Hikaru Amatsuga is a middle school student who lives in Kita-Kagami City and one day, he hears cries for help and finds a mechanical lifeform in the shape of an arm named Alma. Before he knew it, Alma ended up connecting (derizing) to his hoodie.
The Mecha-Ude user Aki Murasame transfers into Hikaru's class with the intention to secure Alma. She belongs to an organization named ARMS and she starts following Hikaru and Alma around 24/7 and Hikaru seems completely exhausted because of all of this.
Hikaru and Alma are taken to ARMS HQ. There, Hikaru is told that he is not suited to be a Mecha-Ude user, and Alma falls under ARMS' protection. Meanwhile, multiple incidents where Mecha-Udes are forcibly ripped away from their users occur.
Hikaru was dealing with the fact that he had to still go to school while learning to live with Alma, working as a member of ARMS, and being followed around by Aki.
Aki is finally able to track down the worm-type Mecha-Ude user who killed her family. But the person she finds is her younger twin sister Fubuki, who should've also died in the incident that killed Aki's family. Aki is told the truth about what happened in that incident during her unexpected reunion with Fubuki and she's informed of one other cruel truth.
An incident where most of the Mecha-Ude users in Kita-Kagami City lost their minds and go berserk occurs. Hikaru tries to find out more about what the cause of this is, which is the worm-type Mecha-Ude, Amaryllis. But he's also trying to find out why Aki seems to be working with Amaryllis and if she actually has any part in this incident.
After a fierce battle with Amaryllis, Alma regains his memories. His expression seemed grim, so Hikaru asks him what's wrong, but then Alma runs away. In order to get clues of where Alma may have gone, Hikaru goes to ARMS HQ to see Alma's older brother, Fist. Fist then explains the past of all of the Mecha-Udes including Alma.
100 years ago, the founder of the Kagami Group, Yakumo, ends up losing his life after unsealing Ordela, the originator of all the Mecha-Udes. And in the present, Ordela was once again dormant, but the Kagami Group was trying to use Ordela's powers to open up the gate to the world of Mecha-Udes. Alma is then captured to become the trigger to open the gate and...
In order to seal away Ordela, who had awakened once more, Hikaru and Alma work alongside Kagami, who had acquired an even more powerful human-made Mecha-Ude, and begin their attack! The truth comes to light about Kagami and his older brother Naohito as well as the one who is actually pulling the strings behind all these incidents...
The true masterminds behind everything that happened appear before Hikaru and Alma and they end up being ARMS' leader Aljis, and his Mecha-Ude, Fist. And underneath Aljis's mask, they discover it's actually Yakumo, who should've died 100 years ago. What are they truly after? The curtain rises on Hikaru and Alma's final battle to determine the fate of all the Mecha-Udes and the Mecha-Ude users!
Fist was plotting to take in Ordela's powers and take over this world to make it the new home for the Mecha-Udes. During the battle, Hikaru tries to protect Alma from Fist's attack and ends up close to dying. Kagami then thinks up a plan to save Hikaru.
Kagami successfully revives Hikaru, but he lets his guard down for a second and his body is taken over by Fist, who had obtained Ordela's powers. Hikaru rises again in order to get Kagami's body back. Alma vs Fist... How will this battle between brothers end?
