The decidedly dark James Spader-as-Ultron-narrated preview was supposed to air after next week's Agents of S.H.I.E.L.D. but apparently leaked out early, prompting an on-brand tweet from Marvel Wednesday night.
— Marvel Entertainment (@Marvel) October 22, 2014
Marvel Studios presents "Avengers: Age of Ultron," the epic follow-up to the biggest Super Hero movie of all time. When Tony Stark tries to jumpstart a dormant peacekeeping program, things go awry and Earth's Mightiest Heroes, including Iron Man, Captain America, Thor, The Incredible Hulk, Black Widow and Hawkeye, are put to the ultimate test as the fate of the planet hangs in the balance. As the villainous Ultron emerges, it is up to The Avengers to stop him from enacting his terrible plans, and soon uneasy alliances and unexpected action pave the way for an epic and unique global adventure.
Marvel's "Avengers: Age of Ultron" stars Robert Downey Jr., who returns as Iron Man, along with Chris Evans as Captain America, Chris Hemsworth as Thor and Mark Ruffalo as The Hulk. Together with Scarlett Johansson as Black Widow and Jeremy Renner as Hawkeye, and with the additional support of Samuel L. Jackson as Nick Fury and Cobie Smulders as Agent Maria Hill, the team must reassemble to defeat James Spader as Ultron, a terrifying technological villain hell bent on human extinction. Along the way, they confront two mysterious and powerful newcomers, Wanda Maximoff, played by Elizabeth Olsen, and Pietro Maximoff, played by Aaron Taylor-Johnson, and meet an old friend in a new form when Paul Bettany becomes Vision.
[Updated with the official trailer posted to Marvel's YouTube page]