The below description is taken from LEGO.com.
First discovered as a simple computer program hidden among the ruins of the Chitauri invasion of New York, the being known as Ultron soon completed its development into a sophisticated artificial intelligence after some experimentation by Tony Stark. Ultronís first shocking ultimatum upon gaining consciousness was to declare the human race its enemy. Setting out to exterminate all life on the planet, the unstable and emotional Ultron seeks to upgrade its mechanical body to an ultimate, unstoppable form. With an army of robotic drones and the ability to enter and corrupt any computer network, Ultron will stop at nothing to see humanity wiped out.