From nan-chatte:

The freeter, Yui, found a stranger in a tattered state along the road. His name is Toshiki, a man with a strange appeal. Yui couldn't leave him alone, and so, he accompanied the stranger back to his house. Upon getting there, Toshiki revealed to be wilful, and Yui decided to go along with him. The next morning, however, he found that he has been trapped inside Toshiki's house.