Hikaru Amatsuga is a middle school student who lives in Kita-Kagami City; he finds a mechanical lifeform in the shape of an arm named Alma.

The Mecha-Ude user Aki Murasame transfers into Hikaru's class with the intention to secure Alma.

Hikaru and Alma are taken to ARMS HQ, where Hikaru is told that he is not suited to be a Mecha-Ude user and Alma falls under ARMS' protection; multiple incidents occur where Mecha-Udes are forcibly ripped away from their users.

Hikaru deals with the fact that he has to still go to school while learning to live with Alma, working as a member of ARMS and being followed around by Aki.

Aki tracks down the worm-type Mecha-Ude user who killed her family, but the person she finds is her younger twin sister Fubuki.

An incident where most of the Mecha-Ude users in Kita-Kagami City lose their minds and go berserk occurs.