The theme of the story is not as potent as many other works. Although WWI and WWII play an important part in the story, the theme really revolves around our hidden pasts and how they can return to haunt us.
In the story, Mrs. Dover promises herself to a young man during the first World War but he does not return. She is forced to find a life for herself with her new family, although she is never quite happy or content with her past because of her mysterious promise.
After returning to the house she finds the letter which reminds her of the past. This implies that our pasts have a way of turning up where we least expect it. It also hints that the past will return no matter how we might try to bury it in memory and life without acknowledging it.
Finally, the mysterious taxi cab driver takes off with Mrs. Dover terrified. Once again the past has caught up to the present.