Mr. Calder and Mr. Behrens are agents for Britain's Joint Services Standing Intelligence Committee (JSSIC).
The formal method of addressing is used above because in the numerous stories about these two gentlemen, that is the way in which they are usually referred. Their full names are Daniel Calder and Samuel Behrens.
This absolutely incomparable pair of agents were both born in the early 1910's. After each had attended the college of his choice, often taking courses in other countries to further their educations, both joined around the same time MI-6, probably 1935. Their service during WWII are still classified so actual activity is not mentioned but both were well regarded for their work and would have received numerous medals if such were given to people working in the clandestine world..
After the War ended, both remained agents with MI-6 until the mid 1950's when both were asked to transfer to the JSSIC, a new agency. They were picked because their reputation showed them to be the best there was and the head of the new bureau wanted nothing but the best. The JSSIC had wide latitude as to the missions it would accept and the cases that Mr. Calder and Mr. Behrens were given reflect that, though largely they would hunt down spies which had infiltrated the government institutions or help shore up security measures where feared lax.
Mr. Behrens was fluent in German, French, Greek, Italian, and Russian. Mr. Calder spoke Greek, Italian, Arabic, Russian, and was especially gifted with Albanian and Hungarian. They were both excellent marksmen and superb drivers and incredibly observant investigators.
In addition to being the best of friends, they each had a cottage just down the road from each other in Lamperdown, Kent. They share many of the same qualities. Both are very proper and polite to everyone they meet. They both act with amazing humility, neither wanting to stand out or take care even for things he actually did. And both are ruthless hunters of men, quite willing to kill without the least bit of hesitation if they feel it called for.
As the series progressed, the age of the men increased until at the last of the stories both were near or at retirement age. Throughout it all, though, they remained gentile and unflinchingly lethal.
All the adventures of Mr. Calder and Mr. Behrens came in the form of short stories, of which there were 23. This pair of exceptional agents got their fictional start in the British magazine Argosy in 1962. [This is a totally different publication for the similarly named American magazine. The British one billed itself as The Short Story Magazine, came out every month from 1926 to 1974. To further complicate things, this Argosy was the second British periodical to use that name, the first one going out of business in 1901.]
The last six stories were written after Argosy had ceased operations and were instead published in the Ellery Queen Mystery Magazine as were at least three of those already in Argosy.
The two books mentioned below are compediums of the all the short stories. Another compedium, The Road To Damascus and Other Stories, was published years later in a very limited run but the stories in that group were also in the two books described here.