Hiro Hamada is a 14-year-old robotics genius living in the futuristic city of San Fransokyo. After graduating from high school, he spends much of his free time participating in illegal robot fights. To redirect Hiro, his elder brother Tadashi takes him to the research lab at the San Fransokyo Institute of Technology, where Hiro meets Tadashi’s friends, GoGo, Wasabi, Honey Lemon, and Fred, as well as Baymax, the inflatable healthcare robot that Tadashi created. Hiro also meets Professor Robert Callaghan, the head of the university’s robotics program. Amazed, Hiro decides to apply to the university. To enroll, he signs up for the school’s science fair and presents his project: microbots, swarms of tiny robots that can link together in any arrangement imaginable using a neuro-cranial transmitter. Hiro declines an offer from Alistair Krei, CEO of Krei Tech, to market the microbots, and Callaghan accepts him into the school. When a fire breaks out among the exhibits, Tadashi rushes in to save Callaghan. Hiro rushes in to help, but a sudden explosion apparently kills both Tadashi and Callaghan.
Weeks later, a depressed Hiro inadvertently reactivates Baymax, who follows Hiro’s only remaining microbot to an abandoned warehouse. There, the two discover that someone has been mass-producing the microbots and are attacked by a man wearing a Kabuki mask who is controlling them. After they escape, Hiro equips Baymax with armor and a battle chip containing various karate moves and they track the masked man to the docks. GoGo, Wasabi, Honey Lemon, and Fred arrive, responding to a call from Baymax, and the masked man chases the group. The six escape to Fred’s mansion where they decide to form a high-tech superhero team to combat the villain.