This movie is TERRIBLE. Literally, you have the Elric brothers verse literally everyone else (at some point). Everyone fights in one continuous fight and you have several different groups trying to seize power at the cost of sacrificing one group or another. Fun to spoof, though.
I could try to explain the plot, but there is no plot, really. It boils down to someone wants to do human transmutation, the Elric boys want to stop it but then they kind of help it, and then it kind of happens, but everyone is okay with it.
Because there is no plot, the action is the main draw. And even then, the action has some terribly rushed parts, and nothing makes sense.
It’s better to see this after you finish the series, because the series is a work of genius and this is its dopey cousin relying on the family name for attention. Do not watch before seeing the series.