The FMA storyline, in all iterations, is... complicated. It deals with massive philosophical themes, juggling questions about the nature of truth and the existence of God, and whether consciousness without a body is enough to confirm that a person exists. It involves a civil war, factional political squabbling, a giant government conspiracy, and each of the seven deadly sins made flesh as artificial, superpowered humans called homunculi. That doesn’t even include the very complicated rules for how alchemy -- the practice of which relies primarily on the scientific (not magical) law of equivalent exchange -- works.
Thankfully, focusing on the crux of the original story is what the live-action adaptation does best. That would be of the Elric brothers, Edward and Alphonse, and their quest to repair their bodies following an attempt of using alchemy to bring their mother back from the dead -- a forbidden practice with what turns out to be an impossible goal. As a result of playing with powers they couldn’t understand or control, the brothers were sucked through a portal into another dimension, where they learn the fundamental Truth behind alchemy. For this, they paid a price: Ed lost his left leg and Al lost his entire body, and Ed was forced to sacrifice his right arm in order to keep Al’s soul tied to the material world. He did so by welding it to a giant suit of armor, which Al’s soul is able to manipulate as if it's his own body. Ed’s childhood friend, Winry, outfits him with an automail arm and leg, and he learns that he can now perform alchemy without the use of a transmutation circle, a tool typically essential to the process. He and Al begin a quest for the Philosopher’s Stone, a fabled alchemical object believed to let alchemists bypass the law of equivalent exchange, in the hopes that its powers will allow them to repair their bodies. The military soon recognizes Ed's talents and employs him as the youngest state alchemist in the history of his country, Amestris. His codename? Fullmetal. (Cue the theme music, aptly adapted from the anime for the film by Reiji Kitazato.)
The film condenses a lot of very complicated plotlines nicely, including a brutal tear-jerker of an arc that is a fan favorite despite being seemingly custom-built to make you hate humanity. The casting, in general, works: the major characters -- most important being the Elrics, Colonel Roy Mustang, and Captain Maes Hughes -- were well-cast and done justice. And decisions to conflate certain characters in order to slim down on the anime’s massive world typically worked, with General Hakuro, presented first as an ally and later as the film’s secondary villain, filling roles in a way that cut down on what would have involved a lot of infodump conversations otherwise.