"Is Mystery" is the second segment of the twenty-third episode of Star vs. the Forces of Evil. It premiered on October 3, 2016 alongside "Friendenemies", and is the second segment of the tenth episode in the second season.
Worried about his tadpoles' futures, Buff Frog investigates the hole in the Mewman force field.
As Buff Frog surveys the breached area of the corn field, a monster named Meat Fork knocks him out and carries him away. Buff Frog finds himself in an underground tunnel loaded with corn and is taken to a corn churning device where he is forced to work it along with other imprisoned monsters. Buff Frog tries to get information from a fellow monster on where the corn goes, but all he gets is that the corn "goes down the hole" and that if they disobey they will be thrown down a different hole. The monster is outed by Meat Fork for stealing kernels and he promptly throws himself down the hole as punishment.
At night, Buff Frog sneaks away and climbs to the surface to see that the owner has their own corn field. He falls down another hole and discovers that the corn gets turned into corn meal in a sub level. He finds a room where there is a recreation of Mewni Castle that the rats mess around on. Meat Fork appears to intimidate Buff Frog, before revealing that he wants to leave too. They make it to the top floor, but Meat Fork is attacked by dark magic, revealed to be from Ludo the owner of the corn.
Ludo is not happy to see Buff Frog and has Eagle try to swallow him, but his wand tells him to give Meat Fork's old job to Buff Frog instead. Buff Frog refuses by knocking out Ludo and tries to grab the wand, but Spider webs it up. Buff Frog makes off with Meat Fork and the two manage to escape. After parting ways, Buff Frog wonders of the made the right choice turning down Ludo, but after reminiscing with his children, he decides that he has a new mission now: to warn Star Butterfly.
- Dee Bradley Baker as Mewnian Rats, Bald Eagle, Giant Spider
- Dominic Bisignano as Pinecone Guard
- Mark Gagliardi as One Eyed Monster, Bat Monster, Lizard Guard
- Travis Willingham as Meat Fork
- This is the first episode where Star Butterfly and Marco Diaz are completely absent though the former is mentioned.
- Buff Frog is confirmed to have completely shifted his alliance to Star.
- October 27, 2016 (United Kingdom and Ireland)
- November 18, 2016 (Italy)
- December 23, 2016 (Israel)
- January 13, 2017 (Poland)
- February 12, 2017 (Japan)
- February 17, 2017 (Spain)
- March 8, 2017 (Latin America)
- April 21, 2017 (Southeast Asia)
- October 5, 2017 (Portugal)
- March 11, 2018 (Philippines)