"City of Monsters" is a two-part mid-season special of Big Hero 6: The Series. The first half premiered on September 4, 2019 while the second half premiered on September 5. It is the conclusion of the season two arc of the same name, and is collectively the twelfth and thirteenth episode of the second season.
Karmi and Hiro team up to help treat someone overrun with genetically engineered parasites. Big Hero 6 must fight a city being attacked by monsters.
Big Hero 6 must continue to fight off a city overrun with monsters as Liv's plot comes to its dramatic climax.
- Ryan Potter as Hiro Hamada, Action Figure
- Scott Adsit as Baymax, Red Jacket Guy
- Génesis Rodríguez as Honey Lemon
- Khary Payton as Wasabi
- Jamie Chung as Go Go Tomago
- Brooks Wheelan as Fred
- Haley Tju as Karmi
- Mara Wilson as Liv Amara, Di Amara
- Daniel Henney as Tadashi Hamada
- Ben Feldman as Chris, Waiter
- Dee Bradley Baker as Mayoi
- Riki Lindhome as Wendy Wower
- Naoko Mori as Momakase
- Patton Oswalt as Mr. Sparkles
- Maya Rudolph as Aunt Cass
- Fred Tatasciore as Orso Knox, Bessie Monster
- Kari Wahlgren as Lily
- Jon Rudnitsky as Ned Ludd
- John Michael Higgins as Mini-Max
- Jenifer Lewis as Professor Granville
- Sophie Reynolds as Juniper
- The power-up sound effect of Go Go's hover-disc is the same sound effect used for the proton packs in Ghostbusters.
- According to Fred, Hiro has been "dating" Megan since the events of "Something's Fishy".
- During Karmi's transformation, the scene briefly cuts to a shot of her squishy doll, referencing the shot of a Mickey Mouse figurine in the 1981 film An American Werewolf in London.
- Part Two does not feature the usual intro and instead has a recap of Part One narrated by Fred.