WARNING: Spoilers
The synopsis contains information that may give away important plot points. You may not want to read any further if you've not already seen this title.
In the city of San Fransokyo (a portmanteau of San Francisco and Tokyo), Hiro Hamada (Ryan Potter) is a teenage prodigy, but seems to be unable to use his intellect for anything more than trying to enter illegal, back-alley bot-fights.
His older brother Tadashi (Daniel Henney) wishes to change Hiro's mind, and tricks his brother into coming to the San Fransokyo Institute of Technology, where Tadashi is a student. While there, Hiro is introduced to a number of Tadashi's 'nerd school' friends: Go Go (Jamie Chung), Wasabi (Damon Wayans Jr.), Honey Lemon (Genesis Rodriguez), and Fred (T.J. Miller), and the different inventions they have constructed.
Tadashi then shows Hiro his own project: an inflatable 'nurse' robot, named Baymax (Scott Adsit). Thanks to a chip that Tadashi programmed with numerous medical analysis information, Baymax is quite knowledgeable, impressing Hiro. Once he has performed his task, Baymax returns to sleep mode by being told, "I am satisfied with my care."
The three also meet Robert Callaghan (James Cromwell), the head professor of the Institute, and after he compliments Hiro on the miniature battle-robot he's made, Hiro wants to enter the Institute.
Tadashi then encourages his brother to come up with something that will impress Callaghan at the next robotics exhibit coming up. After Tadashi encourages his brother to 'use that big brain of yours,' Hiro then uses the robotic technology in his battle-bot, and crafts them into thousands of micro-bots, controlled by a neural transmitter. Whatever the wearer thinks, the microbots will do.
Needless to say, Hiro's presentation wins over a number of people at the exhibition, even impressing a high-profile technology guru, named Alistair Krei (Alan Tudyk). Alistair offers to purchase Hiro's microbot invention on the spot, but Callaghan claims that Krei is not a trustworthy man with technology. Hiro then declines Krei's request, but is excited when Callaghan presents him with a letter of acceptance to the Institute.
Hiro, Tadashi, and their friends head off to celebrate, but the mood is cut short when a fire erupts within the exhibition hall. With word that Callaghan is still inside, Tadashi rushes into the building to save him. However, as Hiro watches, an explosion rocks the building, blasting him back.
After Tadashi's funeral, Hiro becomes distant from his friends, still working through his grief. His aunt (and guardian) Cass (Maya Rudolph), tries to encourage Hiro to go to the Institute and take advantage of his acceptance letter, but he seems unwilling to do so.
After accidentally hurting himself in his room, Hiro is surprised when Baymax appears. Apparently, Tadashi had brought him back to their shared bedroom before he died. Baymax attempts to help Hiro, who is reluctant to accept it. However, the mood is broken when Hiro finds one of his microbots wriggling in a pocket of his jacket. Putting it in a specimen container, Hiro tries to ignore it, but Baymax notes inquires if he can try to determine where the 'small robot' wishes to go.
Hiro casually affirms this...not realizing that the robot will follow his commands. Chasing him down, Hiro and Baymax find themselves at an old warehouse. Sneaking inside, Hiro is surprised to find a machine churning out similar microbots, with numerous containers filled with them. However, his surprise turns to shock, when a masked man appears, attempting to kill Hiro and Baymax with the miniature robots.
Hiro first goes to the police station to report what he's found, but the officer taking down his story seems reluctant to believe him. It also doesn't help that Baymax's battery starts to run low. Hiro manages to get him home to be recharged. It is then that Baymax inquires where Tadashi is. Hiro attempts to explain to the robot that his brother is 'gone.' When Hiro explains that the fire was an accident, he then begins to wonder, given what he saw today...what if it wasn't, an accident?
Deciding to figure out who has stolen his invention, Hiro decides to upgrade Baymax. After creating a carbon-fiber suit for the robot, Hiro also programs a new chip that includes numerous martial arts moves, inserting it into an additional slot in Baymax's 'heart' drive, right next to Tasashi's chip of Baymax's programming.
The two then return to the warehouse, only to find it empty. However, Hiro notes his microbot has started to move again, and the two follow its movements to the docks off San Fransokyo. However, something pulls the microbot out of the container, and it disappears off into the misty bay. However, moments later, the masked figure appears, using the microbots as a transport means, and carrying a large object with the symbol of a red bird on it.
Hiro is just about to send Baymax to get the masked man, when they are accosted by Hiro's friends. Baymax's invite for them to help cheer up Hiro got through, but just then, the masked man attacks them. The group pile into Wasabi's car, as the masked man gives chase. In the aftermath, the car ends up driving into the bay, but the group is saved by Baymax, who has shed his armor, and helps the group float to the surface.
After getting on land, Fred leads the group to his home...which happens to be a large mansion owned by his parents (who are away).
While recovering, Hiro shows the group the red bird symbol he saw on the piece of machinery to his friends, but none have seen it. The group soon comes to a consensus, that the man may have been behind Tadashi's death, and Fred believes he knows who it is: Alistair Krei. Fred explains that it was Krei who wanted Hiro's microbot technology, and most likely, has gained it at the expense of Tadashi's life.
Baymax also surprises Hiro, when he explains that during the fight, he scanned the masked man, and has information on his health and vitals. Hiro wants to identify where their adversary is, but he would need to scan everyone in San Fransokyo to find out.
The group then decides to band together to stop Krei, with Hiro helping to design them each special suits based on their specific abilities.
Once Hiro has done this, he and Baymax fly up above the city, and scan the city from an aerial fan. Baymax's sensors pick up their adversary's health vitals, coming from an island in the bay.
The group flies there, to find a deserted base. One room shows the twisted metal remains of some machine, with the red bird symbol on it. The group then goes into a control room, where video shows that the room was used for testing a teleportation project, called "Operation: Silent Sparrow."
Suddenly, their masked adversary appears, and the group attempts to take him down, but are completely unsure what to do. Hiro manages to unmask their foe, but instead of finding Krei...he finds Professor Callaghan.
Callaghan explains that in the fire, he managed to use Hiro's microbot technology to shield himself from the explosion. This infuriates Hiro, who demands that Baymax destroy Callaghan. Baymax claims that his programming doesn't allow this, leading to Hiro pulling out the chip Tadashi put in him. The removal of the chip causes Baymax to follow Hiro's orders, and he charges at Callaghan. However, Hiro's friends attempt to stop Baymax, with the robot fighting back at them, before Honey manages to restore Tadashi's chip to his programming.
Callaghan has escaped during the fight, and Hiro demands that Baymax find him, only for the robot to explain that his scanners are damaged. Bent on revenge, Hiro blasts away with Baymax back to the garage. He attempts to update Baymax's programming, but Baymax refuses to open the port where his chip information is.
In desperation, Hiro finally breaks down over the death of his brother, with Baymax countering that, 'Tadashi is here.' As Hiro watches, Baymax projects video of Tadashi's numerous attempts to get him running, with the 84th test being a success, and Tadashi claiming that Baymax is "going to help a whole lot of people."
Hiro's friends eventually return to the garage where he is, and reveal that they have found further footage of the military test. The footage shows Professor Callaghan also at the test area, and reveal his motives behind his actions. During the test, a human test subject was propelled into the portal. However, the portal became unstable, and Krei ordered it shut down. It turns out that the test subject, was Callaghan's daughter, Abigail (Katie Lowes).
The next day, Krei is revealing the opening of a new facility in the heart of San Fransokyo, when Callaghan appears, using the microbots to piece together the last active portal, over Krei's building. Callaghan claims he's going to ruin Krei's life like he ruined his, as the portal comes to life, and starts to tear apart the building, sucking its pieces into the other dimension.
It is then that Hiro and his friends appear, pleading with Callaghan to stop what he's doing, but the former Professor refuses to listen to reason. The group attempts to stop him, but Callaghan uses the microbots to counter their attacks.
During the fight, Hiro notices that the stray microbots are being sucked up into the portal, and he and the rest of the team work to hack away at their structures, until Callaghan is soon down to a bare minimum.
Baymax manages to remove the control mask from him, but once he's done so, notes a faint life signal from within the portal. Even though it is unstable, Hiro and Baymax head into it.
Within the otherworldly atmosphere, they find the test pod, with Abigail inside. Baymax and Hiro then attempt to maneuver her out of the dimension, but a stray piece of debris from Krei's building slams into Baymax, ripping away much of his suit, and rendering his rocket boosters inoperable.
Baymax claims that he can get Hiro and Abigail to safety by usin