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.