On February 23, 2015, the episode was released early on WATCHDisneyXD.com, the WATCH Disney XD app, and Disney XD on Demand.
When Marco hurts his arm before a big karate tournament, Star tries to fix it with magic but accidentally turns it into a giant tentacle.
Marco is practicing for his Tang Soo Do tournament against Jeremy Birnbaum, a boy that Marco considers his rival due to his wealth. As Star watches Marco prepare, he accidentally breaks his arm. Worried that he will be unable to perform in the tournament, Star tries to look up a way to help him. She reveals that she has a giant spell book that can help them, but that it is difficult to get through as the previous want owners simply add to the book once they have mastered a spell. Nevertheless, she finds what she presumes is a bone fixing spell and instead imbues Marco with a giant sloppy tentacle arm, much to his horror. As Star tries to find a counter spell, the arm begins to uncontrollably smack away everything and impede Marco's every day rituals.
At school, Marco's Monster Arm beats up the local bully Lars who runs away crying. This act earns the admiration of his friends and fellow students including Jackie-Lynn. As Marco starts getting used to Monster Arm, Star continues to look through the spell book and finds the counter spell, but when she tests it on a plant, it blows up. As Marco heads to bed, Star continues to practice the spell all through the night until she finally gets it early in the morning. She runs over to Marco and discovers that he has been practicing with Monster Arm as well. When she offers the spell to him, he angrily rebuffs her as he believes that he has an advantage now and kicks her out of his room. Afterwards, Monster Arm begins to speak and convinces Marco to use him in the tournament while hinting at malicious intentions.
Ferguson, Alfonzo and a very sleepy Star attend the tournament and are surprised to see that Jeremy is an obnoxious eight year old. Marco arrives and proceeds to beat up Jeremy, but when Monster Arm announces his plan to devour him, Marco attempts to fight back to no avail. Ferguson and Alfonzo manage to wake up Star who uses her arm spell, but keeps missing and creates arms all over the room. Marco forces the arms to grab him and Monster Arm and Star successfully cures Marco with Monster Arm claiming that he will return. Jeremy is revealed to have planned to use gold knuckles for the tournament, leaving Sensei disappointed in the two. Marco apologizes, but worries about Monster Arm's return, though Star tells him that it is not an issue.
- Eden Sher as Star Butterfly
- Adam McArthur as Marco Diaz
- Corey Burton as Monster Arm
- Nick Swardson as Sensei Brantley
- Nate Torrence as Ferguson O'durguson
- Jeff Bennett as Francis Smithington
- Dominic Bisignano as Lars Vanderdud, Mr. Birnbaum, Audience Man
- Matt Chapman as Alfonzo Dolittle
- Abby Elliott as Janna Ordonia
- Grey Griffin as Jackie-Lynn Thomas, Hope
- Daron Nefcy as Mrs. Birnbaum
- Joshua Rush as Jeremy Birnbaum
- Nia Vardalos as Audience Girl
- This is the first episode that shows Star wearing her hair differently.
- Despite the Monster Arm's claim that he will return, and the episode ending with it hinting it, it never comes back for the remainder of the series.
- Glossaryck once again makes another, albeit silent, appearance before his actual introduction.
- When Star finally gets the spell right, the hand had five fingers, while the characters in the show normally have four. Additionally, it appears right handed, but when Star is kicked out of Marco's room, it is suddenly left handed.
- In the scene where Marco is fighting the monster arm, Star seems to be aware of the situation, even though at this point she should have fallen asleep.
- April 19, 2015 (Canada)
- April 30, 2015 (United Kingdom and Ireland)
- July 24, 2015 (Germany)
- August 5, 2015 (Australia)
- September 3, 2015 (Israel)
- September 18, 2015 (Latin America)
- October 7, 2015 (Turkey)
- October 21, 2015 (Poland)
- October 24, 2015 (Spain)
- November 22, 2015 (Southeast Asia)
- December 26, 2015 (Portugal)
- January 15, 2016 (Japan)
- "Monster Arm Question". Reddit (August 3, 2019).