Peter Quayle is the head of a British counter-espionage agency.
This middle-aged man, very tall and broad but with a swiftness and grace that seemed surprising, is an expert in the hunting and stopping of enemy agents, something he has done several years prior to the start of the series.
As might seem reasonable for a man responsible for sending men and women into danger, Quayle is friendly towards but seldom friendly with most of those close to him. It is said in the book, paraphrasing from another notable earlier, that Quayle have no real enemies but most of his friends didn't like him that much. If this bothered him, he never let on as he was too busy defending his country. Besides, he had once been an agent himself and is not asking anything of his people he was not ready to give himself.
In his role as leader of the agency which is not named, Quayle has the assistance of several experience and able agents such as Guelvada, a Belgian who is given British citizenship after years of service and Kane, a laughing, hearty man who would gladly buy you a drink if he didn't have to shoot you.
The series was mainly taking place during the Second World War and dealt, naturally, with spies trying to infiltrate the U.K. to learn secrets and perform sabotage. These enemy agents are all German. After the war, the first book deals with war criminals but after that, the scope started to change as the author branched the characters out further.
Quayle's adventures ended in 1951 with the death of the author.