When her grandma goes into a retirement home, Scarlet must sort through her belongings in the cramped flat above a shop in London. Here Scarlet comes across a real treasure: a novel that Gran wrote, about the life of a girl just like Scarlet, who lived in the flat and worked in a cake shop downstairs. But as Scarlet reads about Ruby's life in Victorian London, she realises this is not a piece of fiction - Ruby was real - facing real danger - and a mystery that unravels to the present day.