Can you help me create a plot for my book?

Some sort of feminine mythical creature (elf, fairy, pixie, what have you) takes up residence in his house and refuses to leave. She explains that there is another mythical creature (brownie, boggart, goblin, what have you) hiding in the house somewhere and she must find it because it's her enemy. It's an old manor house he inherited. They try enticing it with various things it likes over the course of several days, and he falls in love with her. But she finds her enemy and leaves, as he never says anything. He must go all the way to the deepest reaches of her equally mythical and fantastic homeland to get her back.