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Alchemy is the understanding the structure of matter, decomposing it, and then reconstructing it into something new. There is only one law to abide by with alchemy-equilvilant exchange, is which something can be gained if something of equal value is lost. Those laws can be broken by a Philosopher's Stone.

Evelyn Joyce never imagined being the bodyguard of Elric brothers who are a quest, searching for the Philosopher's Stone would yank her into a much more devious plot than she could ever imagine.