Seiji Sawamura is the toughest student in his high school. His grades aren't very good because he fights more than he studies, but he tends to protect the weaker students from bullies. A few classmates idolize him; one (Midori Kasugano from a different school) shyly loves him from afar; but most are just afraid of him, which has made it impossible for him to find a girlfriend. In desperation, he says to himself that he will grow old with only his right Demon-hand as lifelong companion — until one morning when he wakes to find a miniature Midori attached to where his right hand used to be.
Over the next two weeks the pair adapt to this sudden and forced closeness. Much mayhem, and some romance, ensue. Seiji does his best to hide Midori from other people; she wears a bandage around her head and pretends to be his injured hand. Midori frequently professes her love to Seiji (even wearing a dress she made and embroidered with "I ? Seiji"), but Seiji fails to notice, even as he frequently laments that he'll never find a girl who loves him.