Monster Arm

"Monster Arm" is the fifth episode of the Disney XD series Star vs. the Forces of Evil, which officially premiered on Disney XD on April 6, 2015.

It was released early on WATCHDisneyXD.com, the WATCH Disney XD app, and Disney XD on Demand on February 23, 2015, alongside "The Other Exchange Student".

Plot
Star Butterfly watches Marco practice Karate and monologue how he's going to defeat Jeremy Birnbaum, when he breaks his hand on a plank of wood. Star, feeling sorry for her friend tries to repair it with her wand, but instead causes the entirety of Marco's arm to turn into a giant tentacle.

Later at school, Lars is picking on one of the unpopular students, when Marco accidentally uses his arm to hit Lars several times, which sends the bully off, crying, and the students welcome Marco as a "Hero".

Star is trying to find a reverse tentacle spell for Marco, but cant seem to find the right spell for it. The next day, Star still cant find the correct spell, and finds Marco working out with the tentacle arm, which makes Marco arrogant. Star calls the arm a "bad influence", and Marco pushes her out of the room. The Tentacle begins talking to Marco, and subtly reveals his evil plans, while a fooled Marco agrees to keep using the tentacle for the tournament.

When the day of the tournament arrives, Star sits next to Marco's friends, and they acknowledge that she is looking terrible, which she explains is due to her trying to find a cure for the tentacle. As the tournament starts, Jeremy is revealed to be a small kid, and Marco begins brutally fighting him, using the tentacle arm. The tentacle becomes sentient again, and begins to coerce Marco into "finishing" Jeremy. Marco, knowing the Tentacle is evil, calls Star for help. Star uses the want to try and return the arm to normal, but it makes other objects around the room turn into hands. Eventually, the Tentacle gets caught by all the various hands, and Star turns Marco's arm back to normal, but not before the Tentacle promises to return.

As Marco's arm is still broken, Jeremy proclaims he wins, before accidentally dropping his Brass Knuckles, which he used to win. The Sensei claims neither has won, due to them both bringing shame to the sport. Marco apologizes to Star, and stresses his concern about the arm returning, and an unsure Star tells Marco he shouldn't worry.

Cast

 * Eden Sher as Star Butterfly
 * Adam McArthur as Marco Diaz
 * Grey Griffin as Jackie