Wild Sheep Chase by Haruki Murakami

It begins simply enough: A twenty-something advertising executive receives a postcard from a friend, and casually appropriates the image for an insurance company’s advertisement. What he doesn’t realise is that included in the pastoral scene is a mutant sheep with a star on its back, and in using this photo he has unwittingly captured the attention of a man in black who offers a menacing ultimatum: find the sheep or face dire consequences.