Inspector Marshall is an agent with the Special Branch.
When we first meet him, in an action taking place in 1918 at the tail end of the First World War, he is a sergeant working for what is referred to as the Special Service of Scotland Yard. It would be in a very serious and dangerous matter involving German infiltrators trying to assassinate then Prime Minister Lloyd George.
Over the next five decades, ending with an adventure in the 60s, he will see a decent series of well deserved promotions which take him to be an Inspector in that organization followed by holding the same rank in the renamed Special Branch. He will see a temporary assignment to the unusually name Ministry of Food, though considering the amount of black marketeering and American-supplied relief prompting large scale stealing, perhaps it is not so odd. This was now in the midst of the Second World War.
Marshall will receive a promotion to Chief Inspector as he is transferred to Special Branch and then shortly thereafter, sometime around the end of the 40s, he will return to his old haunts at C.I.D. but now he is a Superintendent in charge of a lot of people. That will not last more than a couple of years, though as the Cold War starts heating up and his services will again be needed running a segment at Special Branch yet again.
As the 50s come to a close, Marshall will be running the whole show at Special Branch as now he is made the head of the entire department.
During all this activity and all the various adventures that he will either be a part of out in the field or directing the flow of intelligence back at Headquarters, Marshall - whose first name we never learn - will be a vital support to the narrator of all these tales, Bernard Newman - the same man who is credited with penning the stories for publication.
Newman will become a vital part of the professional and personal life of Marshall, becoming his best friend and frequent companion and even best man at Newman's wedding. Regarding his own personal life, however, we learn virtually nothing. While it would not be unusual for a dedicated man like Marshall to "live for his work" whatever else he might be doing when not on the job remains a secret.
Many of the adventures will be either helping or directing or coercing help from a French master spy-catcher named Papa Pontivy (his first name is also a mystery). These two men will also become close friends who trust and rely on each other without hesitation.