In 1870 London, the body of a postal clerk is found inside a pneumatic railway car, and surgeon Samson Light has been accused. Tommy Jones must abandon his many jobs to pursue the witness who can exonerate his former tutor. Inspector Morgan of St Giles Station seems unusually reluctant to pursue the case, so Samson's barrister, wife, and friends must investigate. Clues lead to the General Post Office, the London Pneumatic Despatch Company, the highest realms of the Foreign Office, and inside Clerkenwell Gaol itself. Why was an ordinary clerk killed and, if it wasn't Samson who did it, who did?

This is the third Tommy Jones Mystery, after Murder at Old St. Thomas's and Murder at an Exhibition. All have some interconnected characters but are stand-alone stories.