It is September 1941 and young Freddie Hackett is a message runner - he collects messages from a government office and delivers them to various destinations around London. He sets off one day with a message, along a route of bombed-out houses, and witnesses a murder. Freddie instinctively wants to summon the police, but he has an envelope to deliver first – all communications during wartime could be urgent. When the man who answers the door appears to be the very same person he has just seen kill another, Freddie rushes to the police, but is summarily dismissed. However, he remembers an address in Fitzroy Square, belonging to a private investigator, Maisie Dobbs. Will she believe him and help solve the mystery?