In a world where mutants (evolved super-powered humans) exist and are discriminated against, two groups form for an inevitable clash: the supremacist Brotherhood, and the pacifist X-Men.
When Tony Stark and Bruce Banner try to jump-start a dormant peacekeeping program called Ultron, things go horribly wrong and it's up to Earth's mightiest heroes to stop the villainous Ultron from enacting his terrible plan.