Bruce Blackburn is an agent with Military Intelligence.
Holding the rank of Captain in the U.S. Army, he has been an agent with that organization for some time, though how long is not mentioned. In fact, not very much at all is mentioned about Blackburn. We know by observation that he is still fairly young. He is certainly athletic. He is also quite good looking with black hair and a strong rugged face. Other than that, not much else is available.
Except, of course, that he dies in the first recorded adventure.
His death is faked, of course. So is that of his friend and fellow officer, a lieutenant known as Jackson (what Jackson's first name is we never learn). The two men serve together in G-2 and are of similar age and height and build. This is important to the storyline because as the first case we know about is beginning, someone has leaked a picture of Blackburn to the media and a newspaper splashed it on the front page. Since Blackburn was working mostly undercover, that was a very bad thing to have happen.
So, to compensate for his exposure, Blackburn decides he must die. It is not certain whose idea it was, his or their boss, the Colonel, but the fake car accident was reported to have claimed the lives of both Blackburn and Jackson. The two officers then headed to the office of a plastic surgeon who performed his magic and two weeks later, both report back for work looking like identical twins.
To continue the illusion that Blackburn was dead, they take over an antique store in New York City and use it as their base of operations. There, Jackson routinely runs the establishment while Blackburn goes about his undercover counter-espionage work. Jackson's role is usually a small one, if any. Trailing people seems to be his forte. Their look-alikeness comes into play on a few occasions when Blackburn is being watched or must be in a secret meeting with bad guys and needs to get away for his own work - in comes mirror-image Jackson.
Once or twice Jackson saves the day but it is Blackburn who nabs the bad guys and rescues whatever needs it. Helping out a time or three is Sergeant Gurk, apparently an employee of the antique shop who is also good at following folks or just guarding them. Gurk is there for the first few missions but then disappears (transferred?). Even Jackson plays less and less of a role in the activities though he never disappears for good.
A tad over halfway through the adventures, a young beautiful foreign agent arrives in America to cause all sorts of trouble and plays havoc with Blackburn. Known only as Sonya, she is the daughter of a notorious World War One female agent and is carrying on her mother's line of work. For quite a while it is not stated exactly who she works for except it is European and prone to goose-stepping. Towards the end of the series' run, the word Nazi is finally used clarifying the matter. Sonya will prove to be a resilient and very resourceful adversary and extremely good at escaping after being captured.