After watching the episode, The Russian House, set in June of 1945, I couldn't help but imagine that there must have been some history between Foyle and Elsa. She was quite willing to help him with information, at some risk to herself, and clearly she must have done so in the past. This is my version of their back story.

All upheavals, obstacles and delays behind them, Christopher and Samantha return home after the rather significant events at the charity auction, ready to enter into a new kind of life together.