Originally planned as a reunion between the writer (Leo Marks) and the director (Michael Powell) of Peeping Tom (1960), this was inspired by Marks's own wartime career as an ace code-breaker. However, the notoriety of "Peeping Tom" made it hard to get the project off the ground. Powell became connected with American producer Herbert Brodkin during the making of the TV series Espionage (1963), and hoped that Brodkin's interest would get "Sebastian" made; when it finally was, both he and Marks were replaced. Powell had to be content with a producing credit, whilst Marks was credited solely with the story.