This full length movie is a sort of sequel to the original series. It seems that when Ed went through the gate so that his brother Al could get his body back ended up on a place much like our world. Set in Germany before the Nazis took over.
In that world Ed found the analog to Al, who is an engineer instead of an alchemist. They meet a Romany girl and the story goes from there. Filled with both things we know of from history with just enough twists to make it interesting and of coures tossing between the two worlds so that we know what happened to the real characters, this movie fills in the gaps in a way that is very enjoyable. Well worth watching.