- Big Hero 6
- Doctor Robert Callaghan (Mentioned)
- Professor Granville (First appearance)
- Karmi (First appearance) (Cameo)
Hiro Hamada starts his first day as a student at San Fransokyo Institute of Technology by moving into his brother Tadashi's personal lab. He finds some Tadashi's research among his old things, allowing Hiro to rebuild Baymax. Baymax's skeleton goes crazy and escapes from the lab when Hiro turns it on without running the proper checks first. He follows it throughout the city, but the skeleton ends up in Mr. Yama's hands. Yama offers to return Baymax's skeleton to Hiro if he steals something that looks like a paperweight from the institute's new dean of students, Professor Granville. Hiro turns to Fred to help him steal if from her desk. When they bring the item to Yama, he betrays them and holds Hiro and Fred captive. Hiro figures out a way to escape Yama's prison when Wasabi, GoGo and Honey Lemon suddenly arrive to help them, having realized something was wrong when they saw Hiro stealing the paperweight. They fight off Yama and his goons, and return to Tadashi's lab with Baymax's skeleton with the paperweight. After he returns the paperweight to Granville's desk, Hiro finishes rebuilding Baymax - this time doing it the right way.
Mr. Yama tries to make up for losing the paperweight to his boss, Obake, by showing him that he's mass-produced robotic skeletons similar to Baymax. While impressed, Obake wants Yama to use these skeletons in another attempt to steal the device from Granville. Yama unleashes his Baymaxes on the city, forcing Fred, Wasabi, Honey Lemon and GoGo to suit up again to stop them. Hiro hurries back to the lab to get Baymax, but discovers Yama and some of his Baymaxes combing the school for Granville's paperweight. He has Baymax pretend to be one of Yama's evil Baymaxes so he can take the device before Yama can get to it, but Baymax blows his cover by offering to help Yama when he accidentally hurts himself. Hiro fights Yama to prevent him from taking the device from Baymax, but during the fight the device is tossed into the air and attaches to one of Yama's Baymaxes. The device makes the Baymax much more powerful, strong enough to easily overpower Hiro and Baymax. They escape and regroup with the rest of the Big Hero 6. After Baymax and Hiro suit up like the rest of the team, Hiro has Baymax scan the city for Yama. The scan reveals that the device can amp-up anything powered by electricity to dangerous levels, and that Yama is headed for one of the city's electric train stations. Yama, however, trips when he gets to the station, and the device falls out his pocket and attaches itself to the train. The Big Hero 6 stop the out-of-control train before it reaches Central Station. The device attaches to Baymax, so Hiro flies up with Baymax and destroys the device in the upper atmosphere.
Now that they've narrowly avoided disaster and saved a lot of people, the rest of the Big Hero 6 commit to make their team permanent. The next day, Professor Granville gives Hiro permission to continue using Tadashi's lab. Obake, meanwhile, ominously watches the Big Hero 6 from his lair...
- This episode originally aired as a one-hour television film on Disney Channel and Disney XD, as a prequel to Big Hero 6: The Series, and which would be followed by several shorts up until the show premiers in early 2018.
- Although it also aired on Disney Channel, it's not a part of Disney Channel Original Movies.