The Secret Adversary
Agatha Christie wrote stories featuring the Beresfords between 1922 and 1973, depicting Tommy and Tuppence from before they were married through to their old age with adult children. It was decided to set the series in the 1950s, in the context of the Cold War, which could be made to fit Christie's existing stories. The two novels chosen, "The Secret Adversary" and "N or M?", were each made into three one-hour episodes, for a total of six.