It is loosely based on story of members of resistance organization Obrana národa who were known as Three Kings.

After their country is occupied by Nazi Germany they join resistance organisation Obrana národa.

They lead brave but already lost fight against occupants.

[6] Largest problem for filmmakers was to giver Prague atmosphere of Protectorate of Bohemia and Moravia as locations that weren't changed since World War II almost didn't exist in Prague anymore.

Historian Vilém Čermák acted as a consultant on the series.