Mr. Calder and Mr. Behrens are fictional counter-intelligence agents created by the British mystery and thriller writer Michael Gilbert. The characters appear in 24 short stories, most of which first appeared in either the British magazine "Argosy" or the American "Ellery Queen's Mystery Magazine". The stories were collected in two anthologies, "Game without Rules" (1967) and "Mr. Calder & Mr. Behrens" (1982). One story, "Double, Double", was later republished in an Ellery Queen paperback. Many involve finding and dealing with British traitors or deeply embedded foreign agents; a few take place in Europe; one particularly grim one is set in World War II, during which Mr. Behrens is the bomb-maker in a failed attempt to assassinate Adolf Hitler. The characters also appeared in 16 BBC radio plays, largely based on the short stories.