The filming took place in the underground area of Prague Castle. The architect of the film recalled that it was so cold in the locations that the actors' mouths were constantly steaming. In the end, they solved the situation by bringing in two huge steam generators for heating.
Writer Jirí Hubac wrote this mystery thriller as an allegory of the political situation in August 1968, i.e. the illegal occupation of Czechoslovakia by the Soviet Union. This caused the film not to be broadcast again until the early 1990s and it remains relatively little known even in Czechia.