Classic Comic-book calamity ensues when the MCU's Avengers come up against their greatest challenge yet - an alliance between two super-powered siblings and an unshackled AI bent on bringing about the end of humanity. New additions, Paul Bettany, Elisabeth Olsen and Aaron Taylor-Johnson join an already star-studded cast including all of the staple super-heroes of the Avengers franchise - Robert Downey Jr., Chris Evans and Mark Ruffallo to name a few. Believing the Avengers initiative to be incapable of fending off every threat which could come to the earth, Tony Stark works alongside Bruce Banner and his AI butler, JARVIS, to create a world-wide super AI to help defend the earth when the team of superheroes cannot. However, after seeing the destruction and violence caused by the human race, the AI - known now as Ultron - possesses an army of robotic bodies before wreaking havoc on a high-profile party that Stark is hosting. As the misanthropic Ultron grows in power and recruits more allies to its cause, the Avengers face their toughest challenge yet and must make shocking sacrifices if they are to thwart to malignant AI's plans and save humanity. The Avengers: Age of Ultron - Saturday 10:20pm on BBC1.