Zinnie and her sisters live by their wits in the murky tunnels beneath Edinburgh. They keep out of the way of the authorities and remain undetected. Until, that is, rumours of a ghost bring some prominent figures into the caverns they call home. Among them, a young Arthur Conan Doyle, on the cusp of a new investigation, and Scotland’s first female doctor, Sophia Jex-Blake.

As Zinnie becomes caught up in a world brimming with intrigue, can she help the budding detective find answers while also protecting the sisters she holds so dear?