'It all happens so fast and unexpected that everyone is quite still. The woman steps out of the train and on to the platform. And I, because I have nothing else to do just now, jump up out of my seat and follow her into the crowd...' Fed up with her job and her boy, Billie Bartholomew spends her time trapped in memories and dreams until her curiosity is woken by a woman acting strangely on the underground. They circle each other on the Northern Line revealing more of themselves as they spy on each other.