The villain of “Brotherhood” episode 1 (eponymously titled “Fullmetal Alchemist”) is Isaac McDougal, aka the Freezing Alchemist. In Amestris, the Western Europe-styled nation where “FMA” is set, alchemists can become State Alchemists, “dogs of military” in exchange for exorbitant research grants and privileges. McDougal was one of these State Alchemists, his title being because his alchemy is based around transmuting water. He resigned his commission after the State Alchemists were forced to annihilate the nation Ishval.
By “Brotherhood” episode 1, McDougal has become a fugitive, drawing alchemic transmutation circles around Amestris’ capital of Central City. Convinced the corrupt military must be destroyed for its war crimes in Ishval, his goal is to destroy the city with a tidal wave of ice and assassinate Führer King Bradley. Bradley assigns Col. Roy Mustang, the Flame Alchemist, to capture McDougal. Mustang in turn delegates the task to the Elric brothers.
The brothers grow interested in McDougal because he wields a Philosopher’s Stone, but don’t learn how he got it before he activates his transmutation circles. The Elrics defeat him before he can flatten the city with ice, so McDougal turns his alchemy inward, shaping his own blood into weapons. While he escapes the Elrics, he’s not so lucky against Bradley.
So, why did “Brotherhood” choose to kick off with an anime-exclusive story? Well for one, it’s an action-packed beginning that fits the pacing of the series. However, above all, the goal of the episode is to streamline the narrative.