If you're not familiar with the Fullmetal Alchemist world, understand that it's 1910, and alchemy, the science-magic of turning stuff into other stuff:super hot. Want a necklace? Just find something of similar size, like a loop of rope, draw some runes on it, and...BLING!
Alchemy can also resurrect. But when two naive pre-pubescent boys try using that miracle method to revive dead mama, they botch it hardcore and the arcane magic backfire-mutilates them. One loses an arm and leg, the other is trapped in the body of a metal monstrosity. So now gimp and giant robot embark on a world-hunt for the Philosopher's Stone, an "ooh" and "aah" mystical gem that can revert them back to their normal pube-budding bodies.
With Fullmetal enjoying a lengthy inhabitance on Adult Swim, you might already know this story. And that's also why the Marketing Gods decreed synergy - there shalt be action-RPG tie-in. There shalt be Dual Sympathy.