To be fair, the plot isn't nearly as important to the film as the psychological profiling and characterization that takes place. Albeit the main story has one of my favorite alternate-universe endings, what is important are the people. Maybe explain it like this:
The film follows the lives and psyches of several out-of-work heroes in a dystopian parallel universe. It takes a deeper look at not only the individual thought process of those who would commit themselves to this type of vigilante justice, but also from the other side the mob mentality of those driven by fear who will lash out at anyone who claims a position of power. The characters are driven by their own fears and psychoses to seek out an unseen villain who is murdering the old heroes for reasons unknown, which leads them all to confront themselves and the world they live in, even at the cost of their safety and sanity.
In fact, more people should explain it like this to young kids who express an interest in it. If they still want to see it after hearing this, take them. They're PROBABLY mature enough for it.