In a world full of mysteries, 14-year-old genius Hiro Hamada prefers to explore them through his inventions and rocketry rather than his college lectures. His rambunctiousness and general lack of concern for others around him tends to get him into trouble, often having to rely on his older brother Tadashi to bail him out.

At a showcase hosted by the Sans Fransokyo Institute of Technology, Hiro exhibits his microbots capable of gathering together and transforming into all kinds of large figures. He is also introduced to Baymax at the event, which is Tadashi's entry that can automatically diagnose and treat humans in need of medical assistance. Little did the young genius know that this rotund robot would soon become the last connection to the only person he truly cared about.