At the end of the 2003 anime, Ed somehow manages to defeat the seven deadly sins and Alphonse gets his body back but gets himself transported to another universe in the process. That world is like normal, reality earth where there's no alchemy or alchemy doesn't work there. Ed ends up meeting the Earth-version of his brother named Alfonse. (Notice the change of spelling, Alphonse=Ed's bro/Alfonse=earth version of his bro where they're not related). At the start of the movie, we see Ed trying to help Alfonse in his research and they sort of end up staying together or working with each other. (I have to rewatch the entire thing again so I can remember but that's that and I won't spoil too much)