Big Heroes is a story about a boy named Hiro Hamada, he has that existential crisis that we all go through, and that some never leave behind, about what he wants to be in life. For what is shown is an expert in robotic fights.
His brother Tadashi, one day convinces him to take him to the University so he can see how he is being wasted in nonsense and then Hiro decides that that is what he wants, to live for science. In addition, they present him for the first time to Baymax, which is the work of his brother.
With that decision in his hands, he has to get into the university, which is a very difficult process because of what he sees, for which he invents a device that will gain the attraction of the villain.
When he presents his
invention, everything explodes and his brother dies. He submerges in a great depression and does not leave it until with the help of his friends and Baymax (whose main programming is to cure) he manages to leave it behind.