Mark Blood is an assassin for the CIA.
He was, prior to the commencement of the chronicled events, a Captain in the Marine Corps serving in Vietnam. He had received numerous medals and decorations and had been much written about for his heroism. He was considered one of the very best the military had for infiltration and elimination. Nevertheless, he was disliked by his immediate superior for being the kind of soldier that man knew he never could be and that held the prospect of promotion for Blood unlikely.
As the series begins, Blood is reeling from the new of the murder of his wife during a plane hijacking. They had met and married during a two-month break from Vietnam that Blood had earned and during those two months they were inseparable. She had a "girl-Friday" job in Washington and he was a decorated hero but mainly they seemed to fit so perfectly together. While Blood was large and strong, she was small and soft. When he returned to Vietnam, she was in his mind constantly. When his jerk of a boss told him in a nonchalant manner after a mission in which Blood had been wounded, "Oh, by the way, your wife is dead" Blood lost control. Luck and two soldiers kept Blood from killing the man.
This launches Blood's personal mission to learn the truth and to punish the offenders. An offer by the CIA for employment was ignored as he was intent on justice for his spouse. Eventually, after he had taken care of the people who killer her, he reconsidered the offer and became an agent.
Trained in numerous ways to kill, Blood is able to dispatch an enemy with a gun, knife, or just his bare hands.