Set in the fictional city of San Fransokyo, 14-year-old tech genius Hiro begins school as the new prodigy at San Fransokyo Institute of Technology and sets off to rebuild Baymax. However, his overconfidence and penchant for taking shortcuts leads him and the newly minted Big Hero 6 team into trouble.
Following the victory against Callaghan and the loss of Baymax, Hiro Hamada prepares to attend San Fransokyo Institute of Technology with his friends Go Go Tomago, Wasabi, Honey Lemon, and Fred. During the tour around campus, Hiro notices Tadashi's lab and reminisces about him and Baymax. Wasabi and the others remark they also miss Baymax and Tadashi, but Hiro assures them he is alright, and tells the others he will meet up later. Inside, Hiro leaves Baymax's rocket fist on a table and gives it a fist bump; he then discovers Baymax's healthcare chip within the fist's palm. Ecstatic, Hiro sets off to rebuild Baymax. Before he can get started, Professor Granville, the school's new dean, enters. She informs him that private labs are restricted for upper classmen, and Hiro will have to earn it like Tadashi did. Hiro tries to argue, but Granville sends him off to class.
In the evening, Wasabi, Go Go, Fred, and Honey regroup at Fred's Mansion. Fred tries to convince the others to continue being superheroes, although the others wish to resume to their normal lives. Somewhere else in the city, Mr. Yama is tasked by his overseer Obake to obtain a peculiar sculpture from San Fransokyo Tech.
Back at the institute, Hiro uploads Baymax's chip into a computer, reviving Baymax's conscience albeit confined to the computer. Hiro spends the next few days rebuilding his exoskeleton and simultaneously attending classes. When the exoskeleton is completed, Baymax advises Hiro that Tadashi had to run the body through a week-long series of trials before getting it to work properly. Hiro brushes the caution aside and boots up the exoskeleton. However, the exoskeleton malfunctions and escapes.
Meanwhile, Fred has finally convinced the others to suit up once more and patrol the San Fransokyo streets. They spot a runaway car and catch it, but only turns out to be a man driving his pregnant wife to the hospital. Embarrassed and ashamed for causing trouble, the others ditch patrolling and head back to the institute. Nearby, Hiro is chasing the exoskeleton until it is deactivated by Yama's goons. Yama catches Hiro and he discovers Hiro is a student at San Fransokyo Tech, leading Yama to spare Hiro. He agrees to give the exoskeleton back if Hiro can steal a peculiar sculpture in Granville's office. Enlisting Fred's aid, Hiro gets the access to Granville's lock and bypasses security to her office, retrieving the sculpture and delivers it back to Yama.
However, Yama betrays Hiro and has him and Fred incarcerated while he confiscates the sculpture. Meanwhile, the others learn from Baymax where Hiro and Fred are, and infiltrate Yama's headquarters to rescue them. Together, they defeat Yama's gang and escape with the exoskeleton and the sculpture. The group is upset with Hiro for rushing into danger, but Hiro redeems himself by returning Granville's sculpture. After running the tests and diagnostics, Hiro completes Baymax's body, and Baymax is revived. Meanwhile, Yama is constructing an army of exoskeleton Baymaxes from the information he obtained on Baymax's body.
Meanwhile, Hiro is encouraged by Granville to start focusing on his studies. Fred again tries to convince him and their friends to pursue the superhero lifestyle, and comes up with "Big Hero 6" as their team name. Despite his multiple efforts however, no one of the group develops interest. Just as Fred gives up trying to convince them, the city is suddenly attacked by Yama's Baymax army. The group sees innocents in jeopardy, and decide to don their suits once more. While the others hold the Baymaxes off, Hiro goes back to Tadashi's lab to complete Baymax's suit; during this time, Hiro admits he feels responsible for causing the mayhem. At that moment, they find the Baymaxes were a distraction for the heroes, as Yama has infiltrated the labs to steal the sculpture. Hiro disguises Baymax as one of his doppelgängers to take the sculpture, but the plot fails after Yama identifies Baymax. Yama has his robots hold Baymax down, but Hiro uses Baymax's rocket fist to punch Yama. In the process, the sculpture is chipped, revealing its ability to amplify any electric entity to dangerous levels. It attaches to a Baymax, causing it to throw the original Baymax out the window before it explodes with the energy. Yama escapes with the sculpture while Baymax is dangling from the building. Go Go, Honey, Wasabi, and Fred arrive in time to save Baymax from falling, and Hiro and Baymax properly suit up to join the others in tracking Yama.
Yama takes the monorail, during which the sculpture falls out of Yama's pocket and onto the train that Aunt Cass is on. The train goes dangerously faster and is set to collide with the central station. The team arrives, where Wasabi and Go Go promptly cut off the passenger car from the rest of the train, while Honey and Fred use an elastic concoction to slow it down. Baymax brings the train to a halt and Hiro removes the sculpture, but it then attaches to Baymax and overcharges his systems, causing Baymax to blast off into the sky. Hiro reverse the polarity on his gloves to hurl the sculpture upward and it explodes. However, Baymax's systems shut down and the two are sent falling. Struggling to reach Baymax, Hiro hotwires his systems and brings him back online, narrowly avoiding impact with the ground.
After the day is saved, the group return to their civilian lives. Professor Granville allows Hiro to move into Tadashi's lab to continue his work. Inspired by helping those in need, the six friends unitedly decide to continue their lives as superheroes under the name Big Hero 6.
In an undisclosed location, Obake is examining information on the Big Hero 6.
- Aunt Cass
- Professor Granville (debut)
- Tadashi Hamada (pictured)
- Bluff Dunder (debut)
- This is the official sequel/pilot to Big Hero 6.
- The special takes place during the film's epilogue.