The story I'm writing has multiple story threads, which relate to each other subtly at first, while gradually converging over the course of the story.
The trouble is that one particular thread of the story appears (from the reader's perspective) to have nothing at all to do with the rest. It is relevant, and I believe its relevance will be clear in retrospect, but how do I make this clear to the reader while they're reading it?
The thread concerns two characters and their relationship, the nature and conclusion of which turns out to be significant to the main story. The trouble is that the only character who understands this significance is dead at the time of the story, and since investigating this character's death is a part of main plot, I think it'd be better if the actual reasons for its relevence were revealed to the reader only when revealed to the (living) characters.
What I need is a way to assure the reader that there is a reason that they're getting to know these characters, without actually revealing what that reason is.