When Zoe Walker sees her photo in the classifieds section of a London newspaper, she is determined to find out why it's there. There's no explanation: just a grainy image, a website address and a phone number. She takes it home to her family, who are convinced it's just someone who looks like Zoe. But the next day the advert shows a photo of a different woman, and another the day after that.

Is it a mistake? A coincidence? Or is someone keeping track of every move they make . . .