Peter Quayle is the head of a British counter-espionage agency.
This middle-aged man, said to be 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 for 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 one of the recorded adventures, 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 lets 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, which, considering what he does in fact ask of his people is a lot says quite a bit about the kind of agent he was.
In his role as leader of this unnamed sub-agency, 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.