A delinquent finds a girl abandoned in a box... and takes her home...

Touma Outa wakes up from a bad dream one morning and when he sees his reflection, there are horns growing out of his head. Suddenly, he is pounded in the head by a cute, hammer-carrying knight from the demon world. The horns shrink, but he is not off the hook yet, for the little knight, Pukateriosu, is there to prevent him from transforming completely into the Demon King whose soul has entered Outa's body. The PukaPuka hammer can return him to normal but it takes time to charge. In the meanwhile, Outa must try not to think worldly thoughts or be kissed, but it's easier said than done with his crush Shirayuki Himeka, the little demon Garbelion, and even Pukateriosu around...

The story centers on an 11-year-old boy who hates the lack of unique individuality in the world, and hates that he himself lacks individuality. One day, he finds that everything that might give a person individuality — such as their faces, names, and personal information — have been erased from his sight, as if by a marker pen. He later learns that this is a fatal disease, and that he will not live past 12.

He rejoices, having acquired the unique trait of "tragic main character," and moreover takes great pleasure in being the first to die from this disease so the disease will be named after him. At least, until a girl appears with the same disease. He quickly becomes disappointed that the girl seems to have a more unique spark to her, and her life expectancy is even shorter than his. To top it off, he learns that the disease might end up being named after her. He resolves to kill her before that happens.