Summary: Hiro Hamada is a 14-year-old robotics genius living in the futuristic city of San Fransokyo who 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. 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 neurocranial transmitter. At the fair, Hiro declines an offer from Alistair Krei, CEO of Krei Tech, to market the microbots, and Callaghan accepts him into the school. At the end of the day, a fire breaks out among the exhibits and Tadashi rushes in to save Callaghan, the only person left inside. The building explodes moments later, killing Tadashi.
Weeks later, a depressed Hiro, in mourning for Tadashi's death, inadvertently activates Baymax, the
inflatable healthcare robot that Tadashi created; the two find Hiro's only remaining microbot and follow it to an abandoned warehouse. There they 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.
The group tracks the masked man, whom they suspect to be Krei, to an abandoned Krei Tech laboratory which was used for
teleportation research until a test pilot was lost in an accident. The masked man attacks, but the group subdues him and knocks off his mask – revealing him to be Callaghan, who had stolen and used Hiro's microbots to shield himself from the explosion and allowed Tadashi to die. Enraged, Hiro removes Baymax's healthcare chip, leaving only the battle chip, and orders him to kill Callaghan. Honey reinstalls the healthcare chip at the last second, preventing Baymax from carrying out the kill order. Callaghan escapes, and Hiro leaves with Baymax, intent on avenging Tadashi.
Back home, Hiro tries to remove the healthcare chip again, but Baymax stops him and states that vengeance is not what Tadashi would have wanted. To calm him down, Baymax shows Hiro videos of Tadashi running numerous tests during Baymax's development as a demonstration of Tadashi's benevolence and legacy. Hiro remorsefully apologizes to his friends, who reassure him they will catch Callaghan the right way.
Video footage from the laboratory accident reveals that the lost test pilot was Callaghan's daughter Abigail, and that Callaghan is seeking revenge on Krei. Callaghan interrupts Krei at a public event and attempts to destroy his headquarters using Krei's teleportation portal. After a lengthy battle, the team deprives Callaghan of his microbots and the mask, saving Krei, but the portal remains active. Baymax detects Abigail inside, alive but in hyper-sleep, and leaps into the portal with Hiro to rescue her. They find Abigail's pod, but on the way back out, Baymax is struck by debris, damaging his armor and disabling his thrusters. Baymax uses his armor's rocket fist to propel Hiro and Abigail back through the portal before it closes. Callaghan is arrested while Abigail is taken to the hospital.
Some time later, Hiro discovers Baymax's health care chip clenched in the rocket fist. He rebuilds Baymax's body, and the six friends continue their exploits throughout the city. During the end credits, a series of newspaper headlines reveals that the university has awarded Hiro a grant and dedicated a building in Tadashi's honor, and that the team has continued protecting the city. In a
post-credits scene, Fred discovers a hidden cache of superhero equipment in his family mansion. His father, a retired superhero, returns from vacation and says, "We have a lot to talk about."
