If you have seen Batman V Superman: Dawn of Justice, then you already have a pretty clear understanding as to how the whole thing plays out.
Borrowing footage liberally from The Dark Knight trilogy, Superman Returns, and Dawn of Justice, the preview shows us a version of Batman V Superman fronted by the predecessors of Ben Affleck and Henry Cavill.
That fan trailer is a phenomenal example of what could have happened if cohesive superhero universes had started just a little bit sooner.
The trailer even cuts Wonder Woman in on the action for a few awesome shots.