In the futuristic city of San Fransokyo, 14-year-old robotics genius Hiro Hamada (Ryan Potter) has taken to competing in underground robot fights, where he cleverly deceives notorious gangster Yama (Paul Briggs) to up the stakes significantly. Hiro's robot, designed for modular and versatile configurations, triumphs over Yama's champion. However, Yama's lackeys soon confront Hiro, only to be saved at the last moment by his older brother Tadashi (Daniel Henney), who arrives on his scooter.
Despite having graduated high school at just 13 years old, Hiro finds himself lost in a life of illegal gambling, disheartening Tadashi. After their arrest, the brothers are bailed out by their legal guardian, Aunt Cass (Maya Rudolph), who has cared for them for a decade. Feeling disillusioned with traditional education, Hiro is resistant to college, a concern for Tadashi who decides to take Hiro to the prestigious San Fransokyo Institute of Technology.
At the institute, Hiro meets Tadashi's close-knit circle of friends: the fierce and driven Go Go Tomago (Jamie Chung), the meticulous Wasabi (Damon Wayans Jr.), the curious chemistry expert Honey Lemon (Génesis Rodríguez), and the comic-relief slacker and mascot Fred (T.J. Miller). Here, Hiro also encounters Baymax (Scott Adsit), an inflatable healthcare companion that Tadashi invented, who is still in developmental stages and armed with an impressive arsenal of medical knowledge—over 10,000 procedures stored on his chip.
Tadashi introduces Hiro to his mentor, the groundbreaking Prof. Robert Callaghan (James Cromwell), who turns down Hiro's ambitions, stating that being part of the program is not something to take lightly. Aiming to inspire Hiro, Tadashi encourages him to participate in the university’s annual showcase, where Hiro can potentially impress Callaghan.
After a failed attempt, Hiro captivates the attention of Alistair Krei (Alan Tudyk), a tech mogul who wishes to purchase his revolutionary project: a swarm of tiny Microbots that connect in various configurations via a neural transmitter. However, before the celebration can begin, tragedy strikes when a fire engulfs the building, leading to Tadashi’s courageous—yet fatal—attempt to save Callaghan.
Lost in grief and isolation, Hiro pushes away everyone, throwing away his acceptance letter, and spirals into despair. It’s only when he inadvertently activates Baymax that he begins to confront his pain. Hiro soon discovers that one remaining Microbot begins to move, leading him and Baymax on a quest to uncover the truth behind his brother’s death. This search takes them to a warehouse operated by a mysterious masked villain known as Yokai, who is discovered to be mass-producing Microbots.
Determined to find justice for Tadashi, Hiro arms Baymax and convinces him to join the battle against Yokai, enduring thrilling confrontations that lead Hiro to enlist the aid of his friends. Together, they combat the rogue tech entrepreneur and uncover more than they bargained for, learning that Yokai is actually Callaghan, who orchestrated the tragic fire to conceal his own misdeeds.
Conflict arises as Hiro seeks revenge against Callaghan, leading to a clash between his thirst for vengeance and the compassionate nature that Tadashi instilled in Baymax. As the story unfolds, Hiro grapples with the ethical dilemma of avenging his brother’s death, and together with his friends, they confront a series of trials that include discovering Callaghan's motives linked to a long-lost test pilot—his own daughter, Abigail.
Ultimately, Hiro learns the importance of teamwork and sacrifice, as he, Baymax, and their friends strive to save Abigail and defeat Callaghan amidst a chaotic battle involving an unstable teleportation portal. In a poignant conclusion, Hiro realizes the essence of humanity and heroism through Baymax, leading to the formation of the Big Hero 6, a team of high-tech superheroes dedicated to protecting their city.
Their journey ends with Hiro rebuilding Baymax, cementing their legacy as protectors while a post-credits scene hints at further adventures hidden within Fred’s opulent mansion, where he stumbles into a secret superhero lair and reconnects with his own enigmatic father.
