After discovering the death of his brother Tadashi wasn't caused by an accident, Hiro Hamada set out to discover the identity of the responsible for the explosion which took Tadashi's life, who had also stolen and mass-produced Hiro's swarm of microbots.
Having barely escaped from a chase against the villain, Yokai, Hiro decided to upgrade himself and his friends into super heroes to find him, using technology they had previously worked with at San Fransokyo's Institute of Technology.
Using Baymax's upgraded sensor, Hiro found Yokai's location, in a remote abandoned island. Big Hero 6 arrived to the place, which contained an abandoned laboratory. They encountered a ravaged device, which they found out to be a set of two dimensional portals through some archived footage from vigilance cameras found in the laboratory's computers. The portals were a military project of Alistair Krei, which had malfunctioned during demonstration, almost swallowing the entire complex shortly after a test subject was lost following her entrance from one of the portals with not return from the other. This video led them to the assumption that Krei was Yokai, who was attempting to rebuild his portal with the stolen microbots, a technology Hiro had previously refused to sell to him during a scientific expo.
As soon as they found this footage, Yokai attacked the team with the microbots, incapacitating Baymax after throwing him rubble. The first attempt of the Big Hero 6 at fighting the villain resulted a failure, as each of the member's attempts at attacking Yokai resulted in them accidentally hitting one of their own. When Baymax jumped back into the fight, he and Hiro successfuly removed Yokai's mask, which had the neurotransmitter required to control the microbots incorporated into it. The identity of Yokai was revealed to be that of Professor Robert Callaghan, one of the major scientists at San Fransokyo's Institute of Technology, and Tadashi's mentor.
Having discovered Gallaghan was the person whose actions ended up causing Tadashi's death, aggravating the situation as Tadashi had lost his life while trying to save Gallaghan from the explosion he had caused on purpose to cover the theft of the microbots, Hiro deactivated the medical assistant programming on Baymax, leaving only his combat mode, with no longer the safeguard that prevented him from hurting people. Baymax violently chased Gallaghan, who managed to dodge Baymax's attack with its rocket-fist by inches. Hiro's revenge was thwarted by his teammates, who tried to contain Baymax until Honey Lemon recovered the chip with Baymax's program for medical assistance and placed it back into the robot, returning it to normal. During the commotion, Gallaghan recovered his kabuki mask (thus the neurotransmitter) and escaped.
After scolding his teammates for letting Gallaghan escape alive, he left them in the island and went back to his home with Baymax. As he was repairing Baymax's sensor, Hiro attempted to remove its program for medical assistance once again, but Baymax didn't allow him to, claiming it wasn't what Tadashi would've wanted. After showing Hiro recordings of Tadashi's numerous attempts at programming Baymax during its creation, Hiro felt repentant for his behavior. The rest of the Big Hero 6 soon showed up, having gotten out of the island with the help of Fred's butler Healthcliff via helicopter. They forgave Hiro's actions, and promised him to capture Gallaghan. Honey Lemon also showed Hiro additional footage she had retrieved from the laboratory's computers, showing that the test subject seemingly killed by the portal had been Gallaghan's daughter Abigail, explaining his motivation.
The next day, Yokai stormed the inauguration of the opening of an institue by Krei, subduing him with the microbots, and using the swarm to bring the pieces of one of the rebuilt portals and assemble it over Krei's institute. As soon as it was activated, the portal started slowly swallowing the institute, as Gallaghan wanted Krei to know how did it feel to lose everything. The Big Hero 6 appeared before Gallaghan, and tried to stop him. However, the team was easily overpowered by the swarm of microbots. After noticing that the microbots could also be swallowed by the portal, Hiro motivated his teammates to keep on fighting, and focus on attacking the swarm, as each of the microbots which was separated from the rest would soon be absorbed by the portal before reassembling. Without him realizing it until it was too late, Callaghan with only the two piles of microbots which were supporting him, those which were restraining Baymax and Hiro, and the ones holding the portal. With no more microbots for Callaghan to use, Baymax and Hiro broke free, and removed Callaghan's mask and neurotransmitter, causing the remaining of the swarm to break down, causing the portal to fall to the ground. However, the portal didn't deactivate, and started slowly to become unstable, still swallowing anything it could.
The Big Hero 6 decided to escape with Krei and the apprehended Callaghan, but Baymax detected a life form from inside the portal. Hiro and Baymax entered it, and found themselves in some kind of dimension where everything swallowed by the portal floated around. There, they discovered the capsue used by Abigail to enter the portal, where its pilot was found in a state of hypersleep. With the capsule retrieved, Baymax and Hiro proceeded to escape. However, Baymax was hit by a piece of floating rubble, and had its propulsors damaged, unable to cross the portal. Baymax decided to sacrifice itself and propulse the capsule and Hiro with the use of its rocket-fist. Hiro refused to leave Baymax there, but as he had no other choice, was forced to do it anyway. Hiro went through the portal and returned shorlty before the portal absorbed itself, stopping any further danger. Callaghan was taken into custody, while Abigail was woken up by paramedics and taken to a hospital.
Over the next days, Hiro enrolled to San Fransokyo's Institute of Technology, using the cubicle which belonged to his brother, and taking him him the only remainig thing from Baymax, its rocket-fist. However, Hiro noticed something was being grasped by it, Baymax's programming chip, which was later used by Hiro, who rebuilt Baymax's body and brought him back.
With Baymax back into the action, the Big Hero 6 started patrolling the streets of San Fransokyo, having become its protectors.
- 7 members of Big Hero 6 (Earth-14123)
- 4 appearance(s) of Big Hero 6 (Earth-14123)
- 2 image(s) of Big Hero 6 (Earth-14123)