In the future super city of San Fransokyo, Hiro, a gifted boy who loves inventions, participated in the entrance competition for the robotics major of the Polytechnic Institute hosted by Professor Robert Callahan with the encouragement of his brother Teddy. He won unanimous praise from the audience, contestants and examiners with his magical micro-magnetic robot, but a sudden disaster destroyed Hiro's dream and life. The fire burned down the exhibition venue, and his brother died in the fire in order to rescue the trapped Professor Callahan. Hiro, who was traumatized physically and mentally, stayed indoors, and the therapeutic robot Baymax left by his brother became his only companion to comfort him. It was originally thought that the micro-robot was also destroyed in the fire, but Hiro and Baymax accidentally discovered that someone was mass-producing his invention in an abandoned factory. Later, his brother's friends Fred and others also joined in. They put on the super soldier combat equipment invented by Hiro and competed with the mysterious opponent with a sinister conspiracy...