"Is Another Mystery" is the first segment of the fifty-second episode of Star vs. the Forces of Evil. It premiered on March 17, 2018 alongside "Marco Jr.", and is the first segment of the seventeenth episode in the third season.
Marco is trying to grab Glossaryck when they end up in the Monster Expert office. To his surprise, Buff Frog is absent, but there is a letter left on the desk. Marco accidentally reads it and is shocked. Meanwhile, Star is stressed out about recent events and tells Tom that she feels she cannot tell anyone about it. Tom, who is trying to be a better boyfriend, tries to get her to open up to him, but is interrupted by Marco who tells them about the letter which says that Buff Frog and his babies have left Mewni. This surprises the two as Buff Frog is illiterate. Star prepares to leave and Tom pulls Marco aside, telling him that he has been trying to have some quality time with Star and would like to take his place on this adventure. Reluctantly, Marco recedes and allows Tom to accompany Star this time.
Star and Tom head to Buff Frog's house to find the place in disrepair. Tom nonchalantly gives up, upsetting Star, so he suggests checking the monster village. They arrive to find the place empty as well and do not know where to look. Suddenly, they are both hit by darts and fall unconscious. They awaken, wrapped in burlap sacks, and begin panicking when various monsters, who have apparently captured them, try to restrain them. After a blind comical fight, the monsters restrain them and tell them to abandon their search for Buff Frog or else they will break Tom's horns. At the last moment, Buff Frog reveals himself and saves Star and Tom.
The letter was revealed to have been written by Katrina, Buff Frog's youngest, and the reason for their messy house was simply due to rough parenting. Buff Frog reveals that he and the other monsters are leaving Mewni. Star is upset and wants them to stay, but they tell her that it is not safe. Tom reveals that he is half Monster and that despite being rich he occasionally still gets treated with fear. While Buff Frog sympathizes, he and the Monsters elect to leave, but promise to return when Star is crowned queen. After leaving, Tom comforts Star, but ruins the moment when he asks if he "did better than Marco".
- Eden Sher as Star Butterfly
- Adam McArthur as Marco Diaz
- Rider Strong as Tom Lucitor
- Jeffrey Tambor as Glossaryck
- Fred Tatasciore as Buff Frog
- Kimaya Thais as Katrina
- Greg Cipes as Horned Mouse Monster
- Mark Gagliardi as Teenage Monster, Two Headed Monster
- Grey DeLisle as Blue Monster, Eyeball Monster
- The episode was to premiere on January 22, 2018, but it was delayed.
- Tom reveals that he has a tail.
- The title is a reference/continuation of the season 2 episode "Is Mystery".
- Buff Frog says goodbye to Star using the same salute he used to greet Ludo during his time in Ludo's employ.
- "Disney Television Animation News post". Twitter. Retrieved on December 31, 2017.