marco.
- You Can Talk?: Marco is naturally freaked out when the Monster Arm starts laying on the charm.