Hiro is a 14 year old robotics genius who uses his skills for profit in dubious underground robot fights. His older (and equally intelligent) brother Tadashi gently steers him into a university career, but this is cut short by tragedy early in the film. Hiro is devastated, but eventually aided - and to some extent guided - by Baymax, a (goofy) health care robot that Tadashi created. Tadashi's friends are eventually roped in to helping find the cause of the tragedy.
The movie is successful on almost every level: Hiro is convincing as a slightly misguided genius, the tragedy is heart-breaking, Hiro's recovery is slow (and the more convincing), and the humour is frequent and marvellous without stepping on the toes of the drama. It's a superbly constructed and hugely entertaining film. And the end-of-film easter egg with Stan Lee is definitely worth making it through the credits.