1938. A man alights from a train at London’s Liverpool Street station, and takes a taxi to Scotland Yard. On arrival there, the cabbie finds that his fare is dead, and it is soon realised that the man has been shot.

The victim worked for the intelligence services. This killing is clearly associated with a much larger investigation into other matters, including treason. Chief Inspector Adair and Inspector Davison are seconded to Special Branch to deal with the murder.

The two detectives help to uncover a web of intrigue involving thugs, murderers – and traitors in high places.