For over a hundred years, the private detective has been a staple of crime fiction. They're the lone crusaders for justice, operating outside the system, putting their lives on the line for their clients with only their own moral compass to guide them.

But the past few years has trashed that noble image. The phone hacking scandal revealed a far less pleasant side of the investigation industry: unscrupulous men and women who'll cheat, lie and break the law without a thought for the innocent lives they ruin.

So what is the reality of this shadowy industry? What sort of person becomes a private investigator? What kind of work do they actually do? And what safeguards are in place to ensure that they operate on the right side of the law?

Denise Mina conducts her own private investigation to find out. She takes to the streets of Glasgow to find out how to trail somebody and meets the men and woman who spend their lives investigating private secrets and public lies. Show less