When Lucy Green sees a stranger drop his wallet on the Tube, it's the beginning of a cat-and-mouse relationship that enlightens, frustrates and arouses her in equal measure. She follows him to give it back, and is soon drawn into a situation she never dreamed possible, behaving in a way she never thought she would. She seems to respond to Ben at some animal level, and it's frightening. Should she follow her head, say goodbye and carry on her way to...