Lynda Day George's character (Casey) is limited to being briefed at the beginning about the mission and driving a car at the end. Her role in the last season's missions are limited by her pregnancy.
The interrogation scene comes directly from the Ipcress File, a book from Len Deighton.
Guest stars Robert Middleton and Peter Mark Richman had 16 years previous been co-stars in Friendly Persuasion (1956).