Sofia the First meets Big Hero 6 is an upcoming Sofia the First crossover.
Somewhere in the city of San Fransokyo, a group of people are gathered in an alley for a bot fight. The reigning champion Yama destroys the opposing bot and challenges someone else to take him on. Entering the ring is 14-year-old boy genius Hiro Hamada (Ryan Potter) with a tiny bot that everyone laughs at. Hiro pays his entry and fights Yama's bot, where the latter breaks Hiro's in three pieces. Hiro pays for a rematch and then shows what his bot can really do. It repairs itself and manages to take Yama's bot apart. Hiro collects his winnings, but an angry Yama threatens him with his goons. Coming to the rescue is Hiro's older brother Tadashi (Daniel Henney) on his motorcycle. Tadashi gets Hiro out of there, but they both get arrested for Hiro betting on the bot fight.
The boys are bailed out by their Aunt Cass (Maya Rudolph). She is upset with them and takes them back to their home, which is right above the cafe she runs. We learn that she's been taking care of them for 10 years after their parents passed away. Tadashi asks Hiro what he is going to do with his life, since Hiro graduated high school at 13. Tadashi then takes Hiro with him somewhere.
The two, along with Sofia the first and her family, who is visiting, go to the San Fransokyo Tech Institute, Tadashi's college. He introduces Hiro to his other genius inventor friends - Go Go Tomago (Jamie Chung), who has invented a motorcycle with electromagnetic wheels; Wasabi (Damon Wayans, Jr.), working with a plasma laser; Honey Lemon (Genesis Rodriguez), a bright and energetic girl who creates a big purple smoke bubble; and Fred (TJ Miller), who is just the school's mascot but has a keen interest in science. Tadashi takes Hiro into a room and puts duct tape on his forearm and rips it off. Hiro yells in pain, which activates Tadashi's invention, a health care robot named Baymax (Scott Adsit). He assesses Hiro's pain level and uses something to get rid of his pain. Baymax deactivates when Hiro says he is satisfied with the results. Tadashi's professor Robert Callaghan (James Cromwell) enters. Hiro recognizes him from his studies and research, which inspires Hiro to pursue an education at this school.
Tadashi tells Hiro that a good way to get into the school is to impress Callaghan by presenting an invention for a showcase at the school. Hiro starts to come up with ideas but is frustrated that he can't come up with anything good. Tadashi inspires Hiro to look at the project from another angle. After spending a lot of time working with Tadashi and his friends, Hiro's latest invention is ready.
It is the night of the showcase. Hiro, Tadashi, and the friends wheel in barrels around the room. Hiro is very tense but Tadashi inspires him to stay cool. Hiro unveils his invention before Callaghan and everyone else - microbots. He takes one out, and hundreds of others spill out of the barrels. Using a neurotransmitter headband, Hiro is able to control the microbots to do anything. Everybody is astonished. After the show, Hiro is approached by Alistair Krei (Alan Tudyk), head of Krei Tech. He is interested in buying the microbots, but Hiro tells him they are not for sale. Callaghan then hands Hiro an envelope to invite him to register for classes.
Outside, Hiro thanks Tadashi for bringing him to the school. Tadashi assures Hiro that he believes in him. Suddenly, people are heard screaming. Hiro and Tadashi run back to the building to see that it's on fire. Upon hearing that Callaghan is still inside, Tadashi runs in, and moments later, there is an explosion. Hiro then calls for his brother.
A memorial is held for both Tadashi and Professor Callaghan. Tadashi's friends and Sofia's family go to his home to console Hiro and Cass. Hiro just stays in his room, depressed.
After a while, Hiro is still in his room mourning his brother. He picks up his little bot and a piece falls on his foot. He yells in pain, and Baymax reactivates. He tries to tend to Hiro's pain, but Hiro says he is fine. Baymax notices the last of Hiro's microbots is moving around in its container. Baymax decides to follow its direction, taking him out into the streets. Hiro and Sofia run after him until they reach a warehouse. In there, they find that the microbot is trying to attach itself to thousands of other microbots that someone else built. These microbots then come together and try to attack them. Hiro and Sofia run with Baymax out of there. Hiro sees a man in a kabuki mask manipulating the microbots advancing toward them. Hiro pushes Baymax out the window, going down with him. Baymax catches Sofia and Hiro and they land safely.
Hiro runs to the police station with Baymax to report on this incident, but the officer has a hard time believing his story. He runs home with Baymax and decides that they must catch the masked villain. Baymax only agrees since he thinks it will improve Hiro's emotional state. Hiro creates a carbon fiber suit for Baymax and incorporates a series of fighting techniques into a chip that Hiro places into Baymax's access port, right next to the health care chip that Tadashi first made for Baymax.
Hiro, Sofia and Baymax follow the microbot again to the city's port to find the other microbots. They don't see that they're followed by a car. In this car are Go Go, Wasabi, Honey, and Fred, as well as Sofia's family, and Baileywick, who were all contacted by Baymax when he assumed Hiro was under emotional distress and needed the support of friends. They are all attacked by the masked villain, who uses the microbots to hurl a giant crate at them. Hiro commands Baymax to fight back, but the villain throws Baymax in the air. The gang hops in the car and Wasabi drives away while the villain chases after them. Wasabi keeps driving too safely (stopping at red lights and whatnot), so Go Go takes the wheel and evades the microbots. They escape the villain, but they end up driving the car into the river. Baymax pulls them all up to the surface.
Fred brings the gang to his home for sanctuary, which happens to be a huge mansion owned by his parents. In his room is a big collection of monster statues, comic books, and action figures. While looking through the comics, Fred deduces that the masked villain must be Alistair Krei, who must have started the fire at the school to steal Hiro's microbots. Baymax then reveals that he scanned the villain at the port, meaning they have a reading on him. Hiro then comes up with a plan to upgrade not only Baymax, but the other four, and Sofia's family, as well.
The plan is to get the villain's mask since he may be hiding the neurotransmitter headband beneath it. Hiro makes suits for the gang and makes weapons for them as they practice getting the mask off Fred's butler Heathcliff. Honey has little balls of tungsten carbide that make foam and harden to trap the villain; Go Go has electromagnetic roller blades and discs; Wasabi has lasers on his arms; and Fred has a monster suit that jumps high and breathes fire. Sofia's suit gives her a power resembling Elsa's from Frozen, Amber has her Princess Butterfly costume, which can launch energy powered butterflies, James's suit give him arm cannons that fire explosive bombs, King Roland gets the power to wield stars of energy, Queen Miranda gets the power to create lasers in all the colours of the rainbow, and Baileywick gets a special watch, that can slow enemies down, also, a black, ninja-like suit that gives him a ninja-style combat, complete with a walking stick that doubles as a staff. Hiro makes himself a suit, and a new one for Baymax that can fly and has a rocket fist. He tests this suit out on Baymax, and they fly all across the city. Baymax determines that Hiro's emotional levels are improving. The two sit high above the city, and Baymax scans everyone in the city. He finds a life form matching the villain on an island further from the city.
Baymax flies the gang to the island. They come across an abandoned laboratory with a ruined machine. They find footage on the computer showing Krei and his scientists showing off two portals they created to make use of teleportation. A woman is preparing herself in a pod to go through the first portal. One scientist tells Krei that he notices an irregularity, but Krei ignores it. The woman goes through the portal, and then the portal becomes unstable. The second portal closes, trapping the woman in there as the scientists are forced to shut down the portal when it starts to suck everything else in. The gang comes to the conclusion that Krei IS the masked villain and that he wanted to steal Hiro's microbots to rebuild the portal, just as the villain attacks the gang. Making use of their suits, they try to catch him, but Honey accidentally gets hit with one of Go Go's discs, which in turn makes Go Go slip on one of Honey's carbide balls, and crash into Miranda, Amber, and Roland. Fred, James, Baileywick, and Wasabi crash into each other as the villain throws them off. Baymax flies Hiro high enough for him to tackle the villain to the ground and Sofia removes his mask. When he turns around, he is revealed to be Professor Callaghan, alive and well. He tells Hiro that he took the neurotransmitter and used the microbots to protect him from the explosion. Hiro tells him that Tadashi lost his life trying to save him, and Callaghan remarks that that was his mistake. Filled with rage over the fact that his brother died for nothing, Hiro orders Baymax to destroy Callaghan. Since Baymax is not programmed for this, Hiro removes the health care chip and leaves the fighting chip, leaving Baymax to turn mad and attack Callaghan. The others try to stop Baymax until Honey opens the access port and puts the health care chip back in its place, just as Callaghan escapes. They are shocked at Hiro for his actions, and he angrily gets Baymax to fly him and Sofia and her family out of there.
Hiro returns home with Baymax to fix his scanner and find Callaghan. He then tries to remove the health care chip again, but the access port won't open. Sofia tries to stop him but to no avail. Baymax tells Hiro that Tadashi wouldn't have wanted this, but Hiro yells that Tadashi is gone. Baymax assures Hiro that Tadashi is "right here" - he turns on a screen on his chest that shows footage of when Tadashi was documenting his creation of Baymax. After 83 failed tests, the 84th one is successful, and Tadashi is ecstatic when Baymax is fully functioning. He says he can't wait to show Hiro and that Baymax will change lives. Seeing Hiro look sad, Sofia, her family, and Baileywick sing 'Hey, What's Going On'. Hiro sheds a tear and he hugs Baymax, upon discovering that killing Callaghan would not be what Tadashi wanted. The others return (they were brought back in a chopper by Heathcliff) and they all hug Hiro. They agree to stand by him and to catch Callaghan properly.
Honey gives Hiro a jump drive that they found with more footage. Just after the portal shut down, Callaghan is shown angrily running toward Krei. Right before the woman entered the pod, Callaghan hugged her. Hiro pauses the video and magnifies her helmet, showing the name Callaghan. They realize the woman was Callaghan's daughter Abigail, and that her father wants revenge against Krei.
Krei holds an event at his company's building, where Callaghan arrives and begins to attack with the microbots. He then recreates the portal, which slowly destroys Krei's building before he plans to throw Krei through the portal. The heroes arrive in time to fight back. Callaghan uses the microbots to attack Hiro's friends and Sofia's family, while Hiro is thrown off Baymax and is nearly sucked through the portal. He gets his friends to see this from another angle, just like how Tadashi showed him. They use their weapons to break free of the microbots. Baymax then saves Hiro. Hiro tells them to send the microbots through the portal so that Callaghan will have nothing to use against them. They succeed in doing so, leaving Callaghan with nothing. He is nearly pulled into the portal, but Baymax grabs him.
The portal falls to the ground and is still active. Baymax then says he detects a life form coming from the portal, and it is a female in hypersleep. Abigail is still alive, and Sofia and Hiro rides Baymax through the portal to rescue her. They find her pod amidst the debris from Krei's building, and they pull her toward the portal. However, a gigantic chunk of debris hits Baymax's armor, leaving him with only half of it. His rocket thrusters are down, and he knows he can't get all of them out. Baymax tells Hiro that he can send him, Sofia, and Abigail back, but Hiro must tell him that he is satisfied with his care. Hiro doesn't want to lose Baymax as well, but Baymax assures him everything will be okay. Hiro tearfully hugs Baymax and says he is satisfied with his care. Baymax releases the rocket fist to push Sofia, Hiro and Abigail through the portal, while Baymax slowly drifts away. Sofia, Hiro and Abigail make it out just before the portal explodes.
Abigail wakes up and is taken to the hospital, while Callaghan is arrested. He is relieved that Abigail is alright, but looks on in shame, knowing he still may never get to see his daughter again.
The city acknowledges the heroics of the gang, unaware of their true identities. Go Go, Wasabi, Honey, and Fred hang out at Cass's cafe with Hiro. In his room, Hiro sets the rocket fist down, when it opens up and reveals that the health care chip is still in place. Using this, Hiro rebuilds Baymax just the way Tadashi made him. The two of them hug.
Hiro closes the film with a voiceover saying that just the way Tadashi wanted to help people, he and his friends and Sofia's family are now going to do the same. They get together in their suits, along with Baymax, to spring into action as...BIG HERO 6!
After the credits: Fred is shown in his home looking at a family portrait. He addresses his father (who looks suspiciously like Stan Lee...) and says he wishes he could have been better. Fred then touches the portrait, which opens up a secret room with gadgets and suits. From behind him, his father appears (it IS Stan Lee!) and holds up a pair of spandex underwear. He shares the same method of underwear use as his son (front, back, inside out, then front to back). The two of them hug, and Fred's dad says they have a lot of catching up to do.
- Hey, What's Going On - Sofia, James, Amber, Miranda, Roland & Baileywick