Simon Larren is a killer for the British Counter-Intelligence Agency.
He had been a killer before, trained in numerous ways of dispatch and highly experienced at it during the Second World War. He didn't kill in anger or for pleasure. He did it methodically, coldly, and efficiently. Working behind the lines with the Resistance, he killed often with a ruthless that unnerved even his teammates. When given an assignment, his superiors knew that Larren would carry it out, whatever it took.
Then the war ended and Larren, after a time, was released from the military. The days of violence were over and he had to put away his skills to find a normal job in a normal world. He did so, getting an office position that neither excited him nor bored him too much. Most importantly, he met a woman and fell in love. They married and life was as it should be.
That lasted several years until he and his wife, entering a pub for a nightcap, ran into an old war buddy. That friend was nervous and edgy but genuinely glad to see Larren after all the past years. A drink or two was had and they all left together. Then the ambush set for the friend took place. The friend was gunned down. So was Larren's wife.
Suddenly, the killer was back in full force. The organization for which the friend had worked offered Larren a chance to complete the mission and get his revenge. He took it and never looked back. Even after the mission was over, there was work to be done, killing work. And Larren was a killer.