John Barrie is Sergeant Cork, one of the early members of London's Metropolitan Police C.I.D. - plain clothes detectives tasked with maintaining law and order in the seething metropolis of Victorian London. Assisted by Bob Marriott (William Gaunt) from his attic at Old Scotland Yard, Cork has to contend with the heavy crimewave created by the massive population boom, industrial expansion and an ever-widening gap between the rich and poor.