Twelve-year-old Addie Brown wants to fit in. But ever since she and her dad moved from small town America to bustling London, life is as bumpy as the Tube she rides to school. Addie's cranky neighbor tells her off but then invites her to tea! It seems as if Addie's best friend in America has already forgotten all about her. Worst of all, Addie's dad doesn't seem any happier than before, even though moving across the pond was his crazy idea.

But the biggest mystery Addie encounters is the lady in the green hat. Who is she? And why does she appear on the train when Addie is missing home – and her mom – the most?