The protagonist, Tomoya, was abandoned by his mother at a tender age. He was taken in by Honjou Arisa after a chance meeting. Four girl adoptees who had joined the Honjou family home for similar reasons also lived there.

Though they were not related by blood, Tomoya treated the girls as if they were his siblings. They lived peaceful lives at the Honjou residence... but one day, he encountered a strange woman in town. She was wreathed in something that looked like iron – and upon seeing her figure, he remembered that he, too, had once harbored such a metallic augment.

"Is that... your Anima?"

A right arm turned to metal... A peculiar corpse discovered at school... A transfer student exuding dark mist... And a foreign warrior wreathed in thick steel armor... Tomoya's peaceful life would soon begin to crumble as disaster came to visit him and his sisters. To protect them, he shall have to once again wreathe his right arm in Anima.