Al's body has been transmuted into a bomb by Kimblee just before his passing, and Scar, in a fit of compassion, prepares Al to be saved by turning him into the Philosopher's Stone. Scar sacrifices his right arm, melding it with Al, and making Al a container waiting for the army's invasion of the city. Ed manages to fight off the two homunculi and with Ryla and Rose, they escape, but Ed knows he has to go back, and returns to the city. Mustang, using information gathered by Armstrong in his mock service of Archer, goes down to Tucker's research room to put a stop to the madness. They are beset by chimeras and though they have little trouble fighting them off, Archer gets away. Before anyone can stop him, he sends the army into Lior. There Scar sacrifices himself to transmute the army into Al's body, making him the Philosopher's Stone.
'Humankind cannot gain anything without first giving something in return. To obtain, something of equal value must be lost. That is Alchemy's first law of Equivalent Exchange. In those days, we really believed that to be the world's one, and only, truth'.
-Alphonse Elric
Edward and Alphonse Elric are two brothers gifted with the ability of alchemy, the science of taking one thing and changing it into another. However, alchemy works on the theory of Equivalent Exchange - for something to be created, something else of equal value must be sacrificed. When their mother dies, Edward decides to do the unthinkable - bringing her back to life by breaking one of Alchemy's biggest taboos and performing Human Alchemy. Thinking they have nothing more to lose, he and Alphonse make their attempt - but something goes horribly wrong. In the process, Alphonse loses his body and Edward loses his leg. Ed manages to save Al by attaching his spirit to a suit of armor, but at the cost of his arm and leg.