No story whatsoever. Avengers worked as well as it did because they built up anticipation by taking the easiest storyline, taking all the main
characters involved, and developing individual storylines in a game plan that took 7 years to complete, from development to release.
None of the Justice League characters have ever even been implied to be involved with one another, in any stand alone film, nor has any of the
behind-the-scenes been meant to imply a union of the characters. At this point, they would have to set up a careful line of DC movies to illustrate
the individual characters involved, before even considering
a consolidation. Because first and foremost, they need to know that DC can find a
Whedon-esque director to ensure that the endeavor doesn't flounder like the overall DC setup (so far as JL characters are concerned) has done so far.
Alone? Decent. As a build up? Man of Steel will determine that, but as it stands now, leave Comic Cinema Consolidated to the Marvel team. They've
proven they can do it. If Sony decides to snatch a piece, it has a lot of groundwork to cover first.
Back to the question - my short answer: that's the wrong question. The better question is, how do they set up the Justice League movie, not even
considering the script? See above.