Kitakagami City is a pretty normal town... except that some people have gotten their hands on Mecha-Ude: powerful, sentient mechanical beings attached to the limbs of their hosts. When middle school kid Hikaru accidentally activates Alma, a mysterious Mecha-Ude with no memory of his past, he forms an unlikely connection with the talking arm. But where did Alma come from? And why are shadowy secret organizations and corporate assassins with deadly Mecha-Ude of their own suddenly chasing Hikaru down, hell-bent on stealing Alma? Hikaru and Alma must learn to work together to uncover the truth behind Alma's identity and prevent him from ending up in the wrong hands (or on the wrong arm)!