"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.
Liv Amara assigns Karmi to a create cure for the parasite species called "parasynths": the special kind of parasites that were created to destroy viruses, but they mutated and started to destroy the healthy zones instead. Karmi tests the sample of the gold-particle infusion which appears to work, but is then destroyed by the swarm. Karmi theorizes the only way to stop the parasynths is to drive them apart from each other, Liv advocates they must find out how fast, the cure is for a special patient.
Meanwhile, Big Hero 6 secretly tails Hiro for safety after begin kidnap in the past, plus the possibility of Liv being a threat. Hiro rejects their protection asserting he is a superhero; Go Go remarks that as a civilian, he is at risk. Back at Sycorax, Karmi suggests micro-robotics to Liv as the solution but she does not possess the aptitude to use this, Liv immediately decides she is no longer of use, resorts to searching for a real robotics expert: Wendy Wower.
At Fred's mansion, Hiro has the team's ultra armors upgraded to provide them with better support. Wasabi, his suit now comes with a plasma shield and plasma dagger; Go Go, maglev discs that can expand to become a hover disc; Honey Lemon, chem-boots with the properties of her Chem-Purse; and Fred, headlights and hearing equipment that doubles as a voice recorder. However, Baymax's armor is missing a piece that is still at SFIT. The two head there and find Karmi studying inside the robotics lab. Karmi admits to studying in order to help with Liv's project. Despite his doubts, she insists she can pick up robotics. Hiro thinks Karmi can't be a robotics and biotech genius, comparing her to Tadashi. But, Baymax reveals Tadashi needed a physician's input to articulate his medical skills. Feeling ashamed for judging Karmi, Hiro offers to help, which she reluctantly accepts. The two create a nano-microbot cure but discover the robots disassemble in the body, Karmi works with a solution to integrate the nanobots with an organic membrane so they can survive, Hiro commands with the neurotransmitter on the microbots to act inside organic matter and they succeed. Hiro reminisces about Tadashi upon the collaborative success of using both robotics and biotech to help. Karmi admits she admired his brother's work, to which Hiro clarifies that Tadashi had help building Baymax.
Big Hero 6 is scouring the streets when they overhear Mr. Sparkles and the Mayoi abducting Wendy Wower, who is sought by Liv for her robotics skills. Go Go calls Hiro to help them with Mr. Sparkles, who has taken Wendy to Torii Gate Bridge, at the bridge, Hiro and Baymax regroup with the team and confront Mr. Sparkles, who is assisted by Momakase and a re-mutated Knox. Big Hero 6 takes on the mutants, they saved Wendy but the mutates got away.
Back at Sycorax, Liv scolds her henchmen, but Momakase reveals Hiro's identity to her and advises separating the leader then the team will be easily defeated. Liv sent her henchmen to leave when Karmi visits her office to inform of the breakthrough with Hiro's help, taking advantage of this, Liv offers her an upgrade. Hiro then receives a call from Karmi for help, Liv is hunting her and she is hiding inside SFIT. Hiro and Baymax arrive only to find bear Bessie waiting at the entrance. Bessie but its electromagnetic blast shuts down Baymax, Hiro goes in to rescue Karmi. While confiscating Baymax, Liv activates the bio-chip implanted on Karmi, she mutates in front of Hiro and escapes with him and the microbots.
Go Go, Wasabi, Honey Lemon, arrive at SFIT campus looking for any trace of Hiro. Now certain that Liv has taken Hiro and Baymax captive, Go Go directs the team to take the fight to Sycorax itself. Once the team arrives at Sycorax, they are approached by Chris, demands him bring them to Liv but he shows them footage of Liv's mutants beginning a descent upon San Fransokyo. The team leaves Hiro behind and rushes out to save the city. Beneath Sycorax, Liv is holding Hiro and Baymax at her mercy with control over the mutated Karmi, she tells Hiro that if he helps her, she will call off the monsters and return Karmi to normal. Liv explains that she needs his nano-microbots to save her patient: Liv Amara.
The mutants invade San Fransokyo, the team split up to cover each villain. Wasabi got High Voltage; Go Go got Mr. Sparkles and the Mayoi; Fred and Mini-Max fight Knox and Bessie, and Honey Lemon battles Momakase
Hiro is confused over the existence of two Livs. Baymax's scan picks up the DNA of both Livs are a genetic match, but one is a clone. "Liv" then shows Hiro footage of Liv prior, where it shows that she created the parasynths to extend human life expectancy. However, she was forced to test the parasynths on herself; the parasynths started deteriorating her, leading to a desperate last attempt to create a clone for a cure while she was in suspended animation. The false Liv introduces herself as Diana Amara/Di, she tells Hiro that her objective is to save Liv and threatens Hiro to cure her or the mutants will continue to destroy San Fransokyo.
Hiro and Baymax proceed to remotely pilot the nano-microbots administered to Liv with Karmi's cure equipped to their microbot avatars. As they approach Liv's heart, Hiro and Baymax begin firing the cure at the parasites and disintegrate them. However, the queen appears, Hiro and Baymax combine their microbot avatars and shoot large doses of the cure, but this provokes the queen to chase after them.
Meanwhile, the rest of the team is slowly wearing down against their opponents. Go Go directs the team to lure the monsters into one place and isolating them from the city. During his fight, Fred comes up with a back-up plan and has Mini-Max leave to execute it. Back at Sycorax, Hiro's microbot manages to force-feed the lead the queen enough cure that disintegrates it, finally purging Liv of the parasynths. Hiro requests Di cure Karmi, but she instead tries to mutate on Hiro now that he is aware of her true intentions and commands Karmi to capture him.
The team regroups at Night Market Square herding the mutants in the center. Momakase then stabs Bessie to fire a massive electromagnetic blast, which disables the ultra armors, forcing the powerless heroes to retreat. Mini-Max returns to the team with Ned Ludd, whom Mini-Max informed about Bessie's whereabouts to get his help, as part of Fred's back-up plan and Skymaxes arrives with the team's normal suits.
Hiro kicks Di's phone out of her hands to keep her control from Karmi and lands near Baymax. Di orders Chris to get the phone, Chris does so, mutating himself in the process, and crushes Baymax.
Ned finds Bessie and manages to tame Bessie back to his side. As Bessie and Ned head back to the woods, the mutants then try to attack them only for Bessie to fire blast at the monsters, shutting down their bio-chips and turning them all normal.
As Karmi has Hiro in her grasp, Hiro implores her to fight Di's control because he cares about her and his friend. Di tries to reinforce her control, but she resists, so Di has Chris finish them off. Regaining control, Karmi fights Chris. Di manages to place the patch on Hiro but loses her phone near Liv, who emerges from her chamber. Di is elated to see Liv well, but she witnessed the scene as a result of Di's actions, she deactivates the bio-chips, Karmi and Chris both turn to humans as Di rebukes Liv's lack of gratitude, but states she is ashamed of what Di did. Just then, the rest of Big Hero 6 arrives at the lab to witness what transpired.
Later, Di and Chris are arrested. Hiro goes to check in with Karmi but discovers her lab empty. Professor Granville informed him that Karmi's parents had decided to move her out of the city after deeming it unsafe. However, Karmi soon posts another Big Hero 6 fan fiction based on the recent event with herself as a superhero named Lab Lady to help Captain Cutie, which Hiro happily reads.
- 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.