Marvel Studios unveils its costliest special-effects demo to date; somewhere beneath the nonstop digital explosions lies a story about superheroes fighting an evil demigod from space. Every now and then, director Joss Whedon (Serenity) executes a quirky camera set-up to assert that the film was created by him and not a team of marketing executives. Of the star-studded cast, only Mark Ruffalo (playing Bruce Banner) and Robert Downey Jr. (as Iron Man) bring any personality to the place-holder dialogue. Overlong, monotonous, violent, and simple-minded, this is like one of those “World’s Biggest Gang Bang” videos, except that no one onscreen appears to be enjoying himself. With Samuel L. Jackson, Scarlett Johansson, and Jeremy Renner.