Before returning to the war, what had Mrs. Drover's fiance promised her?
This story is built around the "sinister troth" and "unnatural promise" that Mrs. Drover makes all those years ago with her then fiance before he goes to war and, to all intents and purposes, dies, leaving her free to carry on with her life and marry again. However, many years later, significantly during the next world war, Mrs. Drover then receives a mysterious letter recalling her to this flashback and a promise that they made together. The flashback gives us more detail as the man says to the younger Mrs. Drover that:
"I will be with you," he said, "sooner or later. You won't forget that. You need do nothing but wait."
The ghostly fiance thus promises that at some point in the future he will come back and the will be reunited, which of course happens at the end of this chilling story.